Sensors

Temperature sensor (Pt-1000)

Figure: Pt-1000 temperature sensor

Specifications

Measurement range: 0 ~ 100 ºC Accuracy: DIN EN 60751 Resistance (0 ºC): 1000 Ω Diameter: 6 mm Length: 40 mm Cable length: ~500 cm

Measurement process

The Pt-1000 is a resistive sensor whose conductivity varies in function of the temperature. The Smart Water board has been endowed with an instrumentation amplifier which allows to read the sensor placed in a voltage divider configuration along with one precision 1 kΩ resistor, which leads to an operation range between 0 ºC and 100 ºC approximately.

The whole reading process, from the voltage acquisition at the analog-to-digital converter to the conversion from the volts into Celsius degree, is performed by the readTemperature() function.

The temperature sensor is directly powered from the 5 V supply, so is no necessary to switch the sensor on, but it is advisable to not keep the Smart Water board powered for extended periods and switch it off once the measurement process has finished.

{
float valuePT1000 = 0.0;
Water.ON();
// A few milliseconds for power supply stabilization
delay(10);
// Reading of the ORP sensor
value_temperature = TemperatureSensor.readTemperature();
// Print of the results
USB.print(F(\"Temperature (celsius degrees): \"));
USB.println(value_temperature);
// Delay to not heat the PT1000
delay(1000);
}

You can find a complete example code for reading the temperature sensor in the following link:

Socket

To connect the Pt-1000 sensor to the Smart Water board a two ways PTSM connector has been placed, as indicated in the figure below. Both pins of the sensor can be connected to any of the two ways, since there is no polarity to be respected.

Figure: Image of the connector for the Pt-1000 sensor

Conductivity sensor

Figure: Conductivity sensor

Specifications

Sensor type: Two electrodes sensor Electrode material: Platinum Conductivity cell constant: 1 ± 0.2 cm^-1^ Cable length: ~500 cm

Measurement process

The conductivity sensor is a two-pole cell whose resistance varies in function of the conductivity of the liquid it is immersed in. That conductivity will be proportional to the conductance of the sensor (the inverse of its resistance), multiplied by the constant cell, in the case of the Libelium sensor around 1cm⁻¹, leading to a value in Siemens per centimeter (S/cm). For an accurate measurement, please take a look at section "Calibration Procedure", where the calibration procedure is detailed.

To power the conductivity sensor an alternating current circuit has been installed in order to avoid the polarization of the platinum electrodes.

In the case of the conductivity sensor the readConductivity() function will return the resistance of the sensor in ohms. In order to convert this value into a useful conductivity unit (uS/cm) function conductivityConversion() will have to be invoked with the calibration parameters of the sensor (please refer to section "API" for more information about how to use this function).

Below we can see a basic code for reading the conductivity sensor using the API functions (for more information take a look at section "API"):

{
// Reading of the Conductivity sensor
cond = ConductivitySensor.readConductivity();
// Print of the results
USB.print(F(\"Conductivity Output Resistance: \"));
USB.print(cond);
// Conversion from resistance into ms/cm
calculated = ConductivitySensor.conductivityConversion(value_cond);
// Print of the results
USB.print(F(\" Conductivity of the solution (uS/cm): \"));
USB.println(value_calculated);
}

You can find a complete example code for reading the conductivity sensor in the following link:

The magnetic field between the two electrodes of the conductivity sensor may be affected by objects close to the probe, so it will be necessary to maintain the sensor at least five centimeters apart from the surroundings.

Socket

To connect the conductivity sensor to its respective socket (highlighted in the image below) it is needed a pigtail to adapt the BNC connection of the sensor to the SMA-RP socket in the board. That pigtail is included when acquiring the Smart Water board from Libelium.

Figure: Image of the connector for the conductivity sensor

Calibration procedure

There are three different Calibration kits for Conductivity: K=0.1, K=1; K=10. The K factor is related to the salinity of the water we want to measure. Each calibration kit takes two solutions:

  • K=0.1

    • around µS 220

    • around µS 3000

  • K=1

    • around µS 10500

    • around µS 40000

  • K=10

    • around µS 62000

    • around µS 90000

The concentration value may vary in each batch with respect to the value shown above, due to the nature of the manufacturing process. That is why we wrote "around". The sticker in each bottle indicates the exact value. Please notice that the software implemented for this calibration procedure is flexible, so it is valid for any concentration values.

The concentration value may vary in each batch with respect to the value shown above, due to the nature of the manufacturing process. That is why we wrote "around". The sticker in each bottle indicates the exact value. Please notice that the software implemented for this calibration procedure is flexible, so it is valid for any concentration values.

In the next table we see the typical conductivity depending on the kind of water we want to monitor:

Table of aqueous conductivities

Solution

µS/cm

mS/cm

ppm

Totally pure water

0.055

-

-

Typical DI water

0.1

-

-

Distilled water

0.5

-

-

Domestic "tap" water

500-800

0.5-0.8

250-400

Potable water (max)

1055

1.055

528

Sea water

50,000 - 60,000

56

28,000

We see as the relation between conductivity and dissolved solids is approximately:

2 µS/cm = 1 ppm (which is the same as 1 mg/l)

In order to get an accurate measurement it is recommended to calibrate the conductivity sensor to obtain a precise value of the cell constant. Although a single point calibration should be theoretically enough, a two point calibration is advisable to compensate for side effects of the circuitry, such as the resistance of the sensor wire or the connector. For a proper calibration two solutions of a conductivity as close as possible to that of the target environment should be used.

Below, the calibration procedure is detailed step by step. For this you will need to have the Waspmote with the Smart Water sensor board sending the information collected from the conductivity sensor through the USB or any communication module and the two calibration solutions to be used:

Figure: Image of the material necessary for the conductivity calibration process. Concentration values may vary.
  1. Turn on the Waspmote with the Smart Water sensor board and the conductivity sensor connected.

  2. Upload the example "Conductivity sensor Reading for Smart Water" to the Waspmote board and make sure of receiving the data in the serial monitor.

  3. Pour the conductivity solutions in two beakers.

  4. Introduce the conductivity probe in the first solution and wait for a stable output. Make sure that the sensor is completely immersed in the solution and that it is not close to the beaker wall, which may affect the field between the electrodes and disturb the measurement. Once the output is steady, annotate the value of the Conductivity Output Resistance obtained. It is really important to give time to the output to get stable, especially the first time we use a sensor. This will take several minutes.

  5. After getting the sensor from the first solution, carefully rinse it (do not dry the sensor, since the platinum black layer of the electrodes could be damaged) and repeat the process explained in step 3 with the second solution.

  6. Introduce the values noted and the conductivity of the calibration solutions in your code, as shown in the next images.

Figure: In this define, you should write the value of the calibration solution used
Figure: In this define, you should write the Conductivity Output Resistance value obtained
  1. The function setCalibrationPoints() is used to configure the calibration parameters.

  2. Upload the code again with the new calibration values obtained from the calibration process.

  3. To know more about the calibration kits provided by Libelium go to the "Calibration Solutions" section.

Operation and maintenance

  1. Any sensor probe needs to be cleaned periodically to remove the possible fouling or other biological material that could appear in the sensor. It should be cleaned with distilled water. A soft towel can be used to dry the sensor and remove biological material.

  2. It is recommended to soak the electrode of the sensor in distilled water to hydrate the electrode before use. If the electrode is not giving correct values, it should be immersed in a solution of 10% of nitric acid or hydrochloric acid with distilled water. Then the electrode must be washed with distilled water as explained before.

Dissolved Oxygen sensor

Figure: Image of the Dissolved Oxygen sensor

Specifications

Sensor type: Galvanic cell Range: 0~20 mg/L Accuracy: ±2% Maximum operation temperature: 50 ºC Saturation output: 33 mV ±9 mV Pressure: 0~100 psig (7.5 Bar) Calibration: Single point in air Response Time: After equilibration, 2 minutes for 2 mV Cable length: ~500 cm

Measurement process

The galvanic cell provides an output voltage proportional to the concentration of dissolved oxygen in the solution under measurement without the need of a supply voltage. This value is amplified to obtain a better resolution and measured with the analog-to-digital converter placed on the Smart Water board. Below, a sample of code to read the sensor is shown (for more information take a look at section "API"):

{
// Reading of the DO sensor
value_do = DOSensor.readDO();
// Print of the results
USB.print(F(\"DO Output Voltage: \"));
USB.print(value_do);
// Conversion from volts into dissolved oxygen percentage
value_calculated = DOSensor.DOConversion(value_do);
// Print of the results
USB.print(F(\" DO Percentage: \"));
USB.println(value_calculated);
}

The value returned by the readDO() function for this sensor is expressed in volts. For a conversion into a percentage of oxygen saturation function DOConversion() will have to be used, introducing the calibration value in volt as an input. Take a look at section "API" for more information about how to call this function.

One of the drawbacks from using a galvanic probe is that it consumes a very small amount of the oxygen it reads. Therefore, a small amount of water movement is necessary to take accurate readings, approximately 60 ml/min.

You can find a complete example code for reading the Dissolved Oxygen sensor in the following link:

Socket

To connect the dissolved oxygen sensor to its respective socket (highlighted in the image below) it is needed a pigtail to adapt the BNC connection of the sensor to the SMA-RP socket in the board. That pigtail is included when acquiring the Smart Water board from Libelium.

Figure: Image of the connector for the dissolved oxygen sensor

Calibration procedure

The calibration process for the dissolved oxygen sensor can be divided into two parts. The first one corresponds to a single point calibration, which should be enough for most applications. In the second one, the calibration is extended to a second point, which leads to a more accurate value, although it implies a high leap in complexity. This second point is specially advisable if the sensor is going to operate in an environment with a low oxygen concentration.

Figure: Image of the material necessary for the dissolved oxygen calibration process

First point:

  1. Turn on the Waspmote with the Smart Water sensor board and the dissolved oxygen sensor connected.

  2. Upload the code "[Dissolved Oxygen Sensor Reading]{.underline}" and make sure the data from the sensor is being received properly in the serial monitor.

  3. To get a saturated value of the sensor, just clean the sensor with distilled or de-ionized water, carefully rinse it and dry it with a paper cloth. Once in air, wait for the output stabilization. Once the measured value is steady, write it down. If the sensor has been deployed in a placement with difficult access, instead of getting it out it is possible to bubble air in the fluid until the sensor reaches saturation, though it is a less reliable method. It is really important to give time to the output to get stable, especially the first time we use a sensor. This will take several minutes.

  4. This value corresponds to a saturated output (100% of dissolved oxygen). In case of a single point calibration, introduce this value in the code as shown in the image below, while introducing a 0 for ZERO_VALUE, or add it to the conversion in the software at reception. Otherwise, go on with the second point procedure.

  5. Upload the code again with the new calibration values obtained from the calibration process.

Figure: In this define, you should write the calibration value obtained

Second point:

  1. Once obtained the first point of the calibration, it is possible to extend it to a second point to increase the accuracy of the measurement. To obtain this new calibration values a saturated solution of sodium sulfite will be required (take a look at section "Calibration Solutions").

  2. Pour the solution in a beaker and introduce the sensor, making sure it is completely immersed but not touching the walls nor the bottom of the beaker.

  3. The output voltage of the sensor will start to drop. It will take a few minutes until it reaches a stable measurement, close to zero volts. When this value has been achieved, write it down, get the sensor out of the solution and carefully rinse it.

  4. Add the second calibration point in the place of ZERO_VALUE or to the conversion in the reception and come back to normal operation.

  5. Upload the code again with the new calibration values obtained from the calibration process.

  6. To know more about the calibration kits provided by Libelium go to the "Calibration Solutions" section.

Maintenance kit

Figure: Dissolved oxygen maintenance kit

The Dissolved oxygen probe reacts with oxygen in the water: the more oxygen it reacts with, the more the probe is depleted of its electrolyte solution. Typically, a dissolved oxygen probe will last ~2 years before the electrolyte is depleted (results will vary, it depends on different use cases). When the electrolyte is depleted, the probe will read very low numbers. The best practice is to replace the electrolyte solution and Teflon membrane every 2 years.

If your dissolved oxygen probe has not been in use for more than one year, the Teflon sensing membrane can dry out and the internal electrolyte solution could leach out of the probe. The dissolved oxygen maintenance kit will get your dissolved oxygen probe back in working order quickly. Although it is not necessary to replace the sensing membrane or electrolyte solution during normal operation; the membrane can be damaged if it is hit with fast moving debris in the water.

Probe maintenance:

  1. Unscrew the pre-membraned cap from the tip of the probe and discard (figure 1).

  2. Remove the cap from the bottle of DO Electrolyte Solution. Remove the 10 mL syringe from packaging and attach hub needle to end of syringe as shown in fig 2.

  3. Use needle and syringe to withdraw 2 mL solution from bottle as shown in fig 3. Be carefully with air bubbles into syringe. It is necessary to avoid inserting bubbles into the sensor.

  4. Insert the needle into each of the four holes surrounding the silver cathode. Inject solution until it leaks out of a fill hole (fig 4). If the solution spills, do not worry. In case your dissolved oxygen sensor has a solid white residue, it will be good to inject more solution to clean the sensor.

  5. Some light flicks on the sensor are enough to make the solution move inside the sensor. Be careful, only light flicks.

  6. Replace the cap by threading on sensor clockwise (opposite of fig 1). Please, pay attention to avoid bubbles between the sensor and the membrane cap.

Figure: Probe maintenance process

pH sensor

Figure: Image of the pH sensor

Specifications

Sensor type: Combination electrode Measurement range: 0~14 pH Temperature of operation: 0~80 ºC Zero electric potential: 7±0.25 p Response time: \<1 min Internal resistance: ≤250 MΩ Repeatability: 0.017 PTS (percentage of slope ): >98.5 Noise: \<0.5 mV Alkali error: 15 mV Reader accuracy: up to 0.01 (in function of calibration) Cable length: ~500 cm

Measurement process

The pH sensor integrated in the Smart Water sensor board is a combination electrode that provides a voltage proportional to the pH of the solution, corresponding the pH 7 with the voltage reference of 2.048 V of the circuit, with an uncertainty of ±0.25 pH. To get an accurate value from these sensors it is necessary both to carry out a calibration and to compensate the output of the sensor for the temperature variation from that of the calibration moment. Once the sensor has been calibrated, these two tasks are carried out in the pHConversion() function of the API.

If a reading of the sensor is performed without invoking pHConversion(), the value obtained will be the voltage read by the analog-to-digital converter in volts. This function may be called using the calibration parameters or just the theoretical values, take a look at section "API" for more information about how this function must be employed. In the code below a basic example for reading this sensor is shown:

{
// Read the pH sensor
value_pH = pHSensor.readpH();
// Read the temperature sensor
value_temp = temperatureSensor.readTemperature();
// Print the output values
USB.print(F(\"pH value: \"));
USB.print(value_pH);
USB.print(F(\"volts | \"));
USB.print(F(\" temperature: \"));
USB.print(value_temp);
USB.print(F(\"degrees | \"));
// Convert the value read with the information obtained in calibration
value_pH_calculated = pHSensor.pHConversion(value_pH,value_temp);
USB.print(F(\" pH Estimated: \"));
USB.println(value_pH_calculated);
}

You can find a complete example code for reading the pH sensor in the following link:

Socket

Like the other combination electrodes (oxidation-reduction potential sensor), the pH probe can be connected to sockets marked in the image below, which share the same characteristics. Having the sensor a BNC connector, a pigtail to adapt it to the SMA-RP sockets of the board (included when purchasing the Smart Water sensor board) must be used.

Figure: Image of connectors suitable for the pH sensor

Calibration procedure

A periodic calibration is highly recommended for the pH sensors if an accurate measurement is desired. If the sensor is going to be deployed in an environmental with a changing temperature or the calibration is going to be carried out under a different temperature from the operation conditions, it will also be required a temperature compensation to update the sensitivity of the sensor to the actual conditions.

The required material for the pH sensor calibration consists of a Waspmote and Smart Water sensor board, the pH sensor to be calibrated (plus a Pt-1000 sensor if temperature compensation is going to be applied) and three pH buffer solutions, one of 7.0 pH and two of higher and lower values (4.0 pH and 10.0 pH). Note that for a proper calibration all the buffers must be at the same temperature, being a temperature the closest possible to that of operation or, if this one is not known, of approximately 25 ºC. The following list includes the complete calibration process:

Figure: Image of the material necessary for the pH calibration process
  1. Turn on the Waspmote with the Smart Water sensor board and the pH sensor and the Pt-1000 connected.

  2. Upload the code "[pH Sensor Reading]{.underline}" and make sure the data is being correctly received through the USB or another communication module.

  3. Pour the solutions in three beakers. The 4.0 pH solution is red, the 7.0 pH solution yellow and the 10.0 pH solution blue. It is recommended that the solutions are at the temperature that will be found at the installation environment.

  4. Introduce the pH sensor and the Pt-1000 in the 7.0 pH buffer solution and wait for a stable measurement, which may take a few minutes. Make sure the sensors are completely immersed in the solution. When there is a stable output for the sensors, annotate the value in volts obtained. It is really important to give time to the output to get stable, especially the first time we use a sensor. This will take several minutes.

Figure: This value in volts should be annotated for each calibration solution
  1. Get the sensor out of the solution and rinse it gently, preferably with distilled or de-ionized water, and introduce it in the 4.0 pH solution, which will cause an increase in the output voltage, along with the Pt-1000 sensor to check that all the solutions are at the same temperature. Again, wait for the stabilization of the output values and write them down.

  2. Repeat step 3 with the 10.0 pH solution, which should make the sensor output voltage fall below that for the 7.0 pH solution. Under 25 ºC the outputs expected for these solutions are 2.048 V for 7pH, 2,227 mV for 4.0 pH and 1.868 mV for 10.0 pH), with the possibility of finding a difference of a few tenths of millivolts for each value and a change in the sensitivity owing to the difference of temperature.

  3. Significantly different values after the exposure of the sensor to the solutions may be caused by a bubble in the sensitive bulb, especially if it is the first calibration after shipment. Shaking the sensor downward like a clinical thermometer will remove them, solving the problem.

  4. Introduce the calibration values in the measurement code as shown in the images below.

Figure: In this define you should write the value in volts obtained with the pH solutions
  1. Upload the code again with the new calibration values obtained from the calibration process.

  2. To know more about the calibration kits provided by Libelium go to the "Calibration Solutions" section.

Operation and maintenance

  1. When using the sensor for the first time or when it has been a long time without use, it is recommended to immerse the sensor in a 3.3 mol/L KCl (potassium chloride) solution for 2 hours.

  2. Before using the sensor it is recommended to clean it with distilled water and then dry the excess with filter absorbent paper.

  3. If the electrode gets dirty, soak it in acetone for 8 hours. After that clean it carefully with distilled water.

  4. Do not use the sensor in a non-aqueous solution.

  5. The BNC connector should be kept clean and dry.

Oxidation-reduction potential sensor

Figure: Image of the oxidation-reduction potential sensor

Specifications

Sensor type: Combination electrode Electric Potential: 245~270 mV Measurement range: 0 ~ ±1999 mV Reference impedance: 10 kΩ Stability: ±8 mV/24 h Cable length: ~500 cm

Measurement process

Like the pH sensor, the ORP probe is a combination electrode whose output voltage is equivalent to the potential of the solution, so it will share the connection sockets with that sensor. The output of the circuitry to which it is connected is directly read from the analog-to-digital converter of the Smart Water sensor board, being the 2.048 V reference subtracted to obtain the actual oxidation-reduction potential in volts (in this case, since this parameter is directly a voltage it is not necessary to call a conversion function). Below is shown a code to read this sensor:

{
// Reading of the ORP sensor
value_orp = ORPSensor.readORP();
// Apply the calibration offset
value_calculated = value_orp - calibration_offset;
// Print of the results
USB.print(F(\" ORP Estimated: \"));
USB.print(value_calculated);
USB.println(F(\" volts\"));
}

You can find a complete example code for reading the ORP sensor in the following link:

Socket

Since the ORP sensor is a combination electrode, it will be possible to connect it to any of the sockets shown in the image below.

Figure: Image of connectors suitable for the ORP sensor

Calibration procedure

Since the sensor output is a straightforward voltage directly measured by the Waspmote\'s analog-to-digital converter there is not a conversion function. Thus, the calibration process will consist in a verification of the proper operation of the sensor with an ORP calibration standard solution, which will lead to the application of a correction offset in the code or in the data processing in the receiver. The procedure to follow is detailed step by step below:

Figure: Image of the material necessary for the ORP calibration process
  1. Turn on the Waspmote with the Smart Water sensor board and the ORP sensor connected.

  2. Upload the code "ORP Sensor Reading" and make sure that the data from the sensor is being received through the USB or another communication module.

  3. Pour the calibration solution in a beaker. Libelium provides a standard solution of 225 mV at 25 ºC.

  4. Rinse the sensor with distilled or de-ionized water and softly dry it with filter paper.

  5. Introduce the sensor into the calibration solution, making sure it stays completely immersed without contact with the beaker walls or bottom, and wait for the output value to stabilize. If the test is being carried out with the solution provided by Libelium at approximately 25 ºC, the output should be around the 225 mV, with a 10%~15% error.

  6. It is really important to give time to the output to get stable, especially the first time we use a sensor. This will take several minutes.

  7. A similar problem to the one mentioned for the pH sensor may appear owed to air bubbles in the sensitive bulb. If this is the case, shaking the sensor downward as stated for that sensor will also solve this problem.

  8. Remove the sensor, rinse it with distilled or de-ionized water again and return it to its working place.

  9. Write down the offset (the obtained value -- 225 mV) and introduce it in the Waspmote code or in the data processing in the receiver. Take into account that there is no conversion function for this sensor in the Smart Water libraries.

Figure: In this define you should write the offset obtained

To know more about the calibration kits provided by Libelium go to the "Calibration Solutions" section.

Operation and maintenance

  1. When using the sensor for the first time or when it has been a long time without use, it is recommended to immerse the sensor in a 3.3 mol/L KCl (potassium chloride) solution for 2 hours.

  2. Before using the sensor it is recommended to clean it with distilled water and then dry the excess with filter absorbent paper.

  3. If the electrode gets dirty, soak it in acetone for 8 hours. After that clean it carefully with distilled water.

  4. Do not use the sensor in a non-aqueous solution.

  5. The BNC connector should be kept clean and dry.

Turbidity sensor

Figure: Turbidity sensor

Specifications

Sensor type: IR optical sensor with optical fiber Measurement range: 0-4000 NTU Accuracy: 5% (around 1 NTU in the lower scale) Robust and waterproof : IP68 Digital output: Modbus RS-485 Power consumption : 820 μA Power supply: 5 V Stocking temperature: -10 to +60 °C Material: PVC, Quartz, PMMA, Nickel-plated brass Cable length: < 300 cm

This sensor is available for Waspmote "OEM" line (as a kit) and for Plug & Sense! line too (as a probe).

In the OEM version, the sensor must be connected in the connector shown in the image of the section below. On the other hand, for the Plug & Sense! version, everything comes connected inside the node and the user just needs to plug the probe to the F bottom socket.

The turbidity sensor is extremely sensitive and the user must treat it with special care in all situations (laboratory tests, development, installation, etc). The sensor must be installed in a solid way and protected from any impact.

Refer to Libelium website for more information.

Since July 2019 this sensor is no longer available for the Smart Water model. It is only available for the Smart Water Xtreme model. Refer to Libelium website for more information

Turbidity socket

The turbidity sensor allows the measure of the temperature, so the Pt-1000 temperature sensor must be disconnected and cannot be used simultaneously with the turbidity sensor.

Figure: Connection of the turbidity sensor on the Smart Water sensor board

Turbidity: the parameter

Turbidity is the haziness of a fluid caused by individual solid particles that are generally invisible to the naked eye. The measurement of turbidity is a key test of water quality. Nephelometers, or nephelometric turbidimeters, measure the light scattered at an angle of 90° by one detector from the incident light beam generated by an incandescent light bulb. Readings are reported in Nephelometric Turbidity Units, or NTUs. NTU has been the traditional reporting unit for turbidity and is still recognized by some as the "universal" unit of measure, regardless of the technology used.

The measurement of the turbidity is important in the next scenarios:

  • Urban waste water treatment (inlet / outlet controls)

  • Sanitation network

  • Industrial effluent treatment

  • Surface water monitoring

  • Drinking water

Measurement process

The Turbidity sensor, is a digital sensor and uses RS-485 output in combination with the Modbus library. The RS-485 standard allows the use of longer wires, and thanks to the use of differential signaling it resists the electromagnetic interferences.

Up now, the measurement of the turbidity was not easy and must be done by qualified personal, collecting samples for laboratory exams. Libelium\'s sensor permits automatic metering. According to the sensor\'s manufacturer specifications, the measurement of the turbidity must be done in a light tight pot, the sensor must be in a fixed position and the water container must be clean or the measure may be wrong.

The accuracy of this sensor is about 1 NTU. The WHO (World Health Organization), establishes that the turbidity of drinking water shouldn't be more than 5 NTU, and should ideally be below 1 NTU. This sensor can be used to determine if the turbidity level of the water is under acceptable levels for consumption, but can\'t be used to determine the exact value of turbidity, because this values is measured in specialized laboratories using special equipment.

The sensor takes some time to get stable values. The correct way to measure the turbidity using this sensor is to take samples for approximately 60-90 seconds and then make the mean between the measured values. Libelium, provides the necessary examples included in the Waspmote IDE.

The Turbidity sensor is calibrated in factory and verified in Libelium. Basically, the provider performs measurements with a range of normalized chemical solutions, which have a known and exact NTU value. This allows them to generate calibration data which is hard-coded inside the sensor to improve the accuracy of the sensor.

In the code below a basic example for reading this sensor connected to the Smart Water sensor board is shown:

{
// Start a new measure
turbiditySensor.readTurbidity();
// Get the Turbidity Measure
float turbidity = turbiditySensor.getTurbidity();
}

You can find a complete example code for reading the turbidity sensor in the following link:

The OEM Turbidity Sensor Kit includes:

  • Turbidity Calibration Kit (low) and Turbidity Calibration Kit (high)

The placement of the sensor is important to get a correct turbidity measurement. The sensor must be placed in a fixed position, you must make sure that light cannot interfere with the optical part of the sensor. Otherwise, sun or light can affect the values. It is necessary a minimum distance, about 3-4 centimeters, between the sensor and the bottom of the beaker.

Figure: Turbidity sensor image wrongly and correctly place

Calibration of the sensor

Important: Libelium provides this sensor calibrated, but a periodic recalibration of the sensors is highly advisable (every 6 months approximately) in order to maintain an accurate measurement along time. The good recalibration process of the sensor is responsibility of the user. Libelium provides standard calibration solutions for some turbidity values; these solutions are optional but highly recommended.

Libelium can provide 2 different turbidity calibration kits, each one is composed of 2 solutions which will provide 2 reference points:

  • Low turbidity: about 0-10 NTU

  • High turbidity: about 10-40 NTU

Figure: Turbidity calibration kit

In the Waspmote Development section you can find complete examples about using this board.

Go to: https://development.libelium.com/waspmote/code-examples

Calibration solutions

Libelium provides several calibration solutions to calibrate the sensors.

pH Calibration kit

Characteristics:

  • 4.0 pH (red), 7.0 pH (yellow), 10.0 pH (blue) ±0.02 pH at 25 ºC

  • 125 ml each

This kit includes three buffer solutions of 4.0 pH, 7.0 pH and 10.0 pH, of colors red, yellow and blue respectively. The calibration process is described in section "Calibration procedure", when handling them pay attention to the information provided in the MSDS.

Figure: Image of the pH calibration kit

Conductivity calibration kits

Characteristics:

  • 3 kits for K = 0.1, K = 1 and K = 10

  • around 0.22 mS, 3 mS, 10.5 mS, 40 mS, 62 mS and 90 mS at 25 ºC

  • 125 ml each

Six solutions for sensor calibration are included within these 3 kits, so the probe can be calibrated in a way range of conductivities. The conductivity values of these solutions are around 0.22 mS, 3 mS, 10.5 mS, 40 mS, 62 mS and 90 mS.

Figure: Image of the 3 conductivity calibration kits. Concentration values may vary.

ORP Calibration solution

Characteristics:

  • 225 mV ±2 mV at 25 ºC

  • 100 ml each

The ORP calibration solution provides a 225 mV output at 25 ºC (beware that it may change at different temperatures) which facilitates the adjustment of the sensor output to the actual values of oxidation-reduction potential. Note that this buffer will keep its properties for 30 days once open. It is recommended to store refrigerated.

Figure: Image of the ORP calibration solution

Dissolved Oxygen calibration solution

Characteristics:

  • 0 mg/ml at 25 ºC

  • 100 ml

In the case of the dissolved oxygen sensor Libelium provides a solution of 0 mg/ml adequate to test the sensor. Though it provides a very good approximation for the zero output, it is not recommended for calibration.

Figure: Image of the dissolved oxygen calibration solution

Remember to read carefully the material safety data sheets you can find in the "Safety guides" section of this guide, in order to take the corresponding precautions when manipulating these solutions and dispose them in the appropriate way.

The liquid has an expiration date. If you are using an outdated liquid, you will get wrong values.

Turbidity calibration kits

Characteristics:

2 kits for low and medium/high turbidities:

  • around 0, 10 and 40 NTU

  • around 200 ml per solution

The 2 turbidity kits enable the calibration in 2 different measurement ranges: low and medium/high turbidity. The exact value of NTU is printed in each sticker. The user can re-calibrate the sensor periodically, getting 2 reference points with one kit and 3 points with 2 kits.

Figure: Image of a turbidity calibration kit

Remember to read carefully the material safety data sheets you can find in the "Safety Guides" section of this guide, in order to take the corresponding precautions when manipulating these solutions and dispose them in the appropriate way.

General considerations about probes performance and life expectancy

When developing a new application with the Smart Water sensor board the conditions of the environment the sensors are going to operate in will deeply affect the durability and behavior of the probes. Thus, it is highly recommended to carry out an exhaustive study of the characteristics of the location of the device and perform all the laboratory tests required in order to assure the correct election of the sensors and of the way they will be deployed. Libelium provides standard sensors which have been largely tested and will work in most of the environments, but keep always in mind that if they are subjected to harmful chemicals present in certain specific scenarios they may be irreversibly damaged. Below a few tips regarding the setup of the sensors are listed:

Sensor deployment

The main problems regarding the setup of the sensors concern both the way and the place they are deployed in. First of all, they must be installed in a way in which there is no interference between the sensor and near objects, making sure that the sensing parts (the bulb of the pH and ORP sensors, the membrane of the dissolved oxygen probe and the electrodes of the conductivity sensor) are not in touch with the objects nearby. In the case of the conductivity sensor, as stated in the section about this sensor, take into account that it will have to be placed at certain distance from other objects in order to not interfere with the sensor magnetic field.

Figure: Image of two sensors wrongly and correctly placed

Secondly, it must be made sure that the sensors are completely submerged in the liquid all the time or the sensors may give an incorrect output. This problem may mainly appear in locations where the volume of water is variable owing to changes in the flow in rivers or canals or to the action of tides in seas. Another variant of this problem is given in locations where there is a continuous entry of air in the water, owing to the waves formed in the surface, jumps of the water flow, etc., which may generate bubbles that, in contact with the sensing part of the sensor, distort the output signal.

The best method to avoid all these problems is to select a location where a minimum level of steady water is available all along. If the location where the sensor is going to be deployed does not meet these requirements and it is not possible to find a more proper place it will be necessary to build a protection system to ensure that the sensor is completely immersed and that there is not an airflow disturbing the measurement.

Figure: Image of several situations with the sensor incorrectly installed
Figure: Example of installation of a complete mote

Recalibration

A periodic recalibration of the sensors is highly advisable in order to maintain an accurate measurement along time in order to correct changes owed to a drift output, polarization or wear.

Even though manufacturers generally recommend a calibration before every measurement, it is not feasible at all when sensors are deployed in a remote location. Nevertheless, it is not really necessary unless an extremely accurate value is required, for a general purpose application a much more spread set of recalibrations should be enough.

This way, the frequency of the recalibration process will be determined by both the accuracy required in the given application and the environment in which the sensors will be operating. The more accurate measurements required, the more often will be necessary to recalibrate the sensor. As well, an aggressive environment with harmful chemicals or with an important variation of the conditions of the parameter under measurement and its temperature will lead to a faster loose of precision, while more steady conditions will allow the user to spread the recalibrations along time.

This recalibration process, which will basically consist in the repetition of the calibration indicated for each sensor in its own section, will be different depending on the place where the conversion into useful units is performed. In case it is the mote itself which carries out this conversion, it will be necessary to provide the code with a calibration option allowing the visualization of the output values under calibration the introduction of the new coefficients in the conversion function. On the other hand, if the conversion is being performed in reception the software must be ready to interpret the calibration data and update its conversion algorithm with the new values arrived.

Life expectancy

If they are not subject to harassing environments Smart Water sensors may keep on functioning for periods of several months, providing the required recalibrations are performed to maintain the accuracy demanded by the application. Tests carried out at Libelium facilities have shown that sensors working for at least six months have not suffered an important variance in their output and still provide an accurate output when calibrated.

However, the chemical processes given in the sensor measurement will finally end up the sensor life. In the case of the pH, ORP and dissolved oxygen, the depletion of the solution of both the reference and measurement electrodes and the wear of the sensitive bulb or membrane are the principal reasons for sensor failure. In the case of the conductivity sensor, the polarization of the electrodes (attenuated by the application of an alternating supply current but not completely avoided), the accumulation of dirt in them and the wear of the platinum black layer are the most significant sources of damage.

We can summarize that both recalibration and lifetime of the sensor probes depend on three main factors:

  1. Water environment: corrosive chemicals, salt, dirt, extreme temperatures, strong flow currents decrease the lifetime.

  2. Usage: the more the probes are used the sooner they need to be changed due to the depletion of the substances used as reference and measurement electrodes.

  3. Time: event in perfect conditions and low usage the chemical reactions that take place in the reference electrodes will stop working.

Owing to all that, the sensor probes will probably have to be replaced between six months and one year after they have been deployed. The process of replacement is really easy as the probes as the probes may be easily unscrew using just the hand.

Figure: Images of the procedure to change the probes for the Smart Water Plug&Sense!

Also beware that if as indicated before the sensors are placed in a chemically or physically aggressive media, with for example temperatures close to the extremes of the operating range, extreme air humidity (especially near salty water), strong flow of water or with presence of corrosive chemicals or salt, these wear and depletion processes may accelerate thus severely shortening the life of the sensors. In case of doubt please contact Libelium to get support about the sensors\' durability.

How to detect that the probes are not working properly

There are certain symptoms that will reveal that a sensor is not working properly:

  • A lack of a proper response during calibration process. This is an obvious error which may appear in different ways and in different degree. A noisy output of several millivolts when submerging the probes in the calibration solutions, inconsistent values with the expected output given in section "Calibration Procedure" and never reaching a stable output will be indicatives of a defective of probe.

  • A steady continuous measurement for a long time. It is very rare that these sensors show a continuous value in a real environment as they do in laboratory. Owing to liquid flow, temperature effects or biological action, a slow fluctuation is to be expected. If the measurement is stalled in a given value, the probe will probably be broken.

  • A sudden change in the output of the sensor. The sensors\' reaction is not instantaneous, if there is a leap between two consecutive measurements a problem with the sensor may have occurred (this kind of error may not be detected if a long time takes place between measurements).

  • Values out of range. If the sensor drifts out of the normal operation range it will probably be caused by a failure.

If there are doubts about the correct operation of the sensor it is recommended to carry out a new calibration in order to discard any possible malfunction.

Important summary

  • Due to the chemical nature of the sensors, the user must recalibrate them periodically. The frequency of this recalibration process depends on the accuracy desired and on the environment conditions; this time should be concluded after real tests. A standard recalibration period would be one month, but certain applications may force to recalibrate after a few days.

  • The lifetime of the sensors depends on many factors. The standard expectancy is about one year but harsh environment conditions could be decreased it to some months.