Sensors

Libelium discontinued the Single and Double variants. The Smart Water Ions PRO variant continues with no change.

Soil/Water Temperature sensor (Pt-1000)

  • Measurement range: 0 ~ 100 ºC

  • Accuracy: DIN EN 60751

  • Resistance (0 ºC): 1000 Ω

  • Diameter: 6 mm

  • Length: 40 mm

  • Cable length: ~500 cm

Figure: Soil/Water Temperature (Pt-1000) Sensor

Measurement process

The Soil/Water Temperature (Pt-1000) Sensor is a resistive sensor whose conductivity varies in function of the temperature. The Smart Water Ions Sensor 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 Ions Sensor Board powered for extended periods and switch it OFF once the measurement process has finished.

{
float valuePT1000 = 0.0;
SWIonsBoard.ON();
// A few milliseconds for power supply stabilization
delay(10);
// Reading of the Temperature sensor
float temperature = TemperatureSensor.read();
// Print of the results
USB.print(F(\"Temperature (celsius degrees): \"));
USB.println(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:

https://development.libelium.com/waspmote/swi-01-temperature-sensor-reading

Temperature sensor connection

To connect the Soil/Water Temperature (Pt-1000) Sensor to the Smart Water Ions Sensor 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 Soil/Water Temperature (Pt-1000) Sensor

Reference probes

A reference electrode is an electrode which has a stable and well-known electrode potential. Reference electrodes are critical to acquiring good electrochemical data. Drift in the reference electrode potential can cause quantitative and qualitative errors in data collection and analysis beyond simple inaccuracies in the measured potential.

The Smart Water Ions Sensor Board have 3 different Reference Probes, depending on the ion to be measured.

The next sensors must be used with the Single Junction Reference Probe:

  • Calcium Ion (Ca2+) Sensor Probe

  • Fluoride Ion (F-) Sensor Probe

  • Fluoroborate Ion (BF4-) Sensor Probe

  • Nitrate Ion (NO3-) Sensor Probe

The next sensors must be used with the Double Junction Reference Probe:

  • Bromide Ion (Br^) Sensor Probe

  • Chloride Ion (Cl^) Sensor Probe

  • Cupric Ion (Cu2+) Sensor Probe

  • Iodide Ion (I-) Sensor Probe

  • Silver Ion (Ag+) Sensor Probe

Figure: Reference Probe

The pH (for Smart Water Ions) Sensor must be always used with the Single or the Double Reference Probe.

All the PRO sensors must be used with the PRO Reference Probe (including the pH [PRO] sensor).

The Soil/Water Temperature Sensor is the only sensor in this board which does not need any Reference Probe.

Probes have a length of about 500 cm.

Reference probe connection

One Reference Probe (Single, Double or PRO) must always be connected in the corresponding socket marked as REFERENCE in the Smart Water Ions Sensor Board. Only one Reference Probe can be connected at the same time in the Smart Water Ions Sensor Board. One single-type sensor and one double-type sensor can never be mixed in the same system at the same time. The Reference Probe for PRO ion sensors can never be mixed with the Single or Double references.

Figure: Reference Probe socket

Maintenance solution

Reference probes should be cleaned with deionized water and stored with their plastic plug after every use. Sensors cannot be left in deionized/distiled water for more than 1 minute.

For a correct storage of the reference probes it's recommended to put a drop of the Reference Sensor Probe [PRO] Maintenance Solution (Lithium Acetate at 3 Mol.), inside their plastic plug.

Ion sensors

In this table we can see the main features of the ions sensors. The ion sensors are divided in two groups depending on the required reference (double, or single junction). In the Smart Water Ions Sensor Board, only one reference can be connected at the same time, so it is not possible to mix different sensor types.

Species

Construction

Concentration Range (mol/L)

pH Range

Temperature Range (ºC)

Dimensions (mm)

Required Reference

Bromide (Br-)

Solid State Half-cell

10^(-1)-10^(-6)

2-11

5-60

Double Junction

Chloride (Cl-)

Solid State Half-cell

10^(-1)-5x10^(-5)

2-12

5-60

Double Junction

Cupric (Cu2+)

Solid State Half-cell

10^(-1)-10^(-6)

2-12

5-60

Double Junction

Iodide (I- )

Solid State Half-cell

10^(-1)-5x10^(-7)

2-12

5-60

Double Junction

Silver (Ag+)*

Solid State Half-cell

10^(-1)-3x10^(-7)

2-8 (Ag+)

5-60

Double Junction

Calcium (Ca2+)

Plastic Membrane Half-cell

10^(-1)-10^(-5)

2.5-11

5-60

Single Junction

Fluoride (F- )

Plastic Membrane Half-cell

10^(-1)-10^(-6)

5-7

5-60

Single Junction

Fluoroborate (BF4-)

Plastic Membrane Half-cell

10^(-1)-3x10^(-6)

2.5-11

5-60

Single Junction

Nitrate (NO3-)

Plastic Membrane Half-cell

10^(-1)-10^(-5)

2.5-11

5-60

Single Junction

* This sensor is also sensitive to Sulfide (S^2-^) ions; take this into account in terms of cross-sensitivity if the monitored water could contain Sulfide. The user could even use this sensor to meter Sulfide ion if he is able to calibrate the sensor by his own means.

The ion sensors have a cable length of ~500 cm.

pH sensor (for Smart Water Ions)

The pH sensor integrated in the Smart Water Ions Sensor Board is specific to be used with this board and in combination with one of the Reference Probes. This pH sensor cannot be used with Smart Water Sensor Board, which integrates another pH sensor, different from the one exposed in this section.

  • pH Range: 0-14

  • Temp. Range (ºC): 5-60

  • Internal Reference Type: Ag/AgCl

  • Dimensions (mm): Ø12x160

  • Reader accuracy: in function of calibration

  • Cable length: ~500 cm

Figure: pH Sensor Probe for Smart Water Ions

PRO Ion Sensors

This is a special line of ion sensors. These sensors are solid state carbon nanotube-based selective electrodes. This feature reduces the maintenance of the sensors and increases their stability on time. Also, these sensors can be combined using a unique reference probe. In this table we can see the main features of the PRO ion sensors.

Ion

Sensitivity

Temp(ºC)

pH

Lineal Range

Interferences

Ammonium Ion (NH4+) Sensor Probe [PRO]

-54 ± 5

5 - 50

4 - 8,5

0,09 - 9000 mg/L

K (-0,8); Na (-2,7); Mg (-3,2); Ca (-4)

Bromide Ion (Br-) Sensor Probe [PRO]

-54 ± 5

5 - 50

1 - 12

0,4 - 8000 mg/L

Cl (-2,7); OH (-4,5)

Calcium Ion (Ca2+) Sensor Probe [PRO]

24 ± 5

3,5 - 8

3,5 - 8

0,4 - 4000 mg/L

NH4 (-3); K (-3,6); Na (-3,7)

Chloride Ion (Cl-) Sensor Probe [PRO]

-54 ± 5

5 - 50

2 - 12

1,5 - 35000 mg/L

Error presence of Ag or S

Cupric Ion (Cu2+) Sensor Probe [PRO]

-54 ± 5

5 - 50

2 - 7

0,06 - 3200 mg/L

Error presence of Ag or Cl

Fluoride Ion (F-) Sensor Probe [PRO]

-54 ± 5

5 - 50

4 - 8

0,1 - 1900 mg/L

OH (-1); Maintain pH < 8

Iodide Ion (I-) Sensor Probe [PRO]

-54 ± 5

5 - 50

2 - 12

0,1 - 12000 mg/L

Error presence Ag or S; Br (-3,4); Cl (-6)

Lithium Ion (Li+) Sensor Probe [PRO]

-54 ± 5

5 - 50

2 - 12

0,1 - 5000 mg/L

Na (-2,3); K (-2,4) H (-3)

Magnesium Ion (Mg2+) Sensor Probe [PRO]

24 ± 5

5 - 50

3 - 8,5

2,4 - 2400 mg/L

Ca (-1); K (-3,6); Na (-3,9)

Nitrate Ion (NO3-) Sensor Probe [PRO]

-54 ± 5

5 - 50

2 - 11

0,6 - 31000 mg/L

Br (-1,2); NO2 (-1,7); OH (-1,8); AcO (-2,2)

Nitrite Ion (NO2-) Sensor Probe [PRO]

-54 ± 5

5 - 50

4 - 8

2,5 - 1000 mg/L

SCN (-0,2); I (-2,2); ClO4 (-2,4); Br (-3,3)

Perchlorate Ion (ClO4-) Sensor Probe [PRO]

-54 ± 5

5 - 50

1 - 11

1 - 10000 mg/L

SCN (-1,7); NO3 (-1,7); I (-1,7)

Potassium Ion (K+) Sensor Probe [PRO]

-54 ± 5

5 - 50

1 - 9

0,4 - 3900 mg/L

NH4 (-2,1); Ca (-3,9), Li (-4,3); Na (-4,6)

Sodium Ion (Na+) Sensor Probe [PRO]

-27 ± 5

5 - 50

1 - 9

0,1 - 3200 mg/L

K (-2,5); Ca (-3), Li (-3,2)

Silver Ion (Ag+) Sensor Probe [PRO]

56 ± 5

5 - 50

1 - 9

0,1 - 10000 mg/L

Error presence S o Hg

pH Sensor Probe [PRO]

-54 ± 5

5 - 50

0 - 14

0 - 14

-

Smart Water Ions Reference Sensor Probe [PRO]

-

5 - 50

-

-

-

The PRO Ion Sensor Probes are composed of two independent parts: the head (the ion membrane) and the holder. We just need to change the header when it is not working properly due to the maximum lifetime was reached.

Figure: Sensor holder Figure: Ion sensor header

The image below shows how the sensor head must be connected in the holder.

Figure: Connecting the sensor head to the sensor holder

Sensor connection in SOCKET1

Connect the sensor in the socket marked as SOCKET1 in the Smart Water Ions Sensor Board, and connect the corresponding Reference Probe. In this case we are going to use the Calcium Ion (Ca^2+^) Sensor Probe so is necessary the use of the Single Junction Reference Probe. The probes must be connected using the pigtail adapter.

Figure: Ion Sensor and Reference connected to the SOCKET1

Example:

{
// Declare the socket where the sensor will be connected
socket1Class calciumSensor;
// Turn ON the Smart Water Ions Sensor Board
SWIonsBoard.ON();
// Reading of the Calcium sensor
float calciumVoltage = calciumSensor.read();
}

You can find a complete example code for reading from the socket1 in the following link:

https://development.libelium.com/waspmote/swi-03-socket1-sensor-reading/

Sensor connection in SOCKET2

Connect the sensor in the socket marked as SOCKET2 in the Smart Water Ions Sensor Board, and connect the corresponding Reference Probe. In this case we are going to use the Nitrate Ion (NO~3~^-^) Sensor Probe so is necessary the use of the Single Junction Reference Probe. The probes must be connected using the pigtail adapter.

Figure: Sensor connected to the SOCKET2

Example:

{
// Declare the socket where the sensor will be connected
socket2Class NO3Sensor;
// Turn ON the Smart Water Ions Sensor Board
SWIonsBoard.ON();
// Reading of the NO3 sensor
float calciumVoltage = NO3Sensor.read();
}

You can find a complete example code for reading from the socket2 in the following link:

https://development.libelium.com/waspmote/swi-04-socket2-sensor-reading/

Sensor connection in SOCKET3

Connect the sensor in the socket marked as SOCKET3 in the Smart Water Ions Sensor Board, and connect the corresponding Reference Probe. In this case we are going to use the Fluoride Ion (F^-^) Sensor Probe so is necessary the use of the Single Junction Reference Probe. The probes must be connected using the pigtail adapter.

Figure: Sensor connected to the SOCKET3

Example:

{
// Declare the socket where the sensor will be connected
socket3Class FluorSensor;
// Turn ON the Smart Water Ions Sensor Board
SWIonsBoard.ON();
// Reading of the Fluor sensor
float fluorVoltage = FluorSensor.read();
}

You can find a complete example code for reading from the socket3 in the following link:

https://development.libelium.com/waspmote/swi-05-socket3-sensor-reading/

Sensor connection in SOCKET4

Connect the sensor in the socket marked as SOCKET3 in the Smart Water Ions Sensor Board, and connect the corresponding Reference Probe. In this case we are going to use the Chloride Ion (Cl^-^) Sensor Probe so is necessary the use of the Double Junction Reference Probe. The probes must be connected using the pigtail adapter.

Figure: Sensor connected to the SOCKET4

Example:

{
// Declare the socket where the sensor will be connected
socket4Class ChlorideSensor;
// Turn ON the Smart Water Ions Sensor Board
SWIonsBoard.ON();
// Reading of the Chloride sensor
float chlorideVoltage = ChlorideSensor.read();
}

You can find a complete example code for reading from the socket4 in the following link:

https://development.libelium.com/waspmote/swi-06-socket4-sensor-reading/

Connecting various sensors

The Smart Water Ions Sensor Board has been designed to connect four ions sensors simultaneously. As it mentioned in previous sections, there are 3 different Reference Probes, depending on the ions group to be measured.

Important: for higher accuracies, we recommend the calibration of the entire system.

Figure: Multi-ion measurement and calibration

Calibration solutions

Libelium provides several calibration solutions kits to calibrate the ion sensors. These kits are not mandatory, but extremely recommended to measure with good accuracy. The calibration process is described in section "Calibration Process" of this Technical Guide.

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

The calibration process of a pH sensor is described in section "Calibration Procedure" in the Smart Water Technical Guide, when handling them pay attention to the information provided in the MSDS.

Figure: Image of the pH calibration kit

Multi-ion calibration kit

The "Multi-ion" calibration solution allows to calibrate up to 7 different sensor probes. So we can use just three solutions to calibrate 7 Sensor Probes (with the old ones we would have needed 21 different solutions).

Characteristics:

  • 250 ml each

  • Concentrations:

Solution 1 (mg/L)

Solution 2 (mg/L)

Solution 3 (mg/L)

Calcium(Ca2+)

36

180

360

Chloride(Cl-)

75

375

750

Potassium(K+)

39

195

390

Magnesium(Mg2+)

11

55

390

Sodium(Na+)

23

20

230

Ammonium(NH4+)

4

20

40

Nitrates(NO3-)

132

660

1320

Figure: Image of the Multi-ion Calibration Kit

Bromide (Br-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Bromide (Br-) calibration kit

Calcium (Ca2+) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Calcium (Ca2+) calibration kit

Chloride (Cl-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Chloride (Cl^-^) calibration kit

Cupric (Cu2+) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Cupric (Cu^2+^) calibration kit

Fluoride (F-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Fluoride (F^-^) calibration kit

Fluoroborate (BF4-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Fluoroborate (BF4-) calibration kit

Iodide (I-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Iodide (I-) calibration kit

Lithium (Li+) calibration Kit

Characteristics:

  • 1 mg/L, 10 mg/L, 100 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Nitrate (NO3-) calibration kit

Figure: Image of the Lithium (Li+) calibration kit

Nitrate (NO~3~^-^) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Lithium (Li+) calibration kit

Nitrite (NO2-) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Nitrite (NO2-) calibration kit

Perchlorate (ClO4-) calibration Kit

Characteristics:

  • 1 mg/L, 10 mg/L, 100 mg/L at 25 ºC

  • 100 ml each

Figure: Image of the Perchlorate (ClO4-) calibration kit

Silver (Ag+) calibration Kit

Characteristics:

  • 10 mg/L, 100 mg/L, 1000 mg/L at 25 ºC

  • 100 ml each

This solution is very sensitive to the light and must used carefully. Please, read the corresponding safety guide.

Figure: Image of the Silver (Ag+) 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.

Sensors maintenance

Smart Water Ions probes are delicate sensors and a proper maintenance is very important to enlarge the lifetime of the platform and to avoid any damage over them.

Ions sensors should be cleaned with deionized water, shaking the sensor part of the probe carefully in a glass with the deionized water. Also, a soft cloth can be used if there are solid residues.

For cleaning Smart Water Ions PRO sensors, the user should disassemble the cover of the header. Do not touch the membrane of the sensor directly.

Figure: Cleaning a sensor in deionized water Figure: Cleaning a sensor in deionized water

Users must follow the next measures for a proper operation of the sensors:

  • The sensor must be calibrated in increasing order of concentration.

  • With the presence of solid residues in the water, the sensor should > be washed with deionized water frequently, recommended at least > once per week.

  • Do not leave the sensor in deionized water for more than 1 minute.

  • The storing conditions of the sensor are 25 ºC and protected from > solar light.

  • After long periods of storage, the sensor must be conditioned for at > least 2 hours at a concentration of 0.1 M.

  • Avoid direct contact with the membrane of the sensor.

  • The membrane should be kept clean and free of solid residues.

  • Never use in environments where the sensor may be damaged by water > agitation or physical blows.

If a probe gets damaged because of a poor maintenance, it will not be covered by the warranty.

General considerations about probes performance and life expectancy

When developing a new application with the Smart Water Ions 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.

Sensors are not designed for salty water (sea) since seawater has many dissolved ions naturally. Besides, check your accuracy requirements, because sensors are conceived for measuring medium and high ions concentrations.

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 dissolved ions) 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

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 Ions Sensor Board 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.

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: even 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 may be easily unscrew using just the hand.

The PRO sensor boards have proven to be more robust and durable than the normal (Single or Double) sensor boards.

Figure: Images of the procedure to change the probes for the Smart Water Ions 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, strong flow of water or with presence of corrosive chemicals, 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.

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.

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.

Specific recommendations for Smart Water Ions PRO

Storage and conservation

  • Store below 25 ºC in a dry and dark place, avoiding direct sunlight.

  • Store the electrode unplugged from Plug & Sense! or the Smart Water Ions sensor board. The electrode must NOT to be connected to when it is not in use.

  • Electrodes should be cleaned with deionized water and stored dry with their plastic plug. Sensors cannot be left in deionized/distiled water for more than 1 minute.

  • To store reference probes it's recommended to put a drop of the Reference Sensor Probe [PRO] Maintenance Solution (Lithium Acetate at 3 Mol), inside their plastic plug.

  • When an electrode has been exposed to target liquids with high interferences or solid particles, after rinsing and before storing, it must be immersed in the conditioning solution for 20-30 minutes in order to regenerate its sensing area (the tip of the electrode).

  • Electrodes are NOT designed to be immersed all the time. If the user leaves them in a liquid for days or hours, they will deteriorate faster. For the best results, electrodes must be kept outside liquids and immerse them only when new measurements want to be taken.

Before starting

  • The gray circumference marks the maximum immersion level. Never immerse the electrode beyond this gray mark.

  • Always use electrodes with the protective head, in order to avoid the tip of the electrode touches anything (for example, the bottom of the bottles or recipients).

  • Do not touch or manipulate the electrode tip.

Figure: Do not immerse the electrodes beyond the mark

Electrode accommodation

Accommodation is a key step when preparing the electrode. Before the first time use (and after long time storage too) it must be accommodated for at least 2 hours and a maximum of 8 hours. Some electrodes need 12-24 hours.

The accommodation time for electrodes with a frequent use (daily or weekly) is 15 minutes. The accommodation process must be carried out with the probe unplugged from the meter. The accommodation process is done at 0.1 M of the target ion (or alternatively at 1000 mg/L) or highest calibration solution. Remove the rubber cap without damaging the sensing area and immerse the electrode in the accommodation solution for the recommended time.

After the accommodation process, rinse with deionized water, and the electrode will be ready to be calibrated.

When changing from high concentration to low concentration, the response time could change. In some cases, the electrodes must be kept some minutes in the low-concentration solution before obtaining a stable reading in order to start the calibration process.

Figure: Accommodating an electrode

Calibration

  • The ion electrode and the reference electrode need to be in the same solution at the same time.

  • Calibration solutions should be used in ascendant order and all calibration solutions should be at the same temperature.

  • When moving the electrode from one solution to another, it must be rinsed with deionized water and dried with a clean tissue paper. Do not hit the sensing area.

  • Depending on the needed accuracy, you can calibrate with 2 or 3 points. If you need the best accuracy, you should calibrate with 3 points. If you need to choose 2 of the 3 calibration solutions, choose them so the expected concentration of the target liquid is between them.

  • Check the concentration range of your electrode.

Process to calibrate:

Make sure that membranes and holders are clean. Use deionized/distiled water for cleaning.

Figure: Cleaning a sensor in deonized water Figure: Drying the sensor with care

Use the Waspmote IDE examples to read the sensor; the first calibration point that you need to take is with the lower calibration solution. Introduce the ion electrode and the reference probe in the liquid and you will start getting the values.

The values of the sensor stabilize in few minutes (about 5 - 20 minutes). This stabilization time depends on the concentration of the calibration solution (it could take more time in the low calibration solutions). Note that, in low calibration solutions, values could get some noise. When the values show a variation of 0.02 V or less, you can consider the output is stabilized; you have the 1^st^ calibration point, write it down. After a correct the stabilization process, the values could degrade if you are measuring for a long time: in the calibration process, the liquid and electrode start to degrade, so values could deteriorate on time.

It is important to use the same time to obtain each calibration point. If you waited 8 minutes for the 1^st^ point, wait 8 minutes with the next calibration solution. You can leave the calibration more time and afterwards choose the best values, but always choose for each point the voltages corresponding to the same time.

Figure: Calibrating a sensor in a calibration solution

If you plot voltage in a graph, you can detect its stabilization better; you will see something like the next figure.

Figure: Stabilization of an electrode in a three-point calibration process

Calibration graphs could not be exactly like this.

In the calibration process, remember to clean the electrode when moving from a calibration solution to another one. Last, clean the electrode when you finish the whole calibration process (2 or 3 points).

When the process is done, write the calibration values in the code. You can do it like that:

//Concentrations of the calibration solutions used in the process:
#define point1 10.0
#define point2 100.0
#define point3 1000.0
// Read calibration voltage values:
#define point1_volt_Ca 2.163
#define point2_volt_Ca 2.296
#define point3_volt_Ca 2.425

After that, you can upload the new code to Waspmote and start sensing the target liquid.

Measurement process:

  • For better performance, we recommend applying to the calibration process the same conditions that the target liquid has (for example, adjust the solution\'s temperature when calibrating).

  • Solid particles in the target liquid, its color or turbidity do not affect the performance of the electrodes.

  • If the target liquid contains solid particles, rinse the probe properly.

When the calibration process is completed and the new parameters are updated in the code, you can start sensing. In a continuous sensing process, the electrode membrane and liquid could degrade, so the accuracy would not be enough for your application. In this case, a minimal maintenance is necessary to get the best performance: the sensors will have to be cleaned and re-calibrated. The maintenance period can be defined for the output values and the needed accuracy. The maintenance has not a defined period: when the values degrade, you should apply maintenance. Depending on the accuracy that your application needs, the maintenance period will be shorter. Qualified personal is not necessary for the maintenance process.

Additional tips:

  • If you change the sensor holder, you should recalibrate the sensor. Any variation of items or conditions could change the calibration values, so a new calibration process will be necessary.

  • When using the pH sensor and the reference electrode, both electrodes need to be in the same solution at the same time.

  • When you are finished with the sensors, please clean and store them. Remember that, if you want use it again, you need to accommodate and calibrate them.

  • For the best accuracy, we recommend calibrating the sensors before each measurement process.