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Sensors probes

Important notes

The following sections describe the main features and the general usage for all the sensors probes included in the Plug & Sense! Smart Water Xtreme model.
It is important to remark that Smart Water Xtreme is only available in the Waspmote Plug & Sense! line. It is not available for the Waspmote OEM line. Besides, keep in mind that Smart Water Xtreme is not compatible with the former Smart Water or Smart Water Ions models. In other words, the sensor probes described in this Guide are only compatible with Smart Water Xtreme, because its advanced electronics allow these specific sensor integrations (some exceptions are the BME, Ultrasound or Luminosity sensors). .
In order to keep this guide as short as possible, some manufacturer information has been omitted. Libelium encourages the reader to visit the manufacturer websites and to spend some time studying all the technical papers and application notes provided for each sensor. Measured parameters on the great majority of Smart Water applications require a deep knowledge and, what is a more, sophisticated measure techniques to obtain the best accuracy.
The importance of laboratory tests
Additionally, Libelium highly recommends to carry out comprehensive laboratory tests before installing the system on the field, as well as proof of concepts on the field during a reasonable period, before going to a real deploy. Thanks to these good practices, the user will have an idea of the platform behavior, which will be very close to the reality. Parameters like accuracy over time, signal drift or battery drain can be only measured with real tests. As a result, a lot of time will be saved.
Typical scenarios
The Smart Water Xtreme model integrates high end sensors valid for the great majority of smart water applications like fish farming, waste water management or drinking water monitoring. They are developed in a robust and compact design, making them waterproof and allowing to place them completely underwater during long periods. In fact, they should be immersed completely for a good measurement. Take into account if the volume of water changes, like the flow in rivers and canals or sea tides.
Deployment
However, the deployment of the sensor is a matter of concern. First, it is recommended to isolate the sensors from big solids, rocks, walls or any the animal life present to prevent physical damages to the sensor. Besides, they would have to be placed at certain distance from other objects like motors or water pumps, in order to minimize interferences with the measures. Second, variable water flows, bubbles, rapid temperature changes or some chemicals would be avoided as much as possible in order to improve the quality and stability of measures. There are some accessories and solutions to achieve a good installation. Incidentally, it must not be forgotten to store the sensors correctly if they are not going to be used for a certain period. In the following sections more information is given about it.
Maintenance
Always take into account a maintenance factor for each sensor probe. The environmental conditions could affect the sensor behavior and accuracy, therefore it will become mandatory a periodic maintenance for each sensor probe, to watch out things like dirty on sensor probes, measure position or wire connections. The period between these maintenance actions will be different on each application.
Calibration
One of the most striking issues is the difference between maintenance tasks and calibration processes. While maintenance is done by low profile technicians, calibration is done only by skilled engineers with the necessary knowledge about the sensors. Despite Plug & Sense! is a stand-alone device, the Smart Water Xtreme sensor probes will definitely require certain maintenance and calibration. A large number of tips and advices, besides than reference calibration and maintenance periods are given in each sensor section.
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.
Life expectancy
If they are not subject to harassing environments Smart Water Xtreme sensor probes may keep on functioning for periods of several months, providing the required recalibrations are performed to maintain the accuracy demanded by the application.
It can be summarized that both recalibration and lifetime of the sensor probes depend on 3 main factors:
  1. 1.
    Water environment: corrosive chemicals, salt, dirt, extreme temperatures, strong flow currents decrease the lifetime.
  2. 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. 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 OPTOD and PHEHT sensor probes (or their consumable parts) will probably have to be replaced between 6 months and one year after they have been deployed. For the optic sensor probes NTU and MES5 and the CTZN, the period is longer. The process of replacement is really easy as the probes may be easily unscrewed using just the hand.
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 a non-working probe
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.
In any case, please contact our Sales department through the next link if you require more information: https://www.libelium.com/contact.

New Sensors for Waspmote Plug & Sense! Smart Water Xtreme

Table 1: Parameters, units, ranges, resolutions and accuracies of every sensor

Sensor name
Parameters
Units
Range
Resolution
Accuracy
Aqualabo OPTOD
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo OPTOD
Oxygen
% saturation
0,0 to 200,0 % SAT
0,1
± 1 %
Aqualabo OPTOD
Oxygen
mg/L
0,00 to 20,00 mg/L
0,01
± 0,1 mg/L
Aqualabo OPTOD
Oxygen
Ppm
0,00 to 20,00 ppm
0,01
±0,1 ppm
Aqualabo PHEHT
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo PHEHT
pH
pH
0,00 to 14,00 pH
0,01 pH
± 0,1 pH
Aqualabo PHEHT
Redox (ORP)
mV
1000,0 to + 1000,0 mV
0,1mV
± 2 mV
Aqualabo PHEHT
pH
mV
-
-
-
Aqualabo C4E
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo C4E
Conductivity
μS/cm
4 ranges to choose (or automatic): 0-200,0 μS/cm 0 –2000 μS/cm 0,00 –20,00 mS/cm 0,0 –200,0 mS/cm Automatic
0,01 to 1 according the range
± 1 % full range
Aqualabo C4E
Salinity
Ppt = g/Kg
5-60 g/kg
0,01 to 1 according the range
± 1 % full range
Aqualabo C4E
TDS – Kcl (Total dissolved solids)
ppm
0-133 000 ppm
Aqualabo NTU
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo NTU
Nephelometric Turbidity
NTU
0-4000 NTU 4 ranges to choose for Parameters 1 and 2 (or automatic):
  • range 1: 0 / 50 NTU
    (FNU)
  • range 2: 0 / 200 NTU
    (FNU)
  • range 3: 0 / 1000 NTU
    (FNU)
  • range 4: 0 / 4000 NTU
    (FNU)
    -Automatic
0,01 to 1 NTU – mg/L
± < 5 % full range Range 1: ±2,5NTU Range 2: ±10NTU Range 3: ±50 NTU Range 4: ±200NTU
Aqualabo NTU
Nephelometric Turbidity
FNU (1 FNU = 1 NTU)
Same ranges than the previous parameter
0,01 to 1 NTU – mg/L
± < 5 % full range Range 1: ±2,5NTU Range 2: ±10NTU Range 3: ±50 NTU Range 4: ±200NTU
Aqualabo NTU
SS (Suspended Solids
mg/L
0 to 4500 mg/L
0,01 to 1 NTU – mg/L
± < 5 % full range Range 1: ±2,5NTU Range 2: ±10NTU Range 3: ±50 NTU Range 4: ±200NTU
Aqualabo CTZN
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo CTZN
Conductivity
mS/cm
0,0 –100,0 mS/cm
0,1 mS/cm
Check dependency tables
Aqualabo CTZN
Salinity
Ppt = g/kg
5-60 g/kg
0,1 mS/cm
Check dependency tables
Aqualabo CTZN
Conductivity not compensated with temperature
mS/cm
0,0 –100,0 mS/cm
0,1 mS/cm
Check dependency tables
Aqualabo MES 5
Temperature
degrees Celsius
0,00 to + 50,00 ºC
0,01 ºC
± 0,5 °C
Aqualabo MES 5
Sludge blanket
%
0-100 %
0.01 to 0.1 %
0,02
Aqualabo MES 5
SS (Suspended Solids)
g/L
0-50 g/L
0.01 g/L
<10%
Aqualabo MES 5
Turbidity
FAU
0-400 FAU
0.01 to 1 FAU
0,05
Eureka Fluorometer: Chlorophyll a - blue
Chlorophyll a - blue
μg/l
0 to 500 μg/l
6 digits with maximum of two decimals
linearity of 0.99R²
Eureka Fluorometer: Chlorophyll a - red
Chlorophyll a - red
μg/l
>500 μg/l
6 digits with maximum of two decimals
linearity of 0.99R²
Eureka Fluorometer: Phycocyanin (freshwater BGA)
Phycocyanin (freshwater BGA)
ppb
0 to 40,000 ppb
6 digits with maximum of two decimals
linearity of 0.99R²
Eureka Fluorometer: Phycoerythrin (marine BGA)
Ammonium
ppb
0 to 750 ppb
6 digits with maximum of two decimals
linearity of 0.99R²
Eureka Fluorometer: CDOM/fDOM
CDOM/fDOM (Colored Dissolved Organic Matter/ Fluorescent Dissolved Organic Matter)
ppb
0 to 1250 or 0 to 5000 ppb
6 digits with maximum of two decimals
linearity of 0.99R²
Eureka Ion-selective electrodes (ISE's): Ammonium
Ammonium
mg/l
0 to 100 mg/l as nitrogen
0.1
5% or 2 mg/l
Eureka Ion-selective electrodes (ISE's): Nitrate
Nitrate
mg/l
0 to 100 mg/l as nitrogen
0.1
5% or 2 mg/l
Eureka Ion-selective electrodes (ISE's): Chloride
Chloride
mg/l
0 to 18,000 mg/l
0.1
5% or 2 mg/l
Eureka Ion-selective electrodes (ISE's): Sodium
Sodium
mg/l
0 to 20,000 mg/l
0.1
5% or 2 mg/l
Eureka Ion-selective electrodes (ISE's): Calcium
Calcium
mg/l
0 to 40,000 mg/l
0.1
5% or 2 mg/l

Table 2: Applications and measuring principlesTable 2: Applications and measuring principles

Sensor name
Parameters
Applications
Measuring principle
Aqualabo OPTOD
Temperature
  • Industrial and municipal sewage treatment
    plants
Optical measure by luminescence technology
Aqualabo OPTOD
Oxygen
  • Wastewater management (nitrification and
    de-nitrification)
  • Surface water monitoring
  • Fish farming, aquaculture
  • Drinking water monitoring
Optical measure by luminescence technology
Aqualabo PHEHT
Temperature
  • Industrial and municipal sewage treatment
    plants
ORP: Platinum electrode - Ag/AgCl reference
Aqualabo PHEHT
pH
  • Wastewater management (nitrification and
    de-nitrification)
PH: plasticized PONSEL
Aqualabo PHEHT
Redox (ORP)
  • Surface water monitoring
Aqualabo PHEHT
pH
  • Drinking water monitoring
PLASTOGEL®. Electrolyte – Ag/ AgCl reference
Aqualabo C4E
Temperature
  • Industrial and municipal sewage treatment
    plants
Aqualabo C4E
Conductivity
  • Wastewater management (nitrification and
    de-nitrification)*
Electrochemical conductivity sensor with 4 electrodes (2 graphite, 2 platinum)
Aqualabo C4E
Salinity
  • Surface water monitoring
Aqualabo C4E
TDS – Kcl (Total dissolved solids)
  • Drinking water monitoring
Aqualabo NTU
Temperature
  • Urban wastewater treatment (inlet/ outlet
    controls)
Aqualabo NTU
Nephelometric Turbidity
  • Sanitation network
  • Industrial effluent treatment
Nephelometry: Optical IR (850 nm) sensor based on IR diffusion at 90 degrees
Aqualabo NTU
SS (Suspended Solids)
  • Surface water monitoring
  • Drinking water
Aqualabo CTZN
Temperature
  • Urban wastewater treatment
Aqualabo CTZN
Conductivity
  • Industrial effluent treatment
Inductive conductivity sensor regulated in temperature
Aqualabo CTZN
Salinity
  • Surface water monitoring
Aqualabo CTZN
Conductivity not compensated with temperature
  • Sea water
  • Fish farming
Aqualabo MES 5
Temperature
  • Urban Waste water treatment (Inlet/
    sewage water (SS, Turbidity), Aeration basin
    (SS), Outlet
    (Turbidity)
Aqualabo MES 5
Sludge blanket
  • Treatment of industrial effluents (Aeration
    b asin (SS)), Clarifier (Sludge blanket), Outlet
    (Turbidity)
Absorptometry: Optical IR (870 nm) sensor based on IR absorption at 180 degrees
Aqualabo MES 5
SS (Suspended Solids)
  • Sludge treatment (Centrifugation)
Aqualabo MES 5
Turbidity
  • Dredging site (turbidity)
Eureka Fluorometer: Chlorophyll a - blue
Chlorophyll a - blue
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Turner Designs fluorometric sensors, with each tuned to the slightly different wavelengths. Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return. The magnitude of the return light is relatable to the amount of analyte present.
Eureka Fluorometer: Chlorophyll a - red
Chlorophyll a - red
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Turner Designs fluorometric sensors, with each tuned to the slightly different wavelengths. Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return. The magnitude of the return light is relatable to the amount of analyte present.
Eureka Fluorometer: Phycocyanin (freshwater BGA)
Phycocyanin (freshwater BGA)
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Turner Designs fluorometric sensors, with each tuned to the slightly different wavelengths. Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return. The magnitude of the return light is relatable to the amount of analyte present
Eureka Fluorometer: Phycoerythrin (marine BGA)
Ammonium
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Turner Designs fluorometric sensors, with each tuned to the slightly different wavelengths. Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return. The magnitude of the return light is relatable to the amount of analyte present.
Eureka Fluorometer: CDOM/fDOM
CDOM/fDOM (Colored Dissolved Organic Matter/ Fluorescent Dissolved Organic Matter)
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Turner Designs fluorometric sensors, with each tuned to the slightly different wavelengths. Fluorometric sensors emit light at a certain wavelength, and look for a very specific, different wavelength in return. The magnitude of the return light is relatable to the amount of analyte present.
Eureka Ion-selective electrodes (ISE's): Ammonium
Ammonium
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Membrane that is selective for the analyte of ammonium. The electrode’s filling solution contains a salt of the analyte, and the difference between that salt’s concentration and the analyte concentration in the measured water produces a charge separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably with changes in the analyte concentration in the water adjacent the membrane.
Eureka Ion-selective electrodes (ISE's): Nitrate
Nitrate
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Membrane that is selective for the analyte of nitrate. The electrode’s filling solution contains a salt of the analyte, and the difference between that salt’s concentration and the analyte concentration in the measured water produces a charge separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably with changes in the analyte concentration in the water adjacent the membrane.
Eureka Ion-selective electrodes (ISE's): Chloride
Chloride
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Membrane that is selective for the analyte of chloride. The electrode’s filling solution contains a salt of the analyte, and the difference between that salt’s concentration and the analyte concentration in the measured water produces a charge separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably with changes in the analyte concentration in the water adjacent the membrane.
Eureka Ion-selective electrodes (ISE's): Sodium
Sodium
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Membrane that is selective for the analyte of sodium. The electrode’s filling solution contains a salt of the analyte, and the difference between that salt’s concentration and the analyte concentration in the measured water produces a charge separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably with changes in the analyte concentration in the water adjacent the membrane.
Eureka Ion-selective electrodes (ISE's): Calcium
Calcium
  • lakes, rivers, ground water...
  • oceanographic
  • process waters
  • waste waters
  • laboratory research
Membrane that is selective for the analyte of calcium. The electrode’s filling solution contains a salt of the analyte, and the difference between that salt’s concentration and the analyte concentration in the measured water produces a charge separation. That charge separation is measured, relative to the reference electrode, as a voltage that changes predictably with changes in the analyte concentration in the water adjacent the membrane.

Optical dissolved oxygen and temperature OPTOD sensor probe

The Optical dissolved oxygen and temperature OPTOD sensor probe, based on a luminescent optical technology, meets the demands of long term smart water applications. The OPTOD sensor probe measures accurately without oxygen consumption, especially with very low concentrations and very weak water flow. It is designed in a compact, robust and light probe with a stainless steel body.
It is often recommended to use an atmospheric pressure sensor together with the OPTOD sensor probe, due to the degree of solubility of oxygen in water is dependant on the atmospheric pressure. Moreover, the salinity is also related.
Figure: Optical dissolved oxygen and temperature OPTOD sensor probe

Specifications

Dissolved oxygen sensor:
Technology: Optical luminescence Ranges:
  • 0 to 20.00 mg/L
  • 0 to 20.00 ppm
  • 0 -- 200%
Resolution: 0.01 Accuracy:
  • ±0.1 mg/L
  • ±0.1 ppm
  • ±1%
Response time: 90% of the value in less than 60 seconds Frequency of recommended measure: > 5 s Cross sensitivity: Organic solvents, such as acetone, toluene, chloroform or methylene chloride. Chlorine gas.
Temperature sensor:
Technology: NTC Range: 0 °C to +50 °C Resolution: 0.01 °C Accuracy: ±0.5 °C Response time: < 5 s
Common:
Water flow is not necessary Default cable length: 15 m Maximum pressure: 5 bars Body material: Stainless steel (titanium option available on demand for sea water applications) IP classification: IP68 Storage temperature: -10 °C to +60 °C
Figure: Sensor probe parts: (1) membrane cap (consumable), (2) membrane screw and seal, (3) sensor body
Figure: Dimensions of the OPTOD sensor probe

Measurement process

The OPTOD sensor probe provides a digital signal using the SDI-12 protocol.
Reading code:
{
// 1. Declare an object for the sensor
Aqualabo_OPTOD mySensor(XTR_SOCKET_A);
// 2. Turn ON the sensor
mySensor.ON();
// 3. Read the sensor. Values stored in class variables
// Check complete code example for details
mySensor.read();
// 4. Turn off the sensor
mySensor.OFF();
}
During the sensor measurement, there is a small stabilization time of a few seconds, so it is recommendable to wait until the values remain stable over time.
A complete example code for reading this sensor probe can be found in the following link: https://development.libelium.com/sw-xtr-06-optod-sensor-reading

Socket

Connect the OPTOD sensor probe to Plug & Sense! Smart Water Xtreme in any of the sockets shown in the image below.
Figure: Available sockets for the OPTOD sensor probe

Maintenance

Calibration

By default, the sensor probe is factory-calibrated, therefore calibration may not be needed for the first usage. However, to increase the sensor accuracy if deviations are detected, calibration would be necessary. Nevertheless, before carrying out the sensor probe calibration, please bear in mind the next comments:
  • The OPTOD sensor probe comes dry and it needs to be rehydrated for 12 hours in tap water before taking any measure.
  • The sensor and the buffer solutions must have the same temperature, so before starting the calibration process leave all the necessary elements in the same temperature conditions. Besides, wait for sensor temperature stabilization once it has been immersed.
  • During the sensor measurement, there is a small stabilization time of some seconds, so please wait until the values remain stable over time.
  • The buffer solution bottles must be closed properly after the usage, to prevent deviations on the default values.
  • The measured value for dissolved oxygen is automatically compensated with the temperature, air pressure, and salinity (salt content).
  • It is recommended to replace the membrane every 2 years.
The sensor membrane must not be inside the dissolved oxygen buffer solution for more than an hour. Otherwise, it will be damaged and measures will be incorrect. Besides, some chemicals can damage the membrane. Contact our Sales department through the next link if you require more information: http://www.libelium.com/contact.
First of all, ensure that all necessary elements are present. It is important that if a calibration process is started, it should be completed to save the results in the sensor internal memory. Do not abandon the calibration process and always follow the given steps and guidelines to avoid a sensor misconfiguration. If the process needs to be repeated or abandoned, always type the 'Q' command to exit the calibration procedure.
Libelium provides the necessary standard buffer solution to calibrate the Smart Water Xtreme sensor probes. Refer to the calibration solution section for more information.
Figure: Necessary elements for the OPTOD sensor probe calibration
The sensor calibration can be done only on socket E. Owing to that, connect the sensor probe to socket E of the Plug & Sense! Smart Water Xtreme unit to calibrate the sensor, as shown in the image below. Do not use any other Plug & Sense! socket to calibrate a sensor. It will not work.
Figure: Connecting the sensor to the calibration socket
The OPTOD sensor probe allows to calibrate temperature and dissolved oxygen. Please read below the necessary steps to calibrate each parameter.
Temperature calibration
The temperature calibration process is the same for all Plug & Sense! Smart Water Xtreme sensor probes.
It is recommended to calibrate in 2 points. The user can choose any 2 points inside the sensor range, but it is recommended to use 0 ºC (can be achieved using water plus ice) and 25 ºC. Moreover, it is necessary to use an external thermometer as a reference.
Now, upload the temperature calibration example for the corresponding sensor probe. The code uses the serial monitor to assist the user with messages and recommendations. The main steps are described below, but the full details are provided in the code.
Step 1: Type the first calibration point (offset) on the serial monitor and press enter.
Figure: Type the first calibration point on the serial monitor
Step 2: Pour tap water in a clean beaker. Immerse the sensor in water at your selected offset. Remove the black protection cap before immersing the sensor in the buffer solution. Wait until values are stabilized over time and type 'N' to continue. Ensure there are not any bubbles on the sensor membrane to avoid measure disturbances.
Do not discard the black protection cap and keep it for the future. It will be useful if the sensor needs to be stored for a large period.
Figure: Immersing the sensor inside the calibration buffer solution
Step 3: Type the second calibration point (slope) on the serial monitor and press enter.
Figure: Type the second calibration point on the serial monitor
Step 4: Immerse the sensor in water at your selected slope. Wait until values are stabilized over time and type 'N' to continue.
Step 5: Save calibration data into the sensor by typing operators name and date of calibration. Then, the sensor values will be printed on the screen to check if the measures are done correctly.
Figure: Save the calibration data
Dissolved Oxygen calibration
The OPTOD sensor probe allows to calibrate the dissolved oxygen parameter with one or two calibration points. It is recommended that temperature and air pressure remain constant during the calibration process.
Upload the dissolved oxygen calibration example for the OPTOD sensor probe. The code uses the serial monitor to assist the user with messages and recommendations. The main steps are described below, but the full details are provided in the code.
Two points calibration:
With this method, a 0% concentration (offset) and a 100% concentration (slope) are measured, offering great accuracy for small concentrations.
Step 1: Type the desired number of calibration points on the serial monitor and press enter. After that, the first calibration point is automatically set to zero.
Step 2: Pour enough buffer solution in a clean baker to cover the sensor head. Immerse the sensor in the 0% standard buffer solution. Remove the black protection cap before immersing the sensor in the buffer solution. Remove the solution with the sensor so that the oxygen saturation decreases more quickly. Ensure there are not any bubbles on the sensor membrane to avoid measure disturbances. Wait until values are stabilized over time and type 'N' to continue.
Figure: Waiting for stabilization
The sensor membrane must not be in contact with the 0% buffer solution for more than an hour, so minimize the contacting time. Otherwise the membrane will be damaged permanently and incorrect measurements will be obtained.
Figure: Immersing the sensor inside the calibration buffer solution
Step 3: Remove the sensor from the buffer solution and clean it carefully as previously described.
Step 4: Now the second calibration point of 100% can be achieved by placing the sensor approximately 2 centimeters above the water surface and keeping the membrane without water drops that could disturb the measure. Remember to shake the water in order to introduce the maximum amount of oxygen inside water. The next picture shows a diagram.
Figure: Placing the sensor to achieve 100% of dissolved oxygen
Step 5: Wait until values are stabilized over time and type 'N' to continue.
Step 6: Save calibration data into the sensor by typing operator's name and date of calibration. Then, the sensor values will be printed on the screen to check if the measures are done correctly.
Figure: Save the calibration data
One point calibration:
It consists of measuring the 100% of dissolved oxygen as described previously. The one point calibration process is valid for most situations, especially on the field. Remember that any water droplets present in the membrane could distort the measures.

Cleaning the sensor

The OPTOD sensor probe is designed for low maintenance. However, it needs to be cleaned periodically to remove the possible fouling or other biologic material that could appear in the sensor.
Use tap water, soap to rinse the sensor carefully and a soft towel to dry it and remove the biological material.
Figure: Cleaning the sensor probe
The presence of biofilm in the sensor membrane can introduce measuring errors. Use a soft sponge if needed.
It is not necessary to remove the membrane for sensor cleaning.
Finally, if the sensor is not going to be used during a large period, it is important to clean the sensor prior to storing it. Remember to place the protection cap together with a moisture absorbent element (like a piece of cotton).

Installation

It is important to think about a few aspects before installing the sensor on the field:
  • The sensor body should be easily accessible for cleaning, regular maintenance and calibration.
  • The sensor body must be firmly fastened to avoid sensor swing and possible collisions with the surrounding objects that can damage the sensor.
  • If the sensor is installed totally immersed, it should be fastened from the body and not from the cable. The cable is not designed to hold the sensor and it could be damaged.
  • Avoid bubbles around the sensor.
  • For those users interested in measuring directly inside pipes, there are pipe segments with a protected measurement point. As an optional accessory for this sensor, Libelium offers a pipe mounting adapter (available in PVC and in stainless steel) that can be connected to those special pipe segments.
Figure: Typical installation on a pipe
Figure: Another typical installation on a lake
If the sensor is used in a hard environment where animals, solids or other environmental elements can damage the sensor, a protection strainer is available as an accessory of extra protection. Contact our Sales department through the next link if you require more information: http://www.libelium.com/contact.
Figure: Protection strainer accessory
A complete sensor manual can be found on the manufacturer's website.

Application examples

  • Industrial and public sewage treatment plants
  • Wastewater management (nitrification and de-nitrification)
  • Surface water monitoring
  • Fish farming, aquaculture
  • Drinking water monitoring

Calibration report

Together with this sensor we provide a factory calibration report in which the manufacturer ensures that the sensor has passed a calibration procedure with traceability.

pH, ORP and temperature PHEHT sensor probe

The pH, ORP and temperature PHEHT sensor probe combines 3 sensors in one probe, which has been designed to measure under hard conditions like pure snow melting water with low conductivity, lakes, rivers, sea water or even waste waters with high conductivity values.
The PHEHT sensor probe is based on measuring the difference of potential between a reference electrode and a measuring electrode. It includes a long-life reference which increases its lifetime and also it has a high interference immunity. The ORP sensor is thought for normal or modest accuracy applications (fine accuracy is not provided).
Besides, the sensor has a temperature compensation for pH measures carried out by its internal NTC temperature sensor.
Oxidation reduction potential (ORP) and Reduction / Oxidation (Redox) are equivalent terms.
Figure: pH, ORP and temperature PHEHT sensor probe

Specifications

pH sensor:
Technology: Combined electrode Measurement range: 0~14 pH Resolution: 0.01 pH Accuracy: ±0.1 pH
ORP sensor:
Technology: Combined electrode Measurement range: -1000 to +1000 mV Resolution: 0.1 mV Accuracy: ±2 mV
Temperature sensor:
Technology: NTC Range: 0 °C to +50 °C Resolution: 0.01 °C Accuracy: ±0.5 °C Response time: < 5 s
Common:
Default cable length: 15 m Maximum pressure: 5 bars IP classification: IP68 Storage temperature: 0 °C to +60 °C
Figure: Sensor parts: (1) protection strainer, (2) cartridge (consumable part), (3) clamp, (4) sensor body
Figure: Dimensions of the PHEHT sensor probe

Measurement process

The PHEHT sensor provides a digital signal using the SDI-12 protocol.
Reading code:
{
// 1. Declare an object for the sensor
Aqualabo_PHEHT mySensor(XTR_SOCKET_A);
// 2. Turn ON the sensor
mySensor.ON();
// 3. Read the sensor. Values stored in class variables
// Check complete code example for details
mySensor.read();
// 4. Turn off the sensor
mySensor.OFF();
}
During the sensor measurement, there is a small stabilization time of a few seconds, so it is recommendable to wait until the values remain stable over time.
You can find a complete example code for reading this sensor probe in the following link: https://development.libelium.com/sw-xtr-10-pheht-sensor-reading

Socket

Connect the PHEHT sensor probe to Plug & Sense! Smart Water Xtreme in any of the sockets shown in the image below.
Figure: Available sockets for the PHEHT sensor probe

Maintenance

Calibration

By default, the sensor is factory-calibrated, therefore calibration may not be needed for the first usage. However, to increase the sensor accuracy if deviations are detected, calibration would be necessary. Nevertheless, before carrying out the sensor calibration, please bear in mind the next comments:
  • The PHEHT sensor probe comes dry and needs to be rehydrated for 12 hours in a standard pH4 buffer solution before taking any measure.
  • During the calibration process the temperature is not compensated, therefore it must be taken into account. On the contrary, during normal measures the temperature is compensated.
  • It is important that the sensor and the buffer solutions have the same temperature, so before starting the calibration process leave all the necessary elements in the same temperature conditions. Besides, wait for sensor temperature stabilization once it has been immersed.
  • During the sensor measurement, there is a small stabilization time of a few seconds, so please wait until the values remain stable over time.
  • The calibration must be done every 15 days to get a reasonable accuracy in the measurements. However, depending on the application, the time between two calibrations would vary. It is highly recommended to do a test as close as possible to the conditions of the final application to check the sensor drift over time. This will allow adjusting the calibration periods according to the required accuracy.
  • The buffer solution bottles must be closed properly after usage, to prevent deviations on the default values.
Do not place the sensor in distilled water. The sensor will be seriously damaged. Besides, the glass electrode is vulnerable to chemicals like organic solvents, acids and strong bases, peroxide and hydrocarbons.
First of all, ensure that all necessary elements are present. It is important that if a calibration process is started, it should be completed to save the results in the sensor internal memory. Do not abandon the calibration process and always follow the given steps and guidelines to avoid a sensor misconfiguration. If the process needs to be repeated or abandoned, always type the 'Q' command to exit the calibration procedure.
Libelium provides the necessary standard buffer solution to calibrate the Smart Water Xtreme sensor probes. Refer to the calibration solution section for more information.
Figure: Necessary elements for the PHEHT sensor probe calibration
The sensor calibration can be done only on socket E. Owing to that, connect the sensor probe to socket E of the Plug & Sense! Smart Water Xtreme unit to calibrate the sensor, as shown in the image below. Do not use any other Plug & Sense! socket to calibrate a sensor. It will not work.
Figure: Connecting the sensor to the calibration socket
The PHEHT sensor probe allows to calibrate temperature, pH and also check the ORP values. Please read below the necessary steps to calibrate each parameter.
Temperature calibration
The temperature calibration process is the same for all Plug & Sense! Smart Water Xtreme sensor probes. Refer to the previously described temperature calibration section of the OPTOD sensor probe for details.
pH calibration
In the same way as temperature, a two-point calibration is recommended for pH sensor of the PHEHT sensor probe. The offset and slope points can be achieved with the standard buffer solutions provided by Libelium. This calibration method offers the greatest possible level of accuracy and is particularly recommended.
In addition, it is recommended to calibrate first with pH 7 buffer solution and then move to pH 4 or pH 10 depending the range of the measures of the application.
It is important to remark that during the pH calibration process the temperature is not compensated and the pH value for standard buffer solution varies with the temperature, so it is important to carry out the calibration at 25 ºC. If it is not possible, take into account the next tables for temperature compensation. For example, if the buffer solution temperature is 20 ºC, the pH value will be 7.03 instead of 7.01.
Temperature (Celsius)
pH value for standard buffer solution
pH value for standard buffer solution
0
7.13
4.01
5
7.10
4.00
10
7.07
4.00
15
7.04
4.00
20
7.03
4.00
25
7.01
4.01
30
7.00
4.02
35
6.99
4.03
40
6.98
4.04
45
6.98
4.05
Figure: Temperature compensation table for standard buffer solutions
Upload the pH calibration example for the PHEHT sensor probe. The code uses the serial monitor to assist the user with messages and recommendations. The main steps are described below, but the full details are provided in the code.
Step 1: The pH calibration process allows 2 or 3 calibration points. Select the desired points.
Step 2: Type the first calibration point (offset) on the serial monitor and press enter.
Figure: Type the first calibration point on the serial monitor
Step 3: Pour enough buffer solution in a clean baker to cover the sensor head. Immerse the sensor in the pH 7 standard buffer solution. Remove the black protection cap before immersing the sensor in the buffer solution. Wait until values are stabilized over time and type 'N' to continue. The stabilization time for pH measures could take up to 20 minutes.
Do not discard the black protection cap and keep it for the future. It will be useful if the sensor needs to be stored for a large period.
Figure: Immersing the sensor inside the calibration buffer solution
Step 4: Remove the sensor from the buffer solution and clean it carefully as previously described.
Step 5: Type the second calibration point (slope) on the serial monitor and press enter.
Figure: Type the second calibration point on the serial monitor
Step 6: Pour enough buffer solution in a clean baker to cover the sensor head. Immerse the sensor inside the desired standard buffer solution. Wait until values are stabilized over time and type 'N' to continue.
Step 7: Save calibration data into the sensor by typing operator's name and date of calibration. Then, the sensor values will be printed on the screen to check if the measures are done correctly.
Figure: Save the calibration data
ORP calibration
Regarding the ORP calibration, it is done using a two-point calibration. The offset will be the zero value exposing the sensor in the air and the slope will be an ORP standard buffer solution (240 mV).
Upload the ORP calibration example for the PHEHT sensor probe. The code uses the serial monitor to assist the user with messages and recommendations. The main steps are described below, but the full details are provided in the code.
Step 1: The first calibration point (offset) is set to zero and it cannot be changed. So keep the sensor exposed to the air and wait till measure stabilization over time. Then type 'N' to continue. Remember to remove the black protection cap.
Do not discard the black protection cap and keep it for the future. It will be useful if the sensor needs to be stored for a large period
Step 2: Type the second calibration point (slope) on the serial monitor and press enter.
Step 3: Pour enough buffer solution in a clean baker to cover the sensor head. Immerse the sensor inside the ORP standard buffer solution. Wait until values are stabilized over time and type 'N' to continue.
Figure: Immersing the sensor inside the calibration buffer solution
Step 4: Save calibration data into the sensor by typing operator's name and date of calibration. Then, the sensor values will be printed on the screen to check if the measures are done correctly.
Figure: Save the calibration data
Step 5: Remove the sensor from the buffer solution and clean it carefully as described in the next section.

Cleaning the sensor

The PHEHT sensor probe needs to be cleaned periodically to remove the possible fouling or other biologic material that could appear in the sensor.
Before cleaning the sensor, please keep in mind that the crystal electrode used for pH measurement is very fragile. Use tap water, soap to rinse the sensor carefully and a soft towel to dry it and remove the biologic material. Avoid using absorbent paper because the glass electrode is extremely vulnerable to frictions. Moreover, if the ORP sensor of the PHEHT sensor probe is still dirty, use a soft and fine sandpaper to clean the metallic part.
Figure: Cleaning the sensor
The presence of biofilm in the sensor electrodes can introduce measuring errors.
On top of that, the cartridge could be replaced if it is damaged for some reason. Contact Libelium for more information.