In this video, we will discuss the recommended order of EXO sensor calibrations.
Keep in mind that it is best to follow your organization’s standard operating procedures. The purpose of this video is to offer guidance if you are looking for recommended practices, and you can refer to our other EXO University videos for specific procedures for each sensor.
To begin calibration, we recommend starting with the temperature check of the EXO Conductivity/Temperature sensor, to verify the temperature measurement accuracy against a NIST-traceable thermometer. Remember that the temperature sensor cannot be user-calibrated. Performing the temp check first is critical because the measurement from this sensor is used for temperature compensation for most other sensors.
The order of your remaining calibrations depends on what sensors you have.
For instance, the calibration of dissolved oxygen in mg/l is influenced by temperature and conductivity. Also, the depth sensor references changes in the density of water as a function of temperature and salinity. For these reasons, we recommend performing the conductivity calibration at the beginning of your process, before calibrating the rest of your EXO sensors.
If you use a pH/ORP combination sensor, calibrate pH before you calibrate ORP, since ORP can be affected by changes in pH.
The ammonium, chloride, and nitrate ion selective electrodes can be calibrated using conductivity and pH/ORP. The conductivity standard, pH buffers, and Zobell’s solution can negatively affect the ISE’s, so they should be removed from the sonde until after the Conductivity, pH, and ORP calibrations are complete. Also, conductivity and pH are used to calculate ammonia, a parameter of the Ammonium ISE, another good reason to calibrate your ISEs after conductivity and pH/ORP.
With respect to the EXO Turbidity sensor, it references its own internal thermistor for temperature compensation of its measurements, and so it can be calibrated in any order. However, the NitraLED sensor relies on real-time input from the turbidity sensor; so turbidity must be calibrated before NitraLED.
We find that some EXO users prefer to calibrate their fDOM and Total Algae sensors after all their other sensors, since these calibrations require standard dilutions. Working with quinine sulfate for fDOM cal can also be hazardous and requires extra care. Quinine fluorescence is degraded by
the presence of copper and chloride or halide ions, found in seawater, conductivity standards, and Zobell solution. Clean your sensors thoroughly and remove the Central Wiper and any antifouling components before fDOM calibration. Following fDOM cal, be sure to rinse the quinine sulfate from the sensors thoroughly and dispose of it properly.
Dissolved Oxygen and depth are typically the last parameters I calibrate since they do not require a standard solution. In some situations, you may wish to calibrate dissolved oxygen while you’re in the field; if you are deploying your sonde at a significantly different elevation than your lab or office, because the dissolved oxygen measurement is affected by barometric pressure.
You can also calibrate depth in the field so that the sonde can account for local influences on the pressure transducer. Remember to keep the sonde stationary during the depth calibration.
To summarize these sensor interactions, let’s say I have an EXO3 with Conductivity/Temp, DO, pH/ORP, and turbidity installed alongside a central wiper. The order in which I would calibrate would be as follows:
First I would verify my temperature sensor with a check against a NIST-traceable thermometer; then, I would calibrate conductivity, followed by pH, ORP, turbidity, DO, and finally depth.
For more information on how to calibrate each of these sensors, please check out our other EXO University videos, and thanks for watching!