Oakton Recalibration

For the SSI FWC Oakton hosts who have taken ownership of a FWC Oakton field chemistry device, some notes on re-calibration.

Keeping the SSI FWC Oaktons aligned by re-calibration makes it easy to collect consistent FWC field data and allows us monitor the field chemistry data that we are capturing (via the data uploaded to the cloud). If we see local chemistry spikes we can then determine if this relates to a particular Oakton, cross-check if this is a recalibration or field data entry error (it does happen!), or if the spike relates to a field change in water chemistry (eg turbidity, mineral content, etc (that happens too!).

Tailored Oakton recalibration instructions are located here.

Note - we re-calibrate each Oakton device before each field visit.

Oakton Recalibration FAQ

Q. Having issues with recalibrating the conductivity (at the 1413 microseimens/cm standard), why is this?

A. It’s most likely a step in the workflow, and a design shortcoming in the Oaktons… (which is not present with the pH recalibration, there is one step missing...). When in conductivity re-calibration (recalibration instructions) “1413” flashes up as a recalibration option, as this point you have to manually hit enter for the device to “know” that “1413” is the buffer being used (alternative buffer choices also flash up and these will go round and round). This conductivity recalibration is different from that for pH, where the device “knows” when the sensor is in the right buffer (and no intervention to confirm this is needed).

Another known conductivity calibration issue that people encounter is not having enough buffer in the sensor cap to completely cover the conductivity electrodes (the fluid level in sensor cap needs to be slightly higher than the "max" indicator).

Q. Reusing Oakton recalibration chemicals (saving on fluids and costs)?

A. If we are able to store and reuse the "Oakton Conductivity 1413” and “Oakton pH 10.01" recalibration chemicals temporarily (in a small air-tight container), after each recalibration session, that’s great. For conductivity fluids this re-use of recalibration fluids is straight forward. For pH recalibration fluid re-use needs more caution, there may be a ph measurement drift, so... some caveats on pH fluid recycling:

- the measured pH drift with clean (ie not recycled) older chemicals is small/zero - continue to use an open Oakton pH 10.01 bottle until empty.

- if the pH recalibration fluid is recycled for a prolonged period, and/or if the bottle used for storing recycled pH fluid has a larger air space, pH measurements may drift more - please do not use the recycled pH fluid for longer term use and use a smaller air-tight container for storage.

Note - if there continues to be drift on the pH re-calibration (ie if the pH re-calibrates to 10.01 but the same fluid then gives a significantly higher, or lower, value with the same fluid in the cap) then please request replacement "pH sachets" (available from 202103 - contact the SSIFWC Project Lead). These sachets store smaller pH re-calibration volumes, allowing longer life pH re-calibration fluid consistency and no expiry issues (the remainder of the re-calibration procedure remains the same ). See also the "When doing the recalibration I am getting “out of range” buffer errors, or "drift" question below.

Note - in both conductivity and pH re-calibration after chemicals have been reused several times the Oakton will flag a “calibration buffer error", at this point junk those reused fluids and start again with new ones. The chemicals are OK for a flush down the sink into a septic field (chemical alkalinity a little like toilet cleaner), follow a sink disposal with gentle running water to ensure no residues are left there.

Q. Does the temperature of the buffer fluids during recalibration matter?

A. Temperature related changes in readings during recalibration do happen and the Oakton is designed to recognise and compensate for this, so nothing to worry about. The recalibration fluid bottle(s) provide a tabulation showing the impact of temperature on the re-calibration numbers seen on the Oakton device display.

Q. I am getting an odd message on the Oakton screen and am unable to make a measurement

A. variety of error messages “out or range” and an odd flashing screen are not a good sign. Contact John, SSIFWC Project Lead

Q. When doing the recalibration I am getting “out of range” buffer errors, or "drift" (the calibration tick comes up but while the recalibration fluid is still in the cup the sensor then gives a different value what do I do?

A. Step 1. Soak the head with tap water (or pH 4 buffer) for 30 mins to re-condition the sensor. Step 2. start with a new re-calibration fluid sample and ditch the old re-use fluid, it's no longer working.

An apparent “drift”, after the pH re-calibration tick, may actually be a function of the pH fluid temperature in the cap. As per tabulation on the back of the bottle (see above) temperature of the fluid influences the pH measure (the pH re-calibration fluid is only actually 10.01 +/- 0.02 at 25 degC) With a liquid temperature between 10C and 20C the re-calibration fluid (same fluid in cap, after tick) you will get a value between 10.18 and 10.06.

If there continues to be drift on the pH re-calibration (ie if the pH re-calibrates to 10.01 but the same fluid then gives a significantly higher (or lower value with the same fluid in the cap) then please request replacement "pH sachets". These sachets store smaller volumes, allowing longer life pH re-calibration consistency and no expiry issues (from 202103 - the remainder of the re-calibration procedure remains the same ) - contact the SSIFWC Project Lead.

Try the recalibration process again, still getting an error - contact the SSIFWC Project Lead for a device swop!

Q. To save batteries should I switch off the Oakton between various sample sites?

A. While switching off the Oakton is OK this does wear down the touch screen around the "on/off" button, eventually this causes the device to leak (ie leads to the device's end-of-life) so please leave it switched on. The battery life on the Oakton's is long and it will switch itself off after 3 minutes.

Q. Where can I store the Oakton re-calibration chemicals

A. The recalibration fluids are chemicals of the kind that you would use for domestic cleaning and should be stored in a suitable chemicals cupboard (child-safe etc). The chemicals should not be exposed to temperature extremes, nor frost.

Oakton hosts?

Our current overview of the SSI locations of our FWC Oakton hosts and how to become a host are under our Homeoffice page, here.