Accuracy of the pH and temp sensors.

[Dr.Yak]

Hi,

I have some issues with the accuracy of the pH and temperature sensors. I have 4 temperature and pH probes connected to a profilux 3N ex and an expansion box (with the latest firmware upgrade).

With the temperature sensors I have had up to 2 deg C difference between them in the same position in the same tank (I've set the calibration data as per the info supplied with each expansion module). I've managed to overcome this by adjusting them via the profilux software to the reading of a recently calibrated industrial temperature probe.

The real problem is with the pH sensors - it is absolutely critical that we are able to measure the pH accurately to at least within 0.02. Our pH and temp probes have been used for about a week and they are giving wildly different values when placed in the same part of a tank (any tank) far away from any electrical interference. I have calibrated them with fresh scientific grade pH 4 and 7 buffers equilibrated to the tank temperature and I get the following readings (for eg) at 23.5 deg C.
1: 8.16
2: 8.11
3: 8.22
4: 8.05
I also took the pH with our lab pH meter (Eutech 610) calibrated at the same time with the same buffers and temperature and it gives a reading of 8.078 (this calibrates with a 97% slope so the probe is still in good shape).

I am very worried by the differences between the probes - I have recalibrated a number of times and am still getting this difference. There doesn't appear to be anyway to adjust the calibration using the profilux software nor can I seem get the voltage readings so I can calculate any deviation from ideal so I can work out where the problem is (the readings given while the sensor is calibrating cannot be millivolts). I can try clean/regenerate the probes with 0.1M HCl but that seems strange to have to do with "new" probes. During calibration all probes are cleaned with distilled water and kimwipes and if they need to be stored they are stored in 3M KCl.

What can you suggest?

Cheers,
Dave

[Matthias]

to measure the pH accurately to at least within 0.02

This is simply not possible.
As written in our manual the accuracy is 0.1 - under perfect conditions. This matches to your measurements.

If you immerse the probes in the calibration fluids you used (4 and 7) you will see that the results are there even better.

You can increase the accuray when you use the correct fluids for your measurement range. For the measured value of approx. 8 you should use cal. fluids 7 and 9 (or similar)