Pico Technical Support and Discussion Forum

chem · Guest

Hello,
I use 'DrDAQ for pH measurements and the readings are correct with the picoscope software, but not with the picolog recorder, where the reading is stuck in a value around 7 in all cases. The problem is that I want to log pH (and other ionic measurements) over a long period of time and therefore I need to use the daq as alogger, rather than as a display of instantaneous measurements. I have recently updated the software drivers using the links in your website. Could you please offer a solution?

I would also be interested to know if you can provide a Labview or Matlab interface for DrDAQ.

Thank you

Michael · Posted: Wed Feb 22, 2006 11:08 am Post subject:

Hello,

This problem may be down to a hardware fault. Please email us direct at support@picotech.com to discuss this further.

Best regards,
Michael

Guest · Posted: Wed Jun 14, 2006 12:35 pm Post subject: pH calibration on DrDAQ

HI

I am wanting to correctly measure pH via DrDAQ. I have buffer solution of 4,5 and 10.

Does anyone know how I set it up? Someone suggested using scaling and table lookup, but that is only applicable for linear relatioships?

scotts · User Joined: 14 Jun 2006 Posts: 2

HI

I am wanting to correctly measure pH via DrDAQ. I have buffer solution of 4,5 and 10.

Does anyone know how I set it up? Someone suggested using scaling and table lookup, but that is only applicable for linear relatioships?

Michael · Posted: Fri Jun 16, 2006 3:11 pm Post subject:

Hello,

Here is an extract from the DrDAQ help files:

Calibrating for accurate pH measurements
Calibration is only required when accurate (better than 0.5pH)

measurements are required. Calibration should be performed just before the measurements are made. User calibration information is stored separately in PicoScope and PicoLog, so if you wish to measure pH accurately with both programs, 2 calibrations will be required.

To calibrate a pH electrode you will require at least 2 pH buffers (pH4, pH7 and pH10 buffers are widely available). Calibration should be performed with the ambient temperature close to 25°C.

A container of clean water is also required to wash the electrode before moving it from one buffer to another.

1. Connect the pH electrode to DrDAQ and display pH using either PicoScope or PicoLog (depending on which program you are calibrating for).
2. Place the electrode in the first solution and wait for at least 30 seconds for the readings to stabilise (gently stirring helps).
3. Make a note of the reading and repeat the procedure for each of the buffer solutions (do not forget to wash the probe clean before swapping from one buffer to another.
4. You should now have a table of readings similar to the one below:
pH Buffer value at 25°C Measured value from DrDAQ
4.01 4.06
7.00 7.07
10.01 9.92

pH calibration with PicoLog
As already mentioned you will need to note down the measured pH value and compare it with the actual value to create a lookup table. Then follow the steps below:
1. Got to 'Settings' and click on 'Input Channels' on the pull down menu.
2. The converter input type will then be displayed, check settings and click 'OK'. You should now see the DrDAQ measurements box.
3. Select pH and click 'Edit'.
4. Click 'Options' from the Edit measurement box.
5. Now click on 'Scaling' and select 'Look up table' from the menu box.
6. Enter the real measured pH value below Raw and the actual value in 'Scaled' (similar to the table in the previous section)
7. Click 'OK' for all open boxes and verify the calibrated pH readings.

I hope this helps.

Best regards,
Michael

scotts · User Joined: 14 Jun 2006 Posts: 2

Thanks, but I solved the problem!

The problem turned out to be an electrical earthing issue.

I am measuring flowing water discharged to river. When I was running on battery power for the laptop everthing was fine, but if I connected to the mains, it all went wrong! BUT if I removed the probe from the flowing water and placed it in a beaker of the same water it was fine again.

I came to the conclusion that the flowing water must be creating a small static charge due to flowing in plastic tanks and pipes.

I am now using an earth isolating pack, and all is well

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