Product information
14
Copyright © 2013 Pico Technology Ltd. All rights reserved.
usbdrdaq.en r4
2.9.3
DD011 pH Electrode
Before using your electrode, remove the storage bottle.
Size
12 x 120 mm
Operating temperature
0 to 60 °C
Resolution
0.02 pH
Using USB DrDAQ with pH Electrodes
The pH input on USB DrDAQ is a very high-impedance input that is suitable for use
with any standard pH electrode. For most applications no calibration is required: just
plug in an electrode and measure. If, however, you require very accurate pH
measurements, then you should calibrate the probe before use (see below).
If you are using a pH probe and not getting the results you expect then the most
probable cause is a defective pH electrode. If cared for properly, pH electrodes will last
for a number of years (see the instructions supplied with the electrode). If they are
not stored properly, they will be destroyed in a few weeks. Trying to calibrate out
errors from a defective probe will not work and may further increase errors.
pH measurement and temperature calibration
The output from a pH electrode is proportional to absolute temperature (kelvin). A
temperature difference of 10 C will cause the probe output to change by
approximately 4%. This is a major source of error with most pH meters and data
loggers. To minimize these errors, a temperature sensor can be connected to Ext 1
and the DrDAQ will compensate for temperature changes.
Testing pH Electrodes
The best way to test a pH electrode is to use pH buffers (pH 4, pH 7 and pH 10 buffers
are widely available). If you do not have any pH buffers, then distilled deionized water
will have a pH near 7 (some still mineral water bottles have the typical pH printed on
the label). Most fizzy (carbonated) cola drinks have a pH in a 2.5 to 3 range.
With the pH electrode connected to USB DrDAQ, display the pH channel using PicoLog.
Measure the pH of each buffer in turn. Allow 30 seconds for the reading to stabilize
and be sure to wash the electrode in clean water before swapping solutions. If the pH
measured is within 1 pH of the expected value then you can be fairly sure the
electrode is working correctly. If the readings are wrong then the electrode is probably
defective. Replacement electrodes are available from Pico Technology.
If you suspect that the pH input on USB DrDAQ may be defective, then short out the
BNC connector using a terminator plug. The reading on the screen should be pH 7. If it
is not, the most likely reason is that someone has calibrated the probe incorrectly.
Select
File
>
New Settings
in PicoLog to delete the user calibration and return to the
default calibration. If the reading is still wrong then contact technical support at Pico
Technology.
