USB DrDAQ User's Guide
15
Copyright © 2013 Pico Technology Ltd. All rights reserved.
usbdrdaq.en r4
Calibrating for accurate pH measurements
As described above, calibration is only required when accurate (better than 0.5 pH)
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,
two calibrations will be required.
To calibrate a pH electrode you will require at least 2 pH buffers (pH 4, pH 7 and pH
10 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 between buffers.
1. Connect the pH electrode to USB DrDAQ and display the 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 stabilize (gently stirring helps).
3. Make a note of the reading and repeat the procedure for each of the buffer
solutions. Don't forget to wash the probe between buffers.
4. You should now have a table of readings similar to the one below:
pH Buffer value at 25°C
Measured value from USB 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 these steps:
1. Go 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 USB 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.
Storing pH electrodes
If KCl solution is not available then any pH 4 buffer solution will be suitable for storing
electrodes. KCl is recommended because it is the substance inside the bulb of the
electrode. Tap water would be acceptable but is not ideal. Do not under any
circumstances use deionized or distilled water!
