16
109117-1 Rev. A, 10/96
3
3.4.3 Meter Error Codes (cont)
Although slope is useful in generally determining when it is time
to do maintenance on an electrode, Response Time is, by far, the
critical factor affecting accuracy of measurements. When moving
from the 7.00 to the 4.00 buffer during calibration, you should
have a stable value in 10-20 seconds. Give a 7.00 to 10.01
calibration 20-30 seconds for a stable value. If it takes longer, the
electrode response is getting slow and the calibration may not be
accurate. Refer to the manufacturer's instructions for electrode
maintenance.
3.4.4 Electrode Problem Solving
Slow response or unstable readings in calibration buffers is
usually related to the condition of the reference fill solution or its
flow through the junction.
1. The fill solution acts as an electrical conductor between the
reference inside the electrode and the solution under test.
Empty the electrode and refill with fresh fill solution
regularly.
2. The junction is designed to allow the fill solution to leak
out of the tip of the electrode at a slow controlled rate.
To test for proper flow, rinse the tip of the electrode, blot
dry and let the electrode hang in the open air for 15
minutes. If it is flowing properly, salt crystals of the fill
solution will form on the junction. If they do not, refer to
the manufacturer's instructions for junction cleaning or
replacement.
Slow response or unstable readings in samples can be caused by
a number of things. Those listed below assume that everything
was fine in the calibration buffers.
1. Low ionic strength samples will tend to be very drifty. You
are trying to take an electrical measurement in a solution
that is a very poor conductor. Try adding a drop of KCl fill
solution to about 5 mL of the sample. In most cases it will
not change the pH but will give the sample enough ionic
strength for the electrode to finish the reading.
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