This article discusses how to trouble-shoot fluctuating measurements.
Some monitoring systems, such as pH, are
very sensitive and prone to interference. Small
current or voltage surges can cause large fluctuations
Here are some simple tests that you can follow to determine
the cause of measurement problems. We've used
the Microlink 751 multi-function USB unit as an example
but many of the tips also apply to other manufacturer's
systems. Work through the tests one-by-one, eliminating
sources of error as you go.
Check that the Microlink is working
properly. First remove other factors in the system that
might be causing the problem, then check the
Remove any power signal conditioner. This provides
surge protection and noise filtering. It should
remove interference from other equipment but a
faulty one could cause its own problems.
Move the Microlink to another location. If you are
using it in a laboratory for example, take it home.
Connect the Microlink to a different laptop running
on battery power, not on mains power.
Now you can test whether the Microlink is the source
of the noisy, fluctuating, data. Connect together
the first positive input pin, the first negative
input pin and the 0 V input pin. With the
Microlink 751 these inputs are on pins 20, 1 and 19.
You could use paperclips to connect the three pins.
Leave all other inputs unconnected.
You have now shorted out the Microlink: if it is
working correctly it will produce a reading of zero.
If not the fault lies with the Microlink and you
should contact Technical Support and send the unit
back. If the reading is zero then the problem is
very unlikely to be caused by the Microlink and you
can go on to step 2.
Once you have eliminated the DAQ unit as the source of
the fault, test the power supply to the computer. Plug
the laptop into the mains - the reading should still
Reconnect any power signal conditioner. Again, check
that the reading is zero.
Test the sensors one-by-one. For example, place a pH probe in a known
solution and see if the reading is as expected. Keeping
the signal wires short and far away from electrical
machinery helps reduce noise. You may also need to
clean your electrodes.
Move everything back to the original location. If the
readings become erratic go through the tests again.
Disconnect from the laptop and connect to the original
computer. Again, if you now get noisy readings repeat
Finally make your measurements in a real situation.
Discovering at which stage the problem occurs will let you
identify the source and take remedial action.