I have a very simple circuit that is measuring the pulses from a Gems FT-110 flow meter. We are checking the accuracy of the flow meter using a bucket test. All of the results are about double what they should be. When I attach the scope probe to the circuit the flow reading as calculated from the pulses generated is now in line with the bucket test. Note that the probe does not even have to be connected to the oscilloscope for this to happen. Another Gems flow meter RFO-2500P does not have the same problem. It has always been accurate with the same pulse counter program.
FT-110: NPN open collector output, 10K pull-up on signal to 5V, connected to input on Arduino Mega
RFO-2500P: PNP output, no pull-up needed, directly connected to input on Arduino Mega
Both use a 5V power supply. Schematics are in the datasheets. This seems like an impedance mismatch issue to me but I can't grok what the solution is. Secondary question is how do I measure this type of signal without affecting the waveform? What kind of setup/probes do I require? Thanks.
FT-110: NPN open collector output, 10K pull-up on signal to 5V, connected to input on Arduino Mega
RFO-2500P: PNP output, no pull-up needed, directly connected to input on Arduino Mega
Both use a 5V power supply. Schematics are in the datasheets. This seems like an impedance mismatch issue to me but I can't grok what the solution is. Secondary question is how do I measure this type of signal without affecting the waveform? What kind of setup/probes do I require? Thanks.
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