Trying to get full scale readings from a 4mA-20mA sensor

Other Hardware Sensors
1

Hello,

I've looked through the archives and as such I am using a 220ohm resistor to read the voltage of 4mA-20mA humidity sensor that takes 12-24vdc. I've mapped the sensor readings as follows:
senConv = map(senReading, 180, 900, 0, 100)

where
4mA220ohms= .88v
20mA220=4.4v
and for mapping
180= .88v/5v1023
900=4.4v/5v1023

The problem that I am having is that the sensor will not read above 70% humidity which is equal to around 3 volts. Is it possible that the resistor is causing too much of a voltage drop such that the 0-5v scale can not be realized?

Thanks in advance for any feedback you may have to offer.

2

It's possible, but not so likely that a 12-24VDC sensor can't deliver 4.4V into a 4-20mA receiver.

Ideas for debugging:

  • Measure the raw voltage with a DMM -- basically eliminate the software as a source of error
  • Look up the sensor datasheet (or call the mfr. if they're the kind that are willing to talk to you) to see if they have any specs on loading
  • Use a lower-value resistor (100 ohms?) and see if the problem goes away

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