I have an Omega logger with a 0-24mA at a test facility trying to log dissolve oxygen readings. The DO sensors are 4-20mA. Initially I thought this was a scaling issue, but the values in the logger and that from the DO analyser are just not consistent.

for example, a 4mA output from the DO analyser showed 4.75 from logger

while

a 20mA output from DO analyser showed a value of 19.1 on the logger.

The issue must come from the logger, because the loop between the DO sensing probes and analyser has been verified to be working correctly. Screens are fitted on the lines, so any interference from other process equipment is minimised.

I just would like to understand why some dataloggers have a 0-24mA range and what needs to be done to measure sensors that give a 4-20mA signal?

 

I never heard of a 0-24 mA input, but there is no technical reason it could not be done. There is probably some way to scale the input to get the correct engineering units that has not been configured correctly.

--
Bob

 

Many 4-20 mA devices are not limited to 4.0 mA at the low end or 20.0 mA at the high end, and if the measured parameter exceeds the low- or high limit by a small amount the 4-20 mA output will still be proportional.

How small? That would depend entirely on the 4-20 mA device manufacturer's specifications.

So, I would suspect one would just connect a 4-20 mA input and at the min point the logged signal would be 4.0 mA and at the max point the logged signal would be 20.0 mA, and if the measured parameter exceeded the min or max value by a small amount the logged value would be still be proportional.

So, a 0-24 mA datalogger would seemingly give one the ability to log values slightly above or slightly below, and if there was an open in the loop the value would go to zero mA and would also be logged as 0 mA (indicating an open loop?).