The Industrial Raspberry Pi Forum

Hi all,

I want to know I connect my CH4 sensor (0-100%, 4-20 mA) from Bluesens with the AIO Modul from RevPi Connect 4.

From the schema attached, I connected PIN 1 and PIN 2 from bluesens to the 24 V power supply.
PIN 6 from bluesens I connected for with PIN 15 of the AIO Modul and a bridge from PIN15 to PIN13.
And PIN 8 from bluesens and PIN 11 from the AIO Modul I connected to GND from the 24 V power supply.

PiCtory:
Input4Multiplier_i07: 625
Input4Divisor_i07: 10000
Input4Offset_i07: -250

Now I have the problem, that the CH4 quality showes me nearly always the maximum of 20 mA = 100% and is flactuating with the temperature. I measured the CH4 ammount with another device and it should be between 55 and 60% and not 100%.

Can somebody help me and have an idea, what might be wrong?

Thank you for your help.

Hello Wolle, first of all, thank you for the detailed description of the problem. I have understood that the error only occurs with one of the two AIO modules, is that correct?
So when measuring the current, the load resistance must be taken into account. If it becomes too high, the current source may no longer be able to drive the current. Are there any other differences, i.e. cabling or sensors? Can you replace the sensor?

Hello Wolle, so I have made a litle wiring setup which is the part that is connected to the RevPi AIO Module:
The other PINs of the bluesen are then connected in this way:

• PIN 1 & PIN 2 - 24V
• PIN 8 & PIN 11 - GND

The inputs of the AIO module on the front side are galvanically isolated from those of the X4 plug, connected to the 24V power source. I noticed that a single source is used. This could be the source of the error for the imprecise measurement.

• If you can use a second power source that functions independently only for the sensor, it would be possible to isolate interference.
• Can you replace the AIO module and do you have a 2nd AIO module?
• Can you measure in parallel with an oscilloscope or similar device and compare whether the values are the same or different?
  Try to compare the firmware versions of the modules that you can output with "piTest -d" with the current versions here - try updating the firmware if necessary

The instructions with the five initial steps don't exactly sound like plug and play, but I'm curious to see what we can discuss with each other.