cFP-AI-111 and Burkert Flowmeter : Problem.

[KevMartel]

Hello,

I have a Burkert flowmeter (Model No 8012) which have a 4/20 mA output and I want to  use a cFP-AI-111 module to read the signal.  My wiring schematics is in attached piece.

When the flowmeter is connected alone in the CfP, there is not problem, it  works well. However, when I use the burkert along with another instrument, I can see the current read on the flowmeter input increase to 21 mA.

By analysing the interaction between both circuits, I realized that the problem is caused by the fact that there is a common connection between all the inputs on the FieldPoint AI-111.  (COM terminal). Explanation -> The RTD wiring connects -24 V and COM together so the -24 V is now linked to pin 4 on my burkert applying directly 24 V to the 100 ohms resistor inside the AI-111.

Is there a solution to my problem ??

Thank you very much

[RavensFan]

Where is the -24 VDC coming from?  You certainly don't want to connect that to common which should be the 0VDC level.

An analog input module is not the correct module for reading an RTD.  You should be using an RTD module such as the cFP-RTD-122.

[KevMartel]

I'm sorry for the confusion :  -24V is the ground of the 24 V Source. It has the potentiel 0V.

I don't read directly the RTD.  There is a transducer to convert the RTD reading to a proportional 4/20 mA signal.

[Craig_]

So is your sensor running off of -24V to 24V or is it running off of 0V to 24V?

You are correct that the ground of the module is shared, but you should be able to reference things to the same ground level.

[KevMartel]

Hi Craig,

Here is another schematics where the nature  of the problem is more obvious.  Burkert alone is ok but when a second transmitter is connected, 24 V is applied on the Input 1 (24 V / 100 Ohms = 240 mA) and I read an overflow "21 mA".

We already have Burkerts wired to a PLC but all the inputs are independants so it works fine in that case.  

I don't think there is a solution to make it work with the compact Field Point.

If you have one, please share it ! 🙂

Thank you

[AnalogKid2DigitalMan]

Have you tried connecting Pin 4 (output) of the Burkhart to the IN1 terminal instead of COM?

-AK2DM

[KevMartel]

Hi AK2DM,

Yes I have tried and it didn't work. Pin 4 in a sink and cannot source current.

According to the seller, I must return the unit for programming if I want it to be configured in source mode as seen in my attached file. Then, I will be able to connect pin4 to In1.

I wanted to find a solution to avoid related fees, but it seems impossible.

[RavensFan]

Perhaps an optical isolator between the flowmeter and the cFP will solve the problem.