PLC not reading analogue input from flowmeter...

[Mas01]

This particular problem is doing my head in...
In our system we have a minisonic flowmeter. One of these.

It outputs 4-20mA and is an input into the PLC as an analogue input into D34.
When I inject 4-20mA into the PLC from a Fluke 754 processor calibrator, the value of D34 changes as expected (litres/sec).


On the flowmeter, there's an I/O function feature where you can output 4-20mA (even though there's no water flowing)...but when I connect the flow meter to the PLC, I get zero flow reading.


I know the flowmeter is correctly outputting 4-20mA because I connected it to a Fluke. I've even tried resetting the PLC in case that makes a difference. I must be doing something daft. Any suggestions? Thanks

 

[OkiePC]

It will probably boil down to a wiring/polarity issue. Connect your Fluke process meter in series with the flowmeter while it is wired to the PLC and measure the mA and test the flowmeter output signal.

What make/model PLC?

 

[Mas01]

It will probably boil down to a wiring/polarity issue. Connect your Fluke process meter in series with the flowmeter while it is wired to the PLC and measure the mA and test the flowmeter output signal.

What make/model PLC?

Sorted mate. It was the wires being swapped over and thus in the wrong terminal block.


Mitsubishi Q-series PLC. Thanks all sorted.

 

[Saffa]

EDIT: I see you have it sorted.

When you're measuring the 4-20mA output using your fluke, is the PLC input still in the circuit? I.e. flowmeter output through the fluke, then into the PLC input?

Was this working before or is this a new or modified setup?

I've had a 24V powered instrument that was only getting 12V due to an overloaded power supply, it still lit up and operated but it couldn't drive its current loop with only 12V when connected to the 250 ohm PLC input. But it worked fine with a low impedance multimeter input... which was doing the techs head in, he was adamant it was the PLC input because "i can read the mA on my meter but when i connect it to your f'#&£ing PLC input it stops working!"

 

[Mas01]

EDIT: I see you have it sorted.

When you're measuring the 4-20mA output using your fluke, is the PLC input still in the circuit? I.e. flowmeter output through the fluke, then into the PLC input?

Was this working before or is this a new or modified setup?

I've had a 24V powered instrument that was only getting 12V due to an overloaded power supply, it still lit up and operated but it couldn't drive its current loop with only 12V when connected to the 250 ohm PLC input. But it worked fine with a low impedance multimeter input... which was doing the techs head in, he was adamant it was the PLC input because "i can read the mA on my meter but when i connect it to your f'#&£ing PLC input it stops working!"

When I had the fluke connected, the PLC was out of the loop. i.e. it was a direct connection from the flowmeter to the fluke. The flowmeter we've had.in the system for a while but it's not been connected to the PLC until now.