4-20 mA current source not reading properly on AI-110

[FMW]

I am trying to use an AI-110 to read a 4-20 mA current source from a device (tachometer). A multimeter reads 24V across the two output terminals and reads the correct (expected)current flow across the two terminals. However, when I try to hook up the device to the I and COM terminals of an AI-110 the readings are overrange or otherwise very wrong. I have also tried using an AI-111 with the same result. I am mystified why the DMM reads as expected but not the Fieldpoint. The AI-110 is configured as +/- 20 mA range till I get this sorted out.

This must be a noob problem but I'd really appreciate any help.

Thanks in advance.

[Wayne.C]

Exactly which tach are you trying to hook up?

[FMW]

It is a tachometer on a Parr Instruments controller. It has a Parr label with no other OEM markings apparent.

[Wayne.C]

I suspect that you have something miswired to the cFP I/O module.  Would need to see documentation for the Parr controller to verify correct connections.  You have a model # for this thing?

[FMW]

I don't have anything in the way of documentation on this specific aspect of the device. However, doesn't the fact that a DMM can obtain the expected measurements mean something? Seems like the device is doing what it is supposed to but either I do have it wired incorrectly, there is something wrong with the FP config, or the FP module is malfunctioning.

If the tach outpt is behaving as expected (current source, 4-20 ma), should not connecting the outputs to the FP I-in and COM terminals work?

Thanks.

[Wayne.C]

There are two different kinds of 4-20mA outputs that can be found on instruments.  One is what I call a sourcing type where the circuit that regulates the output current is between the instruments internal voltage source and the current out ( + ).  In this case the current out ( - ) is tied directly to the common of the instrument.

The other type is what I call sinking type where the positive of the instruments internal voltage source is tied directly to the current out ( + ) and the current regulating circuitry is between the current out ( - ) and the common of the internal voltage supply. 

These The problem with sinking type is the all the cFP-AI-110 inputs are referenced to the common of the cFP.  If the two current loop signals are the ONLY connections between the cFP and the instrument, it should work ok.  If there are other connections between the cFP and the instrument you may encounter problems.

Can you elaborate a bit on all the connections between the instrument and the cFP?  Are you supplying the 24V to the instrument? 

PS There is actually a third type.  These are true two wire devices.

How many wires on your device?

Message Edited by centerbolt on 04-24-2008 09:27 AM

[FMW]

There are two output terminals (BNC connector).

The outer terminal has a +24V versus the chasis. The inner terminal is +24V versus the outer. It is nearly 0V versus the chasis at 4 mA output but ca. 0.8V versus the chasis at about 12 mA output. Does that help?

Thanks again.

[Wayne.C]

Yes, that definitely helps.  It sounds like you have what I call a sinking 4-20 mA output.  I'm guessing that as the output current increases the voltage between the inner terminal and chassis will continue to increase proportionally.  It also means you have a potential problem with what is referenced to what.

Can you give an overall description of what you are setting up?

I know you have the instrument with the 4-20mA output.  What else is connected to this instrument?  What else is connected to the cFP?

[FMW]

The controllers can autonomously control some lab reactors but has AO for a tach, an AI (0-5 V) for remote agitator control, and a DI to inhibit the heater. I have a PS-5, FP-2010, TC-120, AI-110, AI-111, and TB-10 (2x2 DO, 2x2 0-10V AO). The FP system is to monitor and log data and control certain aspects remotely via FP and LabView. The goal is to monitor rpm, temp (thermocouple), and pressure (loop powered 4-20 mA transducer). Control of the agitator can be accomplished via the device or remotely by a 0-5V signal from the FP TB-10. All the I/O seem to be behaving as expected except the tach mA, of course.

I've done this exact type of configuraton before with the exception that the tach output had been 0-5V rather than 4-20 mA. For this incarnation, I thought the 4-20 mA output of the tach module would be at least comparable in setup ease and performance as the 0-5V but I could then move away from the mixed bag of V/mA FP inputs (requiring an 8-ch AI-110) in favor of an all mA system (and the 16-ch AI-111). Maybe this was a mistake(?).

[Wayne.C]

My best guess, without seeing everything in detail is that the problem is that the common of the FP is already referenced to the common of the instrument through another connection.  You can verify with your meter.  It is a combination of the other I/O signals and the fact that you are using a single PS-5 to power everything.  When you connect the inside terminal of the BNC to the common of the AI-110, you are esentially bypassing the control circuitry in the the 4-20mA transmitter.  I've attached an image that shows a simplified view of what I think your 4-20mA transmitter looks like. 

My personal preference to fix this would be to install a 4-20mA loop isolator.  These are available from several sources.