PLC value different from Flow Meter

Hello,

Why do I have two different readings between transmitter and PLC

We have a flow meter sending 4-20 HART to PLC AI card attached with configuration shown in attachment.

The meter display is showing about 900 scfm, but the online live PLC value is showing about 1600 scfm.

Configuration is with a 250 ohm resistor on the AI inputs terminal block.

The loop current matches the PLC value not the meter display.

Is it possible the PLC AI card is not setup for 4-20mA HART?

When I send a 4-20mA only signal to the PLC, PLC matches.

Thank you

Check how the flow transmitter is set up.
Does it match how you have set up your analog input card. ( 0-15000)

4ma = ??
20ma = ??

Why do you have 0-5V selected on the input card config? Why not just use 4-20mA?

goodoboy said:

Configuration is with a 250 ohm resistor on the AI inputs terminal block.

Click to expand...

I suppose you use an external resistor to convert current to voltage. In the manual, page 60:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf

you can see that each analog channel has a 249 ohms resistor. Usually, you need to put a jumper between RTN and iRTN-x (where x is the channel number) to get the channel working. If you are doing that and adding an external resistor, you'll get a different reading. Take a decision: use the internal resistor or the external one.

As @AGill wrote, set the channel to current.

Is the transmitter set up to scale to 0-15,000cfm = 4-20mA as well as the PLC?

Is this the same flow meter as your other post?

ASF said:

Is the transmitter set up to scale to 0-15,000cfm = 4-20mA as well as the PLC?

Is this the same flow meter as your other post?

Click to expand...

Thank you ASF,

Yes transmitter is scaled 0-15000 scfm. Yes, the same flow meter as my other post.

This problem got me stomped and drained today.

AGill said:

Why do you have 0-5V selected on the input card config? Why not just use 4-20mA?

Click to expand...

Thank you AGill,

I do not understand why this is showing 0-5V as input. A PLC contractor sold this to us this way. All the channels are setup this way.

When we unhooked the transmitter and send a 4-20mA value to PLC AI card channel, we see the correct corresponding flow value that match that mA sent.

I am at home now, is it possible to set the card input to 4-20mA. I am not sure that is an option.

ASF said:

Is the transmitter set up to scale to 0-15,000cfm = 4-20mA as well as the PLC?

Is this the same flow meter as your other post?

Click to expand...

The PLC is setup for input range 0-5V, and scaled 1V=0scfm, and 5V =15000scfm.

nhatsen said:

I suppose you use an external resistor to convert current to voltage. In the manual, page 60:

http://literature.rockwellautomation.com/idc/groups/literature/documents/um/1756-um009_-en-p.pdf

you can see that each analog channel has a 249 ohms resistor. Usually, you need to put a jumper between RTN and iRTN-x (where x is the channel number) to get the channel working. If you are doing that and adding an external resistor, you'll get a different reading. Take a decision: use the internal resistor or the external one.

As @AGill wrote, set the channel to current.

Click to expand...

Thank you,

I am confused after looking at page 60.

Why do we have an external 250 ohm resistor for our 4-20mA input signals? If the AI card already have an internal 250 ohm resistor.

nhatsen said:

As @AGill wrote, set the channel to current.

Click to expand...

Thanks,
Will the PLC AI card read 4-20mA HART as well or just 4-20mA?

I think the transmitter is setup to output 4-20mA HART and the HART (digital) causing incorrect readings on the PLC AI channel.

goodoboy said:

Thanks,
Will the PLC AI card read 4-20mA HART as well or just 4-20mA?

I think the transmitter is setup to output 4-20mA HART and the HART (digital) causing incorrect readings on the PLC AI channel.

Click to expand...

"HART" will not cause an incorrect reading. If the card is not a "HART" card the "HART" portion of the signal will be ignored. It has no effect on the 4-20ma.

1.

goodoboy said:

I do not understand why this is showing 0-5V as input.

Click to expand...

The flow meter's 4-20mA is converted to a 1.0 to 5.0 Vdc signal by the IR drop created when the current goes through the external 250 ohm resistor mounted on the terminal blocks.

That voltage is then fed to each of the AI cards.

The setup is correct. I suspect that the choice of input ranges on the AI card is 0-5Vdc, and that there is not a 1-5Vdc option.

The scaling, Low Signal 1.0V = 0.0 Eng units and High Signal 5.0V = 15000 eng units, is correct because the useable range of this signal is 1-5Vdc.

This is a perfectly legitimate way to split a current signal, by making the current signal into a voltage and using the voltage signals.

The AI cards in this case DO NOT SEE a current signal, they 'see' a voltage signal, so the use of another current dropping resistor will only create a problem, not solve a problem.

2.

goodoboy said:

A PLC contractor sold this to us this way. All the channels are setup this way.

Click to expand...

As far as I can tell, the contractor did it right. Leave well enough alone.

3)

goodoboy said:

I am at home now, is it possible to set the card input to 4-20mA. I am not sure that is an option

Click to expand...

It is not an option. If you change the configuration for current inputs, then you will need to change the field wiring to match, which means re-wiring the current signal in series through the two AI's.

Given that you do not understand Ohms Law, the difference between current and voltage signals, or series/parallel splitting, you're best advised to leave well enough alone.


4) The commment made about voltages "always being flakey" is not true.

AI inputs are ALWAYS voltage inputs.

Signals that are not voltage signals have to be made into voltage signals to be 'read'. That's the reason for the dropping resistor that's required for current inputs - the resistor converts the current to a voltage signal that the AI can use.

Sometimes that dropping resistor is internal, sometimes it is external (like in this case).

understanding on Ohms Law make trouble shooting become easy.

danw said:

1.

The flow meter's 4-20mA is converted to a 1.0 to 5.0 Vdc signal by the IR drop created when the current goes through the external 250 ohm resistor mounted on the terminal blocks.

That voltage is then fed to each of the AI cards.

The setup is correct. I suspect that the choice of input ranges on the AI card is 0-5Vdc, and that there is not a 1-5Vdc option.

The scaling, Low Signal 1.0V = 0.0 Eng units and High Signal 5.0V = 15000 eng units, is correct because the useable range of this signal is 1-5Vdc.

This is a perfectly legitimate way to split a current signal, by making the current signal into a voltage and using the voltage signals.

The AI cards in this case DO NOT SEE a current signal, they 'see' a voltage signal, so the use of another current dropping resistor will only create a problem, not solve a problem.

2.

As far as I can tell, the contractor did it right. Leave well enough alone.

3) It is not an option. If you change the configuration for current inputs, then you will need to change the field wiring to match, which means re-wiring the current signal in series through the two AI's.

Given that you do not understand Ohms Law, the difference between current and voltage signals, or series/parallel splitting, you're best advised to leave well enough alone.


4) The commment made about voltages "always being flakey" is not true.

AI inputs are ALWAYS voltage inputs.

Signals that are not voltage signals have to be made into voltage signals to be 'read'. That's the reason for the dropping resistor that's required for current inputs - the resistor converts the current to a voltage signal that the AI can use.

Sometimes that dropping resistor is internal, sometimes it is external (like in this case).

Click to expand...

Thank you for the detail response. This really help me . Thank you.

We still have the issue and I believe the problem is with the flow meter sending the wrong 4 -20mA then what is shown on the meter display. Maybe the Digital to Anolog converter in the meter is bad.

Maybe you need to check the internal setup of the flowmeter using a HART communicator. You can execute a loop test as well (forcing 0-25-50-75-100% of the range).