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Anonymous Poster #1

Flow Transmitter

06/12/2016 8:59 AM

Using Hart com 475. I simulated 4-20ma for flow transmitter to check output reading in DCS. From field either transmitter process variables function set in linear or sqrt , ( DCS config in Linear) output still linear. Also, after zero sensor trim field value shows ''0%''equivalent to 4ma but in HMI shows 2% equivalent to 4.20ma. Pls help. (transmitter is not online)

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#1

Re: Flow Transmitter

06/12/2016 3:57 PM

This is unclear. Are you saying the value is wrong at the DCS/HMI end with a simulated 4-20mA signal in the field? If so, alter the scaling at the DCS or HMI end to get the required measured value.

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#2

Re: Flow Transmitter

06/12/2016 7:18 PM

1) >From field either transmitter process variables function set in linear or sqrt , ( DCS config in Linear) output still linear.

So? What did you expect?

Stop and think about it.

If the transmitter's transfer function is configured for linear, the transmitter's output spans from 4.0mA to 20.0mA.

If the transmitter's transfer function is configured for square root, the transmitter's output spans from 4.0mA to 20.0mA.

What changes with the transfer function is
- how which percentage of differential pressure creates a given mA value.
- how the receiver (DCS in this case) interprets what the mA values mean. The receiver's interpretation has to match what's coming out of the transmitter.

When you put the communicator in simulation mode, you are forcing the transmitter to output a 4-20mA output value without respect for what the transmitter 'sees' on its physical ports as a differential pressure.

You are not telling the communicator to "force an output equivalent to 57L/min of flow." Rather, you are are telling the communicator "force an output of 12.00mA". So you get 12mA.

The communicator simulation function sounds perfectly functional to me.

2) I am as confused as Jack of all Trades about the 2nd point.

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#3

Re: Flow Transmitter

06/20/2016 5:12 PM

Many flow transmitters have a "square root" variation with flow. Often a linear pressure transducer is across an orifice.

Check the data sheet for your transmitter.

If you have set the HMI for "linear" when "sqrt" is needed, you will not get a correct display value.

Also, if the input amplifier or "Analog to Digital" converter of the DCS is not perfect, then the system may not display zero for 4 mA input. There might be a zero adjust for each channel.

If the DCS displays 0 with 4 mA input from a simulator to DCS terminals, but 2% with 4 mA out of transducer located some length of cable remote from DCS, you might have an interference problem.

Also, if each current is converted to a voltage before measurement by the DCS, a high resistence in the common voltage bus [which has to carry all the transducer currents back to the power source] can cause mixing of the signals.

Supposing you are injecting at DCS, is your simulator connected to the same "common" point as the transducer cable?

If you connect your simulator in place of the flow transmitter at the field end of the cable, does it give the same 2% DCS display "error" for 4 mA as you get when the transmitter outputs 4mA?

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