Flow Transmitter Settings

[ChemGuy2016]

Hi everyone,

I am new to instrumentation, so I'm hoping you can help me out. I'm wondering about setting the maximum flow rate of a flow meter during configuration. If I enter an incorrect flow range, will my readings be incorrect? It seems to me that the flow meter should always output the correct flow rate, regardless of the upper bound...Of course not expected to output values higher than the maximum flow rate entered. But if I enter a maximum flow rate much higher than what the actual maximum flow rate is, does that cause any errors in the reading?

I'd really appreciate your experience responses. Thank you all.

 

[danw2]

I assume you're setting the maximum flow rate because that's what 20.0mA on the 4-20mA output will represent (digital protocols, Profibus, Foundation Fieldbux, have no maximum scale, the value the meter reads is the value the receiver gets).

No, changing the maximum scale value (what the 20.0mA value represents) does not create an incorrect flow rate value. Any given flow rate value will be a different mA value depending on what the maximum scale value is, but it will not be incorrect. So, yes the output will be the 'correct' flow rate.

A very common problem though is a mismatch between what the flow meter's 4-20mA output represents and what the receiver device is configured to interpret that value as. Caveat Emptor.

 

[ChemGuy2016]

danw2 can you please rephrase your response? I am having trouble understanding your explanation.

I also guessed that it had to do with the 4-20mA signal, but the flow rate measurement on the unit does not depend on the transmission signal. That is raw, empirical data based on changes in the magnetic field.

Lets use real numbers. Say my pump can produce a maximum flow rate of 1000 l/hr. I set my flow meter range to 0-1000 l/hr. My readings will be correct. But lets suppose I set the flow meter range to 0-2000 l/hr. The flow meter should still measure 1000 l/hr at 100% pump speed based on the operating principle. Will it locally indicate 1000 l/hr on the unit? And will the output signal still read 1000 l/hr on the PLC, or will it send 2000 l/hr because that is the 20mA value? What if I set my range to 0-500 l/hr? What would be the case then?

 

[zdas04]

What kind of flow meter do you have that reports flow? All of them I know of transmit some parameter (e.g. pressure, temp, and dP for a differential producer, speed for a turbine, frequency for a Coriollis, etc) and the RTU or PLC converts that primitive data to flow. For pressure and differential pressure instruments you input the value that you've calibrated to be 20 mA (e.g., if your pressure range is 0-100 psig, then 4 mA is zero and 20 mA is 100 psig). The key is that the calibrated range is the same both in the instrument and the RTU (I don't know how many times a pressure instrument has been calibrated 0-500 psig and the RTU had 0-50 psig or vice versa, making a mess of flow measurement). Turbine meters have a velocity (provided by the manufacturer) that going above result in very bad correlation to flow rate, input that number as 20 mA.

[bold]David Simpson, PE[/bold]
MuleShoe Engineering

In questions of science, the authority of a thousand is not worth the humble reasoning of a single individual. Galileo Galilei, Italian Physicist

 

[ChemGuy2016]

My mistake. You are right; the flow meter itself does not report the flow. It is transmitted to a local indicator. I am using an electromagnetic flow meter.

 

[ChemGuy2016]

So bottom line, if the flow range is set incorrectly then I will get an incorrect reading. Yes?

 

[danw2]

>But lets suppose I set the flow meter range to 0-2000 l/hr. The flow meter should still measure 1000 l/hr at 100% pump speed based on the operating principle.
answer for an avoided question: At 1000l/hr, the flow meter will output 20.0mA because the 'range' is 0-1000 l/hr.

>Will it locally indicate 1000 l/hr on the unit?
answer: yes, the flow meter will locally indicate the correct 1000/hr flow rate.
The 4-20mA output will be 12.0mA for 1000 l/hr with flowmeter range of 0-2000 l/hr.

>And will the output signal still read 1000 l/hr on the PLC?
answer: It depends on what the PLC is configured for to interpret the range of 4-20mA.
If the PLC is configured to interpret 4-20mA as 0-2000 l/hr, then yes, the PLC reading will match the flow meter reading because both agree on what 4-20mA represents.

>or will it send 2000 l/hr because that is the 20mA value?
answer: The flow meter will not send 20.0mA if the flowmeter's range is 0-2000 l/hr and the flow rate is 1000 l/hr.

>What if I set my range to 0-500 l/hr? What would be the case then?
answer: 1000 l/hr flow rate, or any flow rate over 500 l/hr, will peg the output at 20mA because a flow rate higher than the flow meter's maximum 'range' is overscale.

>So bottom line, if the flow range is set incorrectly then I will get an incorrect reading. Yes?
answer: Apparently the indicator is the 'receiver'. The indicator's scale/range has to match the flowmeter's scale range. If those scale/ranges do not match then you get an incorrect reading.

The 4-20mA has to represent some flow range. The flowmeter's range has to match the receiver's range. And the flow rate has to be within the 'range' to be a viable value.

 

[bimr]

Wouldn't a visit to the flow meter instruction manual provide you the information that you are looking for?

 

[danw2]

The manual will not explain the basics of matching the range on a remote indicator/receiver with the output range of the flow meter. That's outside the scope of what a flowmeter manual covers. The assumption is that stuff is understood by whomever is dealing with the instrumentation.

I suspect the OP is changing the range on the flow meter that has a relatively constant flow rate and then when he sees the remote indicator's values changing drastically because the indicator range has not been changed to match the flowmeter's range.

 

[Compositepro]

All instruments have an accuracy rating that is based on percent of full scale. So doubling the full scale output will double the error on an absolute scale. So if you have a 1% of full scale accuracy on a 10 gpm meter your acurracy at any reading is +/- 0.1 gpm. If you change the full scale calibration to 20 gpm, the accuracy of any reading is +/- 0.2 gpm.

So, yes it is important to choose an instrument range that is matched fairly closely to the measurement range.

 

[ChemGuy2016]

Thanks for the responses. I understand the measurement principle now.
Is there a general rule of thumb of what the maximum flow range should be? I'm thinking some percentage over the expected maximum flow rate. 10%, 20%?...Just to make sure that it can still read if you slightly over the maximum expected flow rate.

 

[jari001]

Are you expecting some high flow rate associated with an infrequent operation (cleaning, switch over, etc) that is outside the normal operating range? As Compositepro mentioned, there will be a trade off on how much "extra" you allow in the measurement range and the amount of error on the measurement; that's usually the limiting factor, minimizing error in the normal operating range, and thus there's no generally applicable rule of thumb I've found or been told because it needs to be assessed for each application.

 

[ChemGuy2016]

Got it. But I'm guessing you'd still want to leave a bit of extra room above your maximum calculated flow rate to account for natural variation in flow conditions. Don't want to max out the flow meter at 20.000 gpm and not be able to read 21 gpm.