Pressure compensation

Theboss

Member
Join Date
Jun 2010
Location
Durban
Posts
159
Good day folks,

When doing pressure compensation on a flowmeter to get mass flow. Should the flowmeter read the pressure input at gauge or absolute pressure?
 
I believe zero psia is one bar in gauge, which is about 14.2 psig. You would have to do conversions if your system gauges were reading in absolute, and the flowmeter was set for gauge. If the flowmeter has features to be able to measure in either, then use the units that are being used elsewhere. Just a thought.
 
I'm not sure which is the correct way to have this pressure in. It's a prowirl 200 by E&H.

Which is the best steam tables to use for saturation and superheated steam?
 
Last edited:
Absolute or gauge doesn't matter. If you're measuring absolute, you and tables are in gauge, just add one bar to your measurement. Or the opposite case, just remove one.

Are you doing the compensation in a PLC?
 
I believe zero psia is one bar in gauge

zero psia is about -1.01325 bar gauge depending on your altitude, temperature and prevailing weather conditions.

The calculations for conversion are based on atmospheric pressure not gauge, so you will be more accurate with absolute pressure sensors.

I didn't look up the manual for your meter to see if it accepts gauge and/or absolute.
 
For any thermodynamic calculation involving ratios you should use absolute pressure and absolute temperature. That includes flow meter compensation for any gas.

Be careful about just blindly adding 14.7 psi (1.013529 bar) to gauge pressure to get absolute. That only applies at sea level. Atmospheric pressure drops roughly 1 psi for every 2,000 feet elevation. Weather changes are less dramatic, and can be ignored for all but the most precise calculations.
 
When you set up the external input on the Prowirl, the zero (4 ma), span (20 ma), and engineering units are required setup values based on calibration of the external pressure instrument. The pressure units are selected from a drop-down list based on your country-specific setup. Assuming the flowmeter has the correct application package it will use this input for the desired mass flow computation.
 
The pressure transmitter that E&H supplies as a part of that vortex meter is an absolute pressure transmitter.
 
The pressure units are selected from a drop-down list based on your country-specific setup.

On second thought, yes, this is a problem. This selection, according to the last Prowirl I set up are, "bar" and "psi" leaving it ambiguous to whether absolute or gauge units. You may need to call E&H if the setup menu in the unit itself has not clarified this. I have good experience with their tech support.
 
How does atmospheric pressure have an influence on your steam supply coming from a boiler?
Because if where to use absolute pressure then we are saying that atmospheric pressure does have an influence.
The boiler is a closed system. If I'm not mistaken.
 

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