Differential Pressure Flow meter

BrianMarxCim

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Join Date
Apr 2016
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Valpo
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6
I have a DP Meter that works fine however if I change the temperature of the fluid then there are discrepancies. If I add a rtd to the line how do I calculate the flow based on the temperature?
 
Water Flow

Is the fluid a gas or a liquid? Are you looking for volumetric or mass flow rate? How do you know there are discrepancies?

Water, We filled a tank with 35 degree water and 95 degree water and had different volumes. I have found many references to this but I haven't found a good formula.
 
I have a DP Meter that works fine however if I change the temperature of the fluid then there are discrepancies. If I add a rtd to the line how do I calculate the flow based on the temperature?

Are you using an orifice plate? Venturi?


What type of instrument is mounted to flow device?

Any chance there is air getting caught in the sampling tube? Have you blown down/ cleaned the sampling tubes?

Is this a new installation or one that has been working for years and just now giving you problems?
 
The best reference for flow meter calculations is probably ASME PTC 19.5. https://www.asme.org/products/codes-standards/ptc-195-2004-flow-measurement-(1)

The constants etc. are different for pitot tubes, orifices, venturis, etc. Your flow meter supplier shouldbe able to provide you with the proper formulas and technical data. Like Lare, I am skeptical that the temperature difference is an issue unless you are looking at some very accurate measurements.

Here is a formula, though: http://www.engineeringtoolbox.com/fluid-density-temperature-pressure-d_309.html

dp flow.JPG
 
The best reference for flow meter calculations is probably ASME PTC 19.5. https://www.asme.org/products/codes-standards/ptc-195-2004-flow-measurement-(1)

The constants etc. are different for pitot tubes, orifices, venturis, etc. Your flow meter supplier shouldbe able to provide you with the proper formulas and technical data. Like Lare, I am skeptical that the temperature difference is an issue unless you are looking at some very accurate measurements.

Here is a formula, though: http://www.engineeringtoolbox.com/fluid-density-temperature-pressure-d_309.html

We are looking for very accurate numbers, Thanks for your help!
 
flow

Are you using an orifice plate? Venturi?


What type of instrument is mounted to flow device?

Any chance there is air getting caught in the sampling tube? Have you blown down/ cleaned the sampling tubes?

Is this a new installation or one that has been working for years and just now giving you problems?

We have created our own venturi, We are looking to get as accurate as is possible.
 
Most DP transmitters have at least two choices for the engineering units of the DP measurement:

DP_eng_units_wc.jpg


I strongly suspect that a Multivariable transmitter that measures the temperature of the water medium will make a dynamic compensation calculation that will adjust for the real temperature vs that declared eng unit temperature.
 
We have created our own venturi, We are looking to get as accurate as is possible.

On that case I assume that you know k (discharge coefficient) and alll others constatnts and math is on PLC

Assuming more and thinking for density is used 1 on calculation, all you need is add density variation to calculation.

p.s Finded this for venturi from technical equation book.

venturi.jpg
 
Last edited:
What level of accuracy are you needing? Our specific process requires very accurate measurement of liquids, and have found that DP flow meters can be off largely from small changes in the fluid. While water is usually consistent density there are fewer issues. For liquids that require a high degree of accuracy, we have moved to coriolis flow meters (mass flow). The downside....they are typically quite expensive....but man are they accurate!

For non-critical flows that use a DP, our engineers feed me some constants based on orifice size, standard temp, head, flows, DB, I cal "CAL" figures. so
P-CAL; T-Cal; H-CAL; F-CAL.

Calculates the Pressure- and Temperature-Compensated Flow
through an orifice from the inputs for HEAD (inches of water), PRESS (psig),
and TEMP (deg F).

The orifice calibration values for head, pressure, temperature, and flow are
entered as inputs H_CAL (inches of water), P_CAL (psig), T_CAL (deg F),
and F_CAL (any desired flow units). *)

VAR
P, PC, T, TC : REAL ;
END_VAR

(*Compute Uncompensated Flow*)
[FI] := [F_CAL] * ( [HEAD] / [H_CAL] ) ** 0.5 ;


(* Convert Pressures to psia *)
P := [PRESS] + 13.200 ;
PC := [P_CAL] + 14.696 ;

(* Convert Temperatures to Rankine *)
T := [TEMP] + 459.67 ;
TC := [T_CAL] + 459.67 ;

(* Orifice Calculation *)
[FLOW] := [F_CAL] * ( ( ([HEAD]/[H_CAL]) * (P/PC) * (TC/T) ) ** 0.5 ) * [MC] ;
 

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