hi,

in using an orifice plate with DP to measure flow of gases, if the square root is not done at the DP transmitter, instead linear relationship is used, would it cause serious error in measurement and hence control? in several cases i have seen, its not doing any harm!

 

Facts:

1) double square rooting, at both the transmitter and receiver produces substantial flow rate error.

2) Failure to square root a DP signal at either the transmitter or receiver produces substantial flow rate error.

My opinion is that

1) it matters not whether the square is extracted at the transmitter or the receiver in terms of 'accuracy'.

2) if the site routinely does wet calibrations (non-loop, individual device), applying a known pressure, then doing the square root extraction at the receiver allows the local transmitter's display and 4-20mA to reflect pressure values, which is convenient for the tech doing the calibration.

 

If you do DP flow you have to do square root somewhere or there will be an error

It is better to do the square root in the transmitter because square root extraction has large gain as you get closer to zero. This is why you have zero-cut off (hard or linear). For example, if you are measuring at 4% of range and have a 0.1% mismatch in the current calibration between the transmitter and the system, this error will be amplified to 0.25% if the square root is extracted in the system. This is a problem with analog signals. Digital communication is better.

Another advantage of square root in the device is that you can scale the local display to indicate the flow reading instead of DP

Cheers,
Jonas

 

thank you david and jonas,i am measuring steam flow rate with orifice & dp.

i have 3 options with the transmitter-

1. use square root extracting and linear up to the application point

2. square root extracting and deactivated up to the application point

3. 2 step linear up to the application point

which one should i use and what should be the application point?
any help would be helpful! any suggestive answer is also welcome. reference to literature is also welcome,any website, book.. well, somebody could tell it right away

 

The square root extraction should be done AFTER the density (pressure & temperature) compensation is applied to the differential pressure measurement. If you look at the flow equation, you will see that this is under the radical with the differential pressure. So unless you have a smart transmitter which also includes the pressure and temperature measurements as well as the differential pressure, it is better to do the calculation in the controller (DCS?) than at the transmitter.

 

Hi

Can you please explain the method to calculate flow from the following Known parameters

pipe Size, differential Pressure, Orifice details, Pressure, Temperature

Thanks

 

thank you otised,
the transmitter concerned didn’t have the measurement for temperature. neither did it transmit pressure. i could use separate tt and an additional pt with relevant changes in the plc code but frankly speaking, i haven’t needed that much accuracy till now

 

Hello,
I would start by referring to the AGA Report 3, 1991.