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My colleagues and I are having somewhat of a debate on what flow units we should be using...
By Scott Barrus on 22 March, 2003 - 10:20 am

My colleagues and I are having somewhat of a debate on what flow units we should be using. We are using differential pressure transmitters to determine the flow of combustion air through a line. The temperature and pressure are being measured at the flow element and being inputted along with the DP meter to a PLC. The PLC then does a calculation to compensate for the actual pressure and temperature. The result of the calculation is then displayed on a screen as SCFM. It is my thought that the raw signal from the DP meter (square root is done at the DP meter) is SCFM based on the "standard temperature and pressure" that the flow element was tested at. These "standards" usually come from the data sheet that come with the flow element. Since the temp and pressure may vary, we measure them and continually compensate our flow measurement to determine the actual flow under the "non-standard" conditions and should express this flow as ACFM. Is this thought correct? Should the flow indication that is compensated using the actual(dynamic) temp and press be expressed as SCFM or ACFM??

I asked this question once and had difficulty putting the worms back into the can!!

You are making a measurement of differential pressure. This pressure will result from the actual conditions in the line at the time of measurement. If you want to solve the orifice plate equations on-line (and I can assure you, you probably don't) you could calculate the actual flow from the DP and operating conditions.

Your next choice is to use a "flow" as computed ahead of time using the design data. This is what is done for you if you are, for example, configuring a DCS with a full-scale flow value which corresponds to the nominal process conditions entered on to the orifice plate spec sheet. It should be relatively straightforward to correct for this based on actual pressure and temperature readings. I can't remember the details off the top of my head, but it's not too difficult -try thr square root of temperature and pressure ratios.

You then have to convert the result you are using to standard conditions - in the usual way.



This is something that I think alot of people take for granted. The actual true answer if you start really digging is that neither is appropriate without further calculation. Think about this and then go back and do a little/alot of research to the base calculations and you will see what I mean.

If there is interest, I almost made it thesis topic, but the stakes were pretty high. I have a full 14 page mathcad file with the proof that was the derivation of the intial ASME flow calc through the theoretical development to the final accepted equations which have small approximations.

In short the display my current industry practices should be in SCFM.

In long, as if this reply is not long enough, the "true" solution would be an interative calculation.

Like I said before, you might answer alot of small process questions if you truly prove all of your meters, especially if you are working with a excess O2 calc/NOx / CO / Boiler efficiency/ etc.

Good luck

By David W Spitzer on 23 March, 2003 - 1:24 pm


The differential pressure transmitter measures the flow in scfm or acfm (see your flow element calculations) based upon the operating pressure and temperature that were used in the flow element calculations. Flow element testing may (and likely was) performed at other conditions. The units of compensated flow are typically the same as in the calculations... but they can be different (depending upon the application).

The information below begs the question of whether the PLC calculations are correct. The net effect of the calculations is to pressure and temperature compensate before taking the square root.

You may want to refer to Chapter 4 and 10 of my book "Industrial Flow Measurement" (available at for more information.

You may contact me off-line if you need further help.

David W Spitzer
845.623.1830 (phone/fax)

PS - "The Consumer Guide to Differential Pressure Flow Transmitters" will be available to ship within the next week or so. While this does not address flow computation issues, you may find it useful in analyzing the flow errors introduced by your transmitter. It can be ordered now at

By Jonas Berge on 19 April, 2003 - 10:06 am

Standard (a.k.a. normal or base condition) is AFTER the compensation. Before the compensation is referred to as flowing conditions.

Jonas Berge