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# Converting inH2O to SCFM

## Converting inH2O to SCFM

(OP)
What is the constant used to convert inH2O to SCFM?

I have a 4-20mA Diff Pressure transmitter calibrated to 0-100inH2O going into a PLC (scaled 0-4095).  The PLC needs to convert it to SCFM.

I'm told the formula is Constant*(Square root of the Diff Press)

But what is the constant? Is this something specific to that particular instrument, like with a mag-meter?

### RE: Converting inH2O to SCFM

It appears that you are trying to measure air or gas flow in a pipe or duct, using a differential pressure transmitter.

The "constant" that you are looking for is determined by a number of factors, such as the pipe diameter, and the fluid density.

Is the transmitter measuring across an orifice plate?

I would suggest that you get some help from your instrumentation sales rep, whoever you buy from.

If you provide all the details of your application, they should be able to calculate a constant for you.

### RE: Converting inH2O to SCFM

SCFM is "Standard Cubic Feet per Minute" and is used to relate flowrate to a standard pressure and temperature.

### RE: Converting inH2O to SCFM

(OP)
The instrument rep can figure it but as NWBeaver said he needs the pipe size, orifice plate info, etc.  Then they can develope the "constant" from that info.

### RE: Converting inH2O to SCFM

And, BTW, it is not a constant unless you square pressure difference. At least, that is so for an ideal gas. Other media may need some other exponent, but still close to 2.

Gunnar Englund
www.gke.org

### RE: Converting inH2O to SCFM

To get flow rate from a DP transmitter assumes that you have a primary flow element mounted in a pipe or duct, something like an orifice plate, an averaging pitot tube (Annubar), or one of several other specialized primary flow elements.

Primary flow elements are designed to produce a certain differential pressure at a certain flow rate as calculated from design information including medium viscosity, density, temperature, upstream static working pressure, and a dozen others.

The manufacturer of the primary flow element provides a "sizing sheet" with the primary flow element that includes all the data used to calculate the flow rate at a given DP.   You need that sizing sheet, because that sizing sheet tells you what flow rate 100"w.c. equals for your specific primary flow element.

And you need to know whether the DP transmitter is configured to output the 'raw' DP or whether it extracts the square root of the DP.

And you should be aware that you cannot get SCFM (standard cubic feet per minute, a mass flow measurement) from just a primary flow element and a DP transmitter, unless your static working pressure and temperature never vary from the design standard, which never happens in the real world.   'Standard' cubic per minute is referenced back to STP.   Your DP transmitter does not measure gas temperature and the static pressure and compensate for those changes, so your engineering units are CFM (a volumetric measurement), not SCFM (a mass flow measurement).

Dan

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