We've built a manure digester and are trying to calculate the methane gas output. We measure the pressure drop across the orifice and then use an online calculator at Efunda.com to get the CFM. Our readings seem too low compared to the BTU's we're seeing in our boiler system. Is there another online calculator out there? Another type of meter? Any ideas?

Thanks...
Keith Wilson

 

I don't know what range of cfm you're looking to measure but I've had good luck with ABB Fisher Porter and Rosemont products in the 0-8500 cfm range.

 

You need to know quite a few variables about the gas to insure the calculations are accurate,
temp, density, composition and several others. There are many other online calculators out there, just do some research.

 

Keith:
Are you also compensating for the pressure sags and swells that come from digester gas.
I assume from what you have said above that you only are reading DP across the orfice plate, you may really need temperature and pressure compensation. The pressure swings can be effecting your readings.

Additionally Digester gas varies in quality, do you have any upstream mechanical filtering of the gas?

 

I would suggest you contact your local Micro Motion rep. They do gas very well and may be able to help you.

Cheers!!
R

 

Almost sure the problem is not the calculator but the sensor principle... let me know what flow and pressure are you tryng to measure.

Also, to avoid P and T compensation, I would suggest a mass flowmeter, maybe thermal in this case (since the pressure I guess is very low).

Daniel