Two ways... 1) Sub-metering at the point of entry gives you an accurate indication of overall consumption and 2) Sub-Metering at selected points within your facility provides a clearer picture of who, when and how much gas is being consumed at specific locations and/or work areas. This is the kind of information you need to analyze your ongoing expenses and to then refine your overall usage. 

Natural gas entering your facility is usually measured with a Positive Displacement (PD) flowmeter supplied by the Gas Company and metered in units of hundreds of cubic feet (CCF). 

What’s important for you to know is that although Natural Gas is measured in Cubic Feet, it’s sold in units of energy referred to as: Therms

To understand the concept of "Therms", you'll need to know a little about British Thermal Units or BTU's for short.  One (1) BTU is defined as the amount of energy required to raise the temperature of one pound of water, one degree (1°) at atmospheric pressure.

One Cubic Foot of natural gas has an average heating capacity of 1,025 BTU's.  This value differs slightly in different locations due to variations in the gas mix, but for our purposes, we'll stick with 1,025.  At that rate, a therm would consist of 97.56 Cubic Feet of Natural Gas or roughly 100 Cubic Feet when rounded up. (CCF... See the tie?) 

Gas Billing - (Finally, we get to the math...)  Your monthly bill is a product of net  consumption in cubic feet, multiplied by a factor representing the BTU content of the gas and then divided by 100,000.  Because the pressure at which your gas is delivered has a huge effect on it's density, this figure is adjusted one more time using a Density Factor. 
The

Example: Acme Cookie Company's gas meter shows a net consumption of 1,500.000 Cubic Feet of gas used last month to fire the cookie ovens. 

(1,500,000 x 1,025) / 100,000 = 15,375.00 Therms

A cubic foot of gas in this example contains about 1,025 BTU's and a Therm is equal to 100,000 BTU's. 

To correct for pressure, the following formula is used to create a factor to compensate for gas density: 

Example: 
(2.5 + 14.7) / 14.7 = 1.170 

In this example, we're assuming a line pressure of 2.5 psig at the meter. 

Your bill would now be figured as follows:

Gas enters your meter at 1/4 PSIG, but the density factor on your bill is based on a system pressure of 2.5 PSIG. Does it make a difference?  Let's check back with the Bean Counters at Acme Cookie Company, who are already steamed over the $17,988.75 gas bill they received last month.  That bill was based on an incoming pressure 2.5 PSIG, but in reality the pressure had dropped to 0.25 PSIG.  Let's do the math one more time... 

15,375.00 Therms @ 0.25 PSI

15,375.00 Therms @ 2.5 PSI

(0.25 + 14.7) / 14.7 = 1.01 (Density Factor)

(2.5 + 14.7) / 14.7 = 1.170

Can this type of billing 
error really happen? 

You bet it can and it happens every day. Pressure variations are inevitable in gas lines. Adjustments are made for new construction, lines are shut down for repairs... The list of reasons is endless! The Gas Companies do their best to correct for these inconsistencies, but without a more accurate way to measure the flow of gas into your facility, you're at their mercy at the end of each billing period.

How can you be sure you're only paying for your actual consumption?  The answer is simpler than you might think...

Gas measurement by mass rather than volume. Thermal Mass Flowmeters from EPI are temperature compensated and insensitive to changes in system pressure, so no additional calculations are required to obtain an accurate measurement of your gas consumption.  You just can’t do that with a volumetric flowmeter!

Thermal Mass Flow Technology...

Thermal mass flowmeters are solid state instruments that use the principle of convective heat transfer to directly measure mass flow.  EPI's sensors consist of two resistance temperature detectors (RTD's).  A forced null, Wheatstone bridge which preferentially heats  one RTD; while the other acts as the temperature reference.  The process gas flow dissipates heat from the first RTD, causing an increase in the power required to maintain a balance between the RTD's. This increase is directly proportional to the molecular rate of flow of the gas.