The most common method of measuring gas is through an orifice meter. Gas flows through a piece of straight pipe with an orifice plate inserted in the middle.
The orifice plate is a steel circular plate with a hole in the middle; the hole is smaller than the internal diameter of the pipe. The plate is placed perpendicular to the gas flow, and is sealed so that all the gas flows through the hole. The hole is "tapered", meaning that the edge of the orifice hole is sharp. The plate must be inserted in such a way as to have the direction of flow of gas from the smaller to larger diameter, i.e. if the gas is flowing from left to right, the sharp edge of the orifice (the smaller diameter) must be at the left. Reversing the orifice plate will give an incorrect measurement.
The orifice plate is actually a highly machined component. Its dimensions must adhere to certain specifications. If the plate is damaged in any way in or around the "hole", it can no longer measure gas rates with any accuracy, and it must be replaced.
The plates come in different sizes for measuring different rates. Each time an orifice plate is changed, the size of the new plate must be recorded because the orifice size is needed for calculating flow rates.
In addition to knowing the size of the orifice plate, there are other parameters, which also must be known and reported. The relationship for calculating flow rate, using an orifice meter, is described by the orifice equation.
Orifice Metering of Natural Gas, Gas Measurement Committee Report #3, American Gas Association, 1992,2003
The Orifice meter calculation changes slightly depending on its physical configuration. FieldNotes™ supports the following configurations
The above equations are referred to as the "Factors Approach" as defined by appendix 3B in AGA 3 Part 3
Symbol 
Description 
Reference 



D 
Meter tube internal diameter, calculated at Tf (FieldNotes™ assumes that the meter tube is constructed from carbon steel) 
Input 
d 
Orifice plate bore diameter, calculated at Tf (FieldNotes™ assumes that the orifice plate is constructed from stainless steel) 
Input 
Tf 
Absolute flowing temperature 
Input 
Pfl 
Absolute flowing pressure 
Input 
hw 
Orifice differential pressure 
Input 
Ts 
Standard Temperature 
Input 
Ps 
Standard Pressure 
Input 
Qv 
Volume flow rate at standard conditions 
3B2 
C 
Composite orifice flow factor 
3B4 
Fn 
Numeric conversion factor 
3B5 
Fc 
Orifice calculation factor (Displayed as the Fb column in FieldNotes™) 
3B7 
Fst 
Orifice slope factor (Displayed as the Fr column in FieldNotes™) 
3B9 
Y1 
Expansion Factor (upstream tap) 
332 
Y2 
Expansion Factor (downstream tap) 
337 
Fpb 
Base pressure factor 
3B10 
Ftb 
Base temperature factor 
3B11 
Ftf 
Flowing temperature factor  3B12 
Fgr 
Specific gravity factor 
3B13 
Ftpv 
Supercompressibility factor 
3B14 
This example is taken from AGA 3 Part 3 Appendix 3C
Symbol  Input  Published  Ver 4.1  Ver 3.xx 
D 
8.071 inches  
d 
4.000 inches  
Tf 
65 F  
Pfl 
370 psi(a)  
hw 
50 inches  
Ts 
50 F  
Ps 
14.65 psi(a)  
Gr 
0.570  
N2 
1.10 %  
CO2 
0 %  
Qv 
608,394 SCF/min  608,405 SCF/min  608,623 SCF/min  
C 
4561.453  n/a  
Fn 
5581.82  5581.82  n/a  
Fc 
0.601767  0.601767  n/a  
Fst 
0.001189  0.001177  n/a  
Y1 
0.998383  0.998383  n/a  
Y2 
n/a  n/a  n/a  
Fpb 
1.000000  0.980757  n/a  
Ftb 
1.000000  1.000000  n/a  
Ftf 
0.995224  0.995226  n/a  
Fgr 
1.32453  1.324532  n/a  
Ftpv 
1.02423  1.024249  n/a 
Before the days of computers, engineers and operators used a complex system of tables to create an orifice coefficient for each size of orifice and various flowing conditions. FieldNotes™ does these calculations automatically, and the correct value of C is always applied to current calculation of gas flow rates. Some field operators use a "Sony Circular Slide Rule" or handed down spreadsheets to calculate gas flow rates. The answers obtained from this means are only approximate, and may differ from those calculated in the FieldNotes™ program.