Clocking gas meter: Measuring BTU of Natural Gas Fired Heating Equipment

Activate the unit being tested, making certain no other gas fired appliance is operating.

 

Clock the amount of time it takes for the smallest dial to make one complete revolution.

 

Using a natural gas meter timing chart (or Table XIII in the National Fuel Gas Code Book, NFPA-54, 1996), cross reference the time and appropriate dial size to determine the BTU input.

 

Check and compare the calculated input with the input rating on the heating unit data plate.

If the unit is under-fired or over-fired by more than 10%, check the gas pressure to the unit with a fluid filled manometer and adjust as necessary.

 

(For example, the unit being tested takes 29 seconds for the ½ cubic foot dial to make one complete revolution.  Using the chart, this translates to 62 cubic feet per hour.  Based upon the assumption that one cubic foot of natural gas has 1,000 BTU’s (Check with your local utility for actual BTU content), the calculated input is 62,000 BTU’s per hour.

 

While a ‘fixed orifice’ timing chart can be used on low pressure (residential) gas meters, high pressure (commercial/industrial) gas meters require that a correction factor be used to convert this reading into standard cubic feet (SCF) based upon the gas pressure at the meter.

 

 

Meter

Pressure

(W.C.”)

 

Correction

Factor

 

Meter

Pressure

(OSI)

 

Correction

Factor

 

Meter

Pressure

(PSI)

 

Correction

Factor

 

Meter

Pressure

(PSI)

 

Correction

Factor

 

Meter

Pressure

(PSI)

 

Correction

Factor

 

1

 

1.002

 

1

 

1.004

 

0.25

 

1.017

 

9

 

1.612

 

45

 

4.061

 

2

 

1.005

 

2

 

1.009

 

0.5

 

1.034

 

10

 

1.680

 

50

 

4.401

 

3

 

1.007

 

3

 

1.013

 

0.75

 

1.051

 

11

 

1.748

 

55

 

4.741

 

4

 

1.010

 

4

 

1.017

 

1

 

1.068

 

12

 

1.816

 

60

 

5.082

 

5

 

1.012

 

5

 

1.021

 

1.25

 

1.085

 

13

 

1.884

 

65

 

5.422

 

6

 

1.015

 

6

 

1.026

 

1.5

 

1.102

 

14

 

1.952

 

70

 

5.762

 

7

 

1.017

 

7

 

1.030

 

1.75

 

1.119

 

15

 

2.020

 

75

 

6.102

 

8

 

1.020

 

8

 

1.034

 

2

 

1.136

 

16

 

2.088

 

80

 

6.442

 

9

 

1.022

 

9

 

1.038

 

2.25

 

1.153

 

17

 

2.156

 

85

 

6.782

 

10

 

1.025

 

10

 

1.043

 

2.5

 

1.170

 

18

 

2.224

 

90

 

7.122

 

11

 

1.027

 

11

 

1.047

 

2.75

 

1.187

 

19

 

2.293

 

95

 

7.463

 

12

 

1.029

 

12

 

1.051

 

3

 

1.204

 

20

 

2.361

 

100

 

7.803

 

The reality of using this method to determine actual Btu input is that occasionally gas meters have been found to not be accurately measuring the quantity of gas passing through the meter.  Additionally, often meters are not located in close proximity to the boiler room which increases the amount of time and effort required to perform this procedure.

Another method of verifying proper burner input is simply by performing a combustion test and determining that the readings are within the acceptable ranges specified by the manufacturer.  If they are, the heating system is operating as designed.