|
|
|
The gas meter
Obviously, the gas meter is designed to determine how much gas is used for billing purposes. The meter is designed to be used with low pressure gas. Residential meters work with 7 inches water column. There are also 2 pound and 5 pound meters for commercial and industrial purposes. We will be describing the residential meter. The gas from the utility is delivered to the meter at various pressures usually around 20 pounds. This pressure must be reduced before it reaches the meter itself. So a regulator is mounted at the inlet to the meter.
As the gas passes thru the meter there are mechanical dials that rotate with the movement of the gas. The top 4 dials are for billing purposes.
As the gas passes thru the meter there are mechanical dials that rotate with the movement of the gas. The top 4 dials are for billing purposes.
Looking at the pic above, you can see on the top left, a 1,000,000 cu ft dial. The next is 100,000 Cu ft. Next is 10,000 cu ft. Next is 1,000 cu ft.
When reading the dials, start with the left dial. The left dial is between 5 and 6 so the number is 5. The next dial is between 0 and 1 so the number is 0. The next dial is between 7 and 8 so the number is 7. The last dial is above 3 about 2 points, so 320. The reading is 507320 cu ft. Divide this number by 100 and you get the number of therms (100,000 BTUs). 5073.2 therms.
The video below shows how this is done.
When reading the dials, start with the left dial. The left dial is between 5 and 6 so the number is 5. The next dial is between 0 and 1 so the number is 0. The next dial is between 7 and 8 so the number is 7. The last dial is above 3 about 2 points, so 320. The reading is 507320 cu ft. Divide this number by 100 and you get the number of therms (100,000 BTUs). 5073.2 therms.
The video below shows how this is done.
Using the gas meter to check firing rate
The gas meter is of use to the service tech because of its ability to determine the amount of gas that passes thru it. Most meters have 6 dials. The upper 4 are of no use to us. They are for billing purposes. The lower 2 dials are usually one cubic foot and 1/4 cubic foot. The one cubic foot dial is used to "clock the meter". When installing a new appliance or replacing a gas control, the gas input must be set according to the model plate to avoid over or under fire of the appliance. In most areas, the utility sets the amount of BTUs in gas at 1000 BTUs per cu ft but you should check with the utility to be sure.
Before beginning, be sure all other gas appliances are off. Fire off the unit to be tested. Once the unit has main ignition, you may begin the test. The procedure here is to determine the amount of time it takes to make a complete rotation of the 1 or 2 cu ft dials. The dials are located below the dials used for determining how much gas was used for billing purposes. The 1/2 or 1/4 cu ft dial moves too quickly to be used for determining gas input, so the 1 or 2 cu ft dial is used.
If you know the time for a rotation, the amount of gas moved in one rotation and how many BTUs are in the gas moved, you know the input.
As an example, if you are using the one cu ft dial, one cubic foot will be moved for each revolution. So if it takes 60 seconds for a rotation, one cu ft will be moved per minute. So, in 60 minutes it will rotate 60 times. If one cu ft of gas equals 1000 BTUs, 1000 X 60 = 60,000 BTU. Appliance ratings are in BTUs per hour. The video below gives an illustration of how to do this.
If you know the time for a rotation, the amount of gas moved in one rotation and how many BTUs are in the gas moved, you know the input.
As an example, if you are using the one cu ft dial, one cubic foot will be moved for each revolution. So if it takes 60 seconds for a rotation, one cu ft will be moved per minute. So, in 60 minutes it will rotate 60 times. If one cu ft of gas equals 1000 BTUs, 1000 X 60 = 60,000 BTU. Appliance ratings are in BTUs per hour. The video below gives an illustration of how to do this.