by Marc Ward
Clay Times Aug. 1996
Q: The guy from the gas company told me my pressure is 7 inches of water column, but I can only get a reading of 4 inches on my water column gauge. Why is this?
A; There's a dirty little secret at work here. Some of the time gauges don't tell you anything that means much. Yea, it means something to you...."at eight hours into the firing , I turn the burners up to 3", but that is a number that may not have much of a relationship to the actual pressure on the line.
There are three types of pressure readings that are commonly used. They are: gauge or differential pressure, pressure drop, and manifold pressure. Now, depending on where your gauge is located on the line and what the situation is, you'll get totally different readings. This is especially true with low pressure. Low pressure is anything under 1 PSI. There are 27.7 inches of water column pressure in 1 PSI (pounds per sq. inch). Natural gas is usually delivered at 7" wc (water column) and household LP (propane) is usually delivered at 11" wc. When I say delivered, I'm speaking of manifold pressure. That's the pressure on the line when nothing is running. It's the pressure the propane regulator or natural gas meter is set to deliver. If you were to have a gauge on the line after the tank/meter but before the valve to the burner, it would be reading the pressure on the line while the burner was off. This is manifold pressure. You need to know this pressure. This is the pressure that all the math dealing with BTU output is based upon. Now, what happens to this gauge in this location when you turn the burner on? The pressure will drop because it is escaping via the orifice. The bigger the orifice, the more it will drop. This is called a pressure drop reading.
If the gauge is between the valve used to turn the burner up and the burner itself, you'll get another totally different reading. This is gauge or differential pressure and the most common placement. If you have a large orifice like the ones in a forced air burner and are using low pressure, the gauge won't read very high. There is not much gas "backed up" between the orifice and the rest of the pipe. This is where the gauge is located. A bigger orifice; less reading on the gauge. A smaller orifice; more reading on the gauge. All these different readings, but your manifold or regulated pressure is constant.
As you move higher in pressure these inconsistencies become less and less. You have much smaller orifices with high pressure so that the effect caused by a smaller hole (orifice) in the end of a pipe doesn't change the internal pressure as much. If you have 25 pounds of pressure (manifold or regulated pressure) and a small orifice, then a gauge located between the burner and the valve will read pretty close to 25PSI.
Gauges are a reference tool so you can repeat a firing. Depending on the pressure that the regulator/meter delivers and where the gauge is located, they may not tell you the actual (manifold) pressure you are using. If your unsure of what your manifold pressure is, ask your gas supplier what the regulator on your propane tank delivers or ask the gas utility what the delivered pressure is on you natural gas service.