Measurement of Pressure with the Manometer
Pressure is defined as a force per unit area - and the most accurate way to measure low air pressure is to balance a column of liquid of known weight against it and measure the height of the liquid column so balanced. The units of measure commonly used are inches of mercury (in. Hg), using mercury as the fluid and inches of water (in. w.c.), using water or oil as the fluid.
Fig. 2-1. In its simplest form the manometer is a U-tube about half filled with liquid. With both ends of the tube open, the liquid is at the same height in each leg.
Note how the needle on that Mag is not resting on zero. Mags must be place level then check the calibration and adjust with that little screw up front. I've seen many Mags still not able to hold calibration.
When checking calibration, you should check on both the high end and low end for the range of that instrument. What I had my regional service depts do was check a gas appliance at maxium turn down and measure against a water manometer, then go to a medium pressure regulator turned down to 1/2 psi and test there. We allowed a variance of 0.02 wci plus or minus, which was the deviation given to us by our mfrs. VP of engineering. We placed tape on the back of each manometer and wrote the correction factor on it. That way if a tech borrowed someone else's truck, he had a reasonable chance of accuracy. Each manometer was numbered and a log kept. We held monthly calibrations and documented them. Did the same with our CO and LEL sniffers (bump test monthly/ annual calibration). When conducting you monthly calibrations, don't forget to inspect every inch of hose and replace as soon as you see cracks or damage. Cut off those ends that stretch out and don't recoil back or get stiff. Be careful with various types of tubing, esp. vinyl as it moves a lot with temp. changes. You can bring in a manometer from a freezing truck and attempt to use it immediately, then get noticeably different readings 30 min. later. Always keep your manometers warm before use. Watch for kinked hoses. When attaching the hose, don't pinch the hose and press onto the barbed adapter and release--you'll pervert the readings. Instead, hold it at the end with the tubing open and twist it on while watching the gauge. You should always zero the manometer seconds before attaching. If it changes more than 0.02 wci during attachment, remove it and do it again. Sometimes its easier to attach the loose hose to the appliance test port first then to your manometer.