Many manometers and all Magnehelic gauges (to my knowledge) have two ports so that you can read the differential pressure all at once. For example, if the return static is -0.3″WC and the supply static is +0.2″WC, the total static is 0.5” WC. By calculating the differential, you come up with the total external static in inches of water column. We use a manometer or a Magnehelic and measure the negative air pressure in the system return side before the blower (and after the filter whenever possible) and the positive pressure supply air side directly after the blower. Static pressure is the pressure exerted in all directions in a contained space it is not the directional force of the air. We also use “WC to check air static pressure on systems. This varies based on manufacture specs, combustion analysis, and meter clocking tests. Gas pressure is usually measured in “WC, so we most commonly set single-stage appliances to 3.5″WC on natural gas and 11″WC on propane. At altitudes over 2000′ above sea level, you will also need to follow manufacturer recommendations to adjust the gas valve and even change orifice sizes, in some cases, due to the effect the lower atmospheric pressure has on the gas. That means that for a manometer or Magnehelic to be properly used, you MUST recalibrate them before each use (many auto-calibrate to zero) to compensate for changes in elevation and barometric pressure. When we measure inches of water column with our tools, we calibrate it at atmospheric pressure or the gauge scale instead of the absolute scale. These low pressures are most often read using a manometer or a Magnehelic gauge. One PSI is equal to 27.71 inches of water column this is why water column is most often used to measure pressures under 1 psi. While some water manometers (water tubes) are still in use, the vast majority are either dial or digital gauges that still use the same scale. Like most units of measure, it has a very simple origin in a water manometer, 1″ of water column is literally the amount of force it takes to raise the column of water by 1”. Low pressures are often measured in inches of water column or “WC.
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