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I bought a Dwyer 2004 (0-4" H2O) Magnehelic gauge on Ebay and an extra metal oil filler cap from the junkyard to make a "Don Foster" crankcase vacuum gauge.
The differential pressure gauge measures the difference between its two ports, labeled "High Pressure" and "Low Pressure". I was under the impression that I plugged the "High Pressure" port and connected my vacuum hose to the "Low Pressure" port via a Brass hose barb fitting to measure the crankcase vacuum.
However, the application engineer at Dwyer straightened me out on this point and said that the proper connection is to leave the "High Pressure" port open to atmosphere (usually thru a small filter to keep dust and crud out). That way the gauge measures the difference in pressure between the outside air and air inside the crankcase. He said that if the "high pressure" port is plugged instead of being left open, the confined air on the "high pressure" side of the diaphragm acts as an additional spring that severely changes the calibration of the gauge. The gauge would actually require much more vacuum to deflect the needle than the advertised reading.
Don, I'm just checking to see how you made the connections on the back of your gauge. If the "High Pressure" port is open then all the readings you have taken are correct. If the port is plugged then there is actually much higher vacuum inside a correctly running and ventilated crankcase than the gauge is reading.
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