I think that a gauge being 100psi off is pretty common.
can they go bad? Yes. I've had one fail before on a dive.
check out this link to see how they work. You'll see they are just a simple mechanical device. If you have a small tolerence change in the mechanics, it can change the reading
When scuba diving, it is important for the diver to know the remaining pressure in the cylinder that supplies the breathing air. As a result, diving regulators are equipped with a submersible pressure gauge which reads the remaining pressure in the cylinder in pounds per square inch of pressure, or PSI. This is accomplished by connecting the mechanical submersible pressure gauge to the regulator with a high-pressure hose. For some, there is some mystery as to how the submersible pressure gauge works. We thought we would provide a couple of special pictures to allow you to easily understand how this device works.
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Submersible Pressure Gauge.....
The picture to the left shows a typical high-quality submersible pressure gauge. Pressure gauges used for scuba diving are special water-proof gauges, designed to prevent the intrusion of water into the gauge, even at the high ambient pressures experienced in diving. The gauge shown on the left is typical of the submersible pressure gauges used for scuba diving. This particular version is marketed by XS Scuba and is manufactured by a famous Italian gauge manufacturer. The construction of the gauge shown is all brass, with a flat glass face plate. The case is sealed water-tight and the front glass is installed with an o-ring to preserve its water-tight integrity. It is connected to the scuba regulator by way of a flexible high-pressure hose which transmits the cylinder pressure from the regulator to the inside of the gauge.
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The Guts of the SPG.....
This photo shows the same submersible pressure gauge with the gauge face removed. The housing of the pressure gauge is a sturdy vessel, constructed completely from brass. All internal components are constructed from brass and other corrosion-resistant materials. High-pressure air flows from the regulator through a rubber hose, attached to the threaded inlet at the bottom of the gauge. The pressurized air enters the gauge through an air-tight tube that bends around the left of the gauge. There is then a flexible coil, leading to the end of the tube at the top of the gauge. This tube is connected to a gear mechanism that rotates as the coil flexes with pressure. The center gear mechanism has a shaft in the center of the gear which rotates as the gear moves. The needle attached to the shaft deflects as the shaft turns. Careful calibration of the pressure tube to the dial face allows the actual pressure to be read directly from the gauge face.
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The Bourdon Tube.....
The picture at the left shows the pressure tube highlighted in red. This tube is known as a Bourdon Tube. Patented in 1849 by Eugene Bourdon in France, the bourdon tube type of gauge is considered the most accurate method of making portable measurements of pressure. A bourdon tube works very much like a limp balloon. As you blow into the balloon, the building pressure causes the balloon to deflect as it fills. Just like the balloon, when pressure flows into the bourdon tube, the tube attempts to straighten, causing a pulling pressure on the end of the tube. This pressure activates the center gear, rotating the shaft and deflecting the needle. When pressure is reduced, the tube relaxes and returns to its original position. It is important to note that the construction of the bourdon tube is rather exact and it always responds to a given pressure with a predictable amount of deflection, making the bourdon tube pressure gauge quite accurate. The Bourdon tube is constructed so that it can withstand thousands of increases and decreases in pressure without alteration of the predictable deflection of the tube.
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A Little Camera Trick.....
This picture shows a graduated gauge face super-imposed on the open pressure gauge with the bourdon tube highlighted red. This should allow you to get a really good idea of how this all fits together. While looking at the "final" assembly, it is important to make a couple of key points. First, many people are under the impression that the entire interior of the gauge is under pressure. This is incorrect. The ONLY place where there is high cylinder pressure in the gauge is inside the bourdon tube. This is a fairly hefty construction, so it is well designed to withstand the repeated increases and decreases in tank pressure. Second, the entire assembly is water-proof and tested to great depths to prevent water intrusion into the gears that operate the gauge. The glass face plate is sealed to the brass case using an o-ring to make this water tight. It is important to note that even if the interior of the gauge was to flood, the gauge would likely continue to operate properly, until corrosion damaged the works beyond function.
but point noted!
