Vibrations will often violently load Bourdon tube pressure gauges. Tense cause pointer flutter and in addition, in the event of sustained loading, harm to the measuring system, even to the point of total failure. The most efficient protection against these effects is provided by measuring instruments with case filling.
The principle is simple: The pressure gauge case is filled up with a liquid, usually glycerine or silicone oil. In the case of vibrations, the case filling optimally dampens the vibrations of the Bourdon tube, transmission mechanism and movement. It therefore prevents pointer flutter, and therefore the displayed measured value remains clearly readable. Concurrently, the measuring system is protected against premature wear, since the fill fluid acts as a lubricant for the moving components. This considerably extends the service life of the pressure gauge.
Forbidden : View of the inside of a pressure gauge with a dampened movement. The circle marks the seat of the pot with the silicone oil that accommodates the pointer pinion.
Highly viscous silicone oil
As an alternative to an instrument with case filling, a pressure gauge with silicone-dampened movement is often chosen. In this design, the pointer pinion moves in a pot of highly viscous silicone oil. Consequently, the pointer also operates largely free of vibration. However, this effect, that is necessary for immobilising the pointer, slows down other moving the different parts of the movement. The result is really a significantly higher wear of these parts than with a pressure gauge with fill fluid.
WIKA confirmed these details some time ago in an internal laboratory test with different pressure gauge versions. Unfilled pressure gauges, pressure gauges with dampened movement and pressure gauges with case filling were put through an endurance test under practical conditions which were harsher than those of the EN 837-1 pressure gauge standard. The investigation produced the next results:
Pressure gauge version (type of dampening)
Zero point offset after 50 hrs / 200 hrs
Condition of the instruments after 200 hours
Unfilled / without dampening
2,3% / 3.0%
Conditionally functional
Dampened movement
2,3% / 66%
No longer functional
Liquid-filled
0,6% / 0,8%
Fully functional
Fast wear
In the test, the unfilled variant turned out to be relatively resistant. However, given the inevitable pointer flutter, such a pressure gauge is not recommended for applications with vibrations. This verdict also applies to the version with dampened movement, particularly in applications with stronger and sustained vibrations. The pointer stability is, in this instance, countered by way of a rapid wear of the other moving parts. This version was, already, no more functional well before the finish of the test.
Figure 2: Cracks in the Bourdon tube or a worn-out link are examples of typical vibration damage in the mechanisms of pressure gauges. Such wear is prevented by case filling.
The pressure gauge with case filling was the only real variant which remained fully operational. As a result of fill fluid, the risk of leakage is frequently used as an argument against this kind of instrument. WIKA cases with filling are therefore designed and handled to lessen the chance of leakage due to vibration to the very least.
Note
For additional information on our selection of pressure gauges, visit the WIKA website.
See also our article
Filling liquids in pressure gauges: Usage and advantages