Vibrations can sometimes violently load Bourdon tube pressure gauges. They cause pointer flutter and in addition, in the event of sustained loading, harm to the measuring system, even to the stage of total failure. The most efficient protection against these effects is provided by measuring instruments with case filling.
The principle is easy: 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. As well, the measuring system is protected against premature wear, because the fill fluid acts as a lubricant for the moving components. This considerably extends the service life of the pressure gauge.
Figure 1: View of the within 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 a musical instrument with case filling, a pressure gauge with silicone-dampened movement is frequently chosen. In this design, the pointer pinion moves in a pot of highly viscous silicone oil. Due to this fact, the pointer also operates largely free from vibration. However, this effect, which is essential for immobilising the pointer, decreases 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 this information some time ago within 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 that have been harsher than those of the EN 837-1 pressure gauge standard. The investigation produced the next results:
Pressure gauge version (kind 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 ended up being relatively resistant. However, given No Strings Attached , 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 cases like this, countered by way of a rapid wear of another moving parts. This version was, already, no longer 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.
Explosive with case filling was the only variant which remained fully operational. As a result of fill fluid, the risk of leakage is frequently used as an argument against this type of instrument. WIKA cases with filling are therefore designed and handled to reduce the risk of leakage because of vibration to the very least.
Note
For more information on our range of pressure gauges, visit the WIKA website.
See also our article
Filling liquids in pressure gauges: Usage and advantages