Pressure gauge calibration is one of the simple field in instrument calibration. It has a simple set up where you can easily execute the calibration. But pressure gauge is also one of the instruments where errors are mostly encountered.
Most errors are either it is out of tolerance or the needle is out of the zero settings. When this happens, do not immediately throw or discard the gauges. Luckily, there are still ways to adjust or repair them.
In this post, I will share to you 5 ways of adjusting a pressure gauge, how to identify them and execute the adjustment. Pressure gauges have so many applications and therefore have also many construction designs. I will present in this post the different set ups depending on its construction.
Below are some reasons why pressure gauges are getting out of whack (out of tolerance or not in the zero settings):
- Changes in the altitude and temperature (due to changes in environmental factors).
- Exposure to over pressure where the bourdon tube is over stressed.
- Overused or over exposed to pressure cycle
- Exposure to high temperatures and excessive vibrations
Before we execute the gauge adjustment or repair, below are some considerations that you should determine in order to save time and effort.
- A reading near its full-scale value is shifted to more than 10%( a span shift)
- Pressure gauges that have a zero shifted to more than 25 % (a zero shift)
- Gauges with signs of leakage and corrosion of its body.
- Gauges with a needle showing error due to excessive friction or wear on its movement (loose)
- With damage sockets and threads.
You can see more in this article from ASHCROFT
5 Techniques to Adjust or Repair a Pressure Gauge
- Opening a vent to release pressure build up
- Opening the Glass (or plastic cover) then removing the needle.
- Adjusting the screw located behind inside the needle mechanism
- Adjusting the knob at the lower side of the pressure gauge
- Adjusting the screw located at the gauge needle pointer itself.
Be reminded that above techniques depend on the construction or design of the pressure gauges.
1. Opening a vent to release pressure build up
This is the simplest technique. Due to changes in temperature and altitude, a pressure inside the pressure gauge will be formed and affect the needle settings to zero. You can correct this by just opening the vent to release the pressure.
Do not forget to return or re-seal the vent. The oil may leak and may damage the gauge.
2. Opening the Pressure Gauge Protective Glass (or plastic cover) then removing the needle.
For a non-oil filled gauges and it is not permanently sealed (most oil filled gauges are permanently sealed where adjustment or repair is not possible).
Remove the glass cover (some are plastic) and remove the needle pointer.
( see at the bottom of the page the tool to remove the cover and needle easily and safely).
To correct the Zero Shifts:
Once the cover is removed, you can now pull the needle. Once the needle is removed, position it on the zero scale then push to return. Verify the reading.
To correct Span Shift:
Once the needle is removed, connect the pressure gauge to a known pressure source (the Electronic Deadweight Tester or Fluke 754 with set of pressure modules) then generate pressure of about 50% of its Full scale. Once the pressure is set, return the needle on the same value as the supplied pressure. For example, generate a 100 psi pressure from the reference standard, then fix the needle on the 100 psi scale reading on the pressure gauge.
Most of the time, returning the needle on the zero position solves this problem.
3. Adjusting the knob at the lower side of the pressure gauge
Some pressure gauges has an adjustable knob in the lower side of the body. This is a diaphragm type pressure gauge that is usually used in water treatment pipings.
Just remove the lock then the knob will freely turn to adjust the needle.
4. Adjusting the screw located inside the pressure gauge mechanism
Some have an adjustable screw inside the body, at the back of the needle. This type ot gauges are used in gas lines.
Access the screw inside the gauge by removing the cover. Turn the screw and return the needle to zero position.
5. Adjusting the screw located at the gauge needle pointer itself
Some has an adjustable screw integrated on the needle itself .Once the cover is removed, use a screw driver and carefully turn the screw to position the needle. See figure above.
Some useful tools.
- Ashcroft 3220 Hand Jack Set Gauge Needle Puller. You can use this tool to remove the needle safely and effectively:
click here to check at Amazon
In removing the needle, be careful not to pull the needle so hard to avoid damaging the pinion mechanism. Some needles are very tight where you can pull the pinion with it. In order to avoid this, you may use this tool.
2. Strap Wrench Set. Use this tool to remove the glass cover safely and easily.
Click here to check at Amazon: Craftsman 2 pc 16” Rubber Strap Wrench Set
Watch the video here at my Facebook Page: How to use the Strap Wrench
There it is, I have shared the different techniques to adjust or repair a pressure gauge. You can save a pressure gauge by following or choosing the right set up suited for a specific pressure gauge. I will add here other types just in case I see a new one.
To learn how to calibrate a pressure gauge, visit this post pressure-gauge-calibration-using-fluke-754-process-calibrator-with-a-set-of-fluke-pressure-modules
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