The analog pressure gauge is one of the simplest, mostly used instruments, and the easiest to calibrate in pressure calibration. It has a simple setup where you can easily execute the calibration. But analog pressure gauge is also one of the instruments where errors are mostly encountered.

Most errors are either “out of tolerance” or the needle is not in the “zero settings”. The following are the commonly encountered problems with the analog pressure gauge:
1. pressure gauge needle stuck;
2. pressure gauge not zeroing;
3. the pressure reading is very far or out of calibration;
4. the worst, the pressure gauge not working at all

When this happens, do not immediately throw or discard the gauges. Luckily, there are still ways to adjust or repair pressure gauges.

In this post, I will share with you the following:
1. 5 ways on how to adjust a pressure gauge
2. The things to consider before making an adjustment or repair
3. how to identify the error in the pressure gauge and execute the adjustment.
4. How to verify a pressure gauge.
5. Some useful tools to use to perform adjustments

Pressure gauges have different applications and therefore have many construction designs.

I will present in this post the different setups for different models of gauges to be adjusted.

Below are some reasons why pressure gauges are getting out of whack (out of tolerance or not in the zero settings):

1. Changes in the altitude and temperature (due to changes in environmental factors).
2. Overstressed bourdon tube due to exposure to overpressure.
3. Overused or overexposed to the pressure cycle
4. Exposure to high temperatures and excessive vibrations
5. Wear in the mechanical parts.
   

Things to Consider Before Performing Pressure Gauge Adjustment or Repair

Pressure gauge errors can be identified during the execution of calibrations where we compare the readings of the pressure gauge against the reference standard or pressure calibrator.

Pressure gauge errors can be calculated using the formula:

ERROR = UUT Reading – Reference Standard Reading

If the identified error is not acceptable or too large based on verification, then an adjustment is necessary.

During the pressure gauge calibration process, we observed the below defects, chances are high that adjustment or repair is impossible.

1. A reading near its full-scale value is shifted to more than 10% (a span shift)
2. Pressure gauges that have a zero shifted to more than 25 % (a zero shift)
3. Gauges with signs of leakage and corrosion of its body.
4. Gauges with a needle showing an error due to excessive friction or wear on its movement (loose)
5. With damaged sockets and threads.
   
   When the above signs are observed, there is a high chance that replacement is the only option.
   

You can read more in this article from  ASHCROFT

The Process of Pressure Gauge Repair

Pressure gauge repair becomes necessary when a gauge no longer responds to adjustment or exhibits physical damage. Repairing a pressure gauge involves a series of steps to restore its functionality and accuracy. To repair it, you would typically follow these steps:

1. Isolate the Gauge: First, shut off the system and isolate the gauge from the pressure source. This prevents any further damage or injury during the repair process.
2. Disassemble the Gauge: Carefully disassemble the gauge, removing the damaged parts. In the case of a cracked glass face, you would remove the glass and any broken components.
3. Replace Damaged Parts: Obtain replacement parts, such as a new glass face, needle pointer, and gasket, if necessary. Ensure that the replacement parts are compatible with your gauge model.
4. Reassemble and Test: Reassemble the gauge with the new parts and ensure proper sealing. Then, verify the gauge’s accuracy using the appropriate reference standard and make any necessary adjustments.
5. Calibrate: Perform calibration on the gauge to ensure it provides accurate readings covering its full range.

By following these repair steps, you can extend the life of your pressure gauges and avoid the expense of replacing them. Regular maintenance and repair help keep your operations running smoothly while minimizing downtime and safety risks.

5 Techniques to Adjust or Repair a Pressure Gauge

1. Opening a vent to release pressure build-up
2. Opening the Glass (or plastic cover) then remove the needle.
3. Rotating the screw located at the face of the scale.
4. Adjusting the knob at the lower side of the pressure gauge
5. Adjusting the screw located at the gauge needle pointer itself.

Be reminded that the above techniques are dependent 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, pressure inside the pressure gauge will build up 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.

This is the simplest of all the adjustment techniques that is usually applicable to most oil-filled and permanently sealed types of analog pressure gauges. If this technique does not fix the problem, then a replacement is the only fix. Check out replacement gauge here >> Pressure Gauges

2. Opening the Pressure Gauge Protective Glass (or plastic cover) then remove the needle.

For non-oil-filled gauges, it is not permanently sealed (most oil-filled gauges are permanently sealed where adjustment or repair is not possible).

But in some cases where it can be opened, remove first the oil inside before removing the glass cover. You need an extra oil replacement just in case.

If you are wondering what is the liquid filled inside the pressure gauge is, it is called glycerine or silicone oil. It is important to replace or refill the pressure gauge oil when removed or if the oil has been drained because of a leak.

What are the advantages of oil oil-filled pressure gauge? Below are some of the reasons why we need to maintain the oil-filled pressure gauge:

1. Oil-filled pressure gauges are more protected from harsh environmental factors and moisture or water condensations. Some pressure gauge mechanical parts get stuck from dust and corrosion build-up resulting in wrong readings, or worse, damage.
   
2. Glycerine Oil-filled pressure gauges are also protected from strong vibrations. Strong vibrations affect the mechanical components. Some needles are getting removed or broken because of these strong vibrations.
   
3. Since the liquid-filled pressure gauge can resist strong vibration, the displayed readings of the needle are more stable, making it easy to read, thus, increasing the accuracy of the pressure gauge.
   
4. Glycerine or silicone oil acts as a lubricant for the mechanical parts especially for the needle pointer making it move smoothly.

(see at the bottom of the page the tool to remove the cover and needle easily and safely).

In some cases where releasing the pressure build-up cannot solve the problem, we can perform the below adjustment by opening the pressure gauge through the removal of its protective cover and removing the needle pointer. With this method, we can correct the zero and span shift of the instrument when it occurs.

A. To correct the Zero Shifts:

Once the cover is removed, perform below steps:
1. Pull the needle using the needle puller (see below).
2. Once the needle is removed, position it on the zero scales then push to return.
3. Verify the reading by supplying pressure

How to perform pressure gauge verification? Read on …

B. To correct Span Shift:

Once the needle is removed, perform the following:
1. connect the pressure gauge to a known pressure source (the Electronic Deadweight Tester or Fluke 754 with a set of pressure modules)
2. Generate a pressure of about 50% of its Full scale.  Once the pressure is set, return the needle to 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.
3. Perform verification to ensure that it is now within the accepted range.

Most of the time, Just performing either A or B  solves the problem.

3. Adjusting the knob at the lower side of the pressure gauge

Some pressure gauges have an adjustable knob on the lower side of the body. This is a diaphragm-type pressure gauge that is usually used in water treatment piping.

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 pressure gauges have an adjustable screw inside the body, at the back of the needle, or within the dial face. This type of gauge is used in gas lines.

Access the screw inside the gauge by removing the cover. Turn the screw either clockwise or counterclockwise to return the needle to zero position.

Perform verification after zeroing.

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5.  Adjusting the screw located at the gauge needle pointer

This applies to some pressure gauge that has an adjustable screw integrated on the needle itself. Once the cover is removed, use a screwdriver and carefully turn the screw to position the needle. See the figure above or watch the video below.

Based on the photo above, I have shown you the location of the screw and the position of the screwdriver, but during the rotation of the screw, you need to hold the needle to rotate the screw and perform the adjustment.

You need to be more careful in this type of adjustment. the needle is connected with the spring inside and it is very sensitive. A strong force while rotating the screw can damage the whole part or pressure gauge.

After performing the adjustments, do not forget to verify the pressure gauge.

Additional tips:  
1. Adjusting the screw located at the Pressure Gauge Dial Face

This is the same as # 4 above but this one is the simplest and easiest to adjust, the adjustment screw is just beside the needle pointer on the dial face, designed by the manufacturer (Zahm and Nagel). It seems difficult at first but once the glass cover is removed, you simply rotate the screw clockwise or counter-clockwise to perform zeroing or any point adjustment.

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Key Takeaways

1. Zero Adjustment

Key Takeaways:

• Zero error is a common issue with analog pressure gauges.
• Adjustment is typically done by removing the front cover and turning the zero adjust screw.
• Always depressurize the gauge before performing any adjustments.

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2. Pointer Removal and Repositioning

Key Takeaways:

• Misaligned pointers can mislead pressure readings.
• Use a pointer remover tool to safely lift and reposition the pointer.
• Ensure the gauge reads zero after pointer adjustment, especially with atmospheric pressure.

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3. Span Adjustment

Key Takeaways:

• Span errors affect gauge readings across the scale.
• Adjust the span screw if the gauge shows consistent over- or under-reading.
• Calibration with a standard pressure source is essential to verify span correction.

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4. Gear Movement Calibration

Key Takeaways:

• Internal gear misalignment causes pointer inaccuracies.
• Re-engage or align gears properly for accurate needle response.
• This step is more complex and may require manufacturer guidelines or expert assistance.

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5. Leak Testing and Sealing

Key Takeaways:

• Leaks in bourdon tubes or connections lead to incorrect readings.
• Use leak detection fluid or bubble solution to locate leaks.
• Seal minor leaks or replace damaged components when necessary.

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How to Verify the Accuracy of Pressure Gauge After Adjustment?

In every adjustment or calibration that we perform, we need to verify if the readings are within the pressure gauge’s acceptable range, tolerance, or acceptance criteria.

Before performing the adjustments, you must consider what tolerance you should follow.

The tolerance or acceptance criteria are based on:
1. manufacturer specification;
2. user or process tolerance.
3. International Standards or Regulatory requirements ( ex: ASTM or ISO Standards)

As an example, we will be using the manufacturer’s specs to verify the pressure gauge. Below are the steps:

1. Determine the accuracy rating (accuracy class) of the pressure gauge. Most pressure gauges have the accuracy class printed on their dial face (see the below photo). This accuracy class has an equivalent MPE (Maximum Permissible Error) where we can calculate the limits of permissible error.

Learn more about the relationship and differences between accuracy, error, tolerance, and uncertainty in my other post on this link>> accuracy-error-tolerance-uncertainty

2. Convert the accuracy class to decimal by dividing it by 100 then multiplying it with the Span or FS reading. This rating is in percent of Full Scale or Span. Based on the photo above, the pressure gauge is labeled with KL 1.6 which means 1.6% of Full Scale and Class 1% equivalent to 1% of Span (see example below).

The difference Between Full Scale and Span in Pressure Gauge Display

Full Scale (FS) reading is equivalent to the maximum displayed value of the pressure gauge. It is from ZERO to the maximum range.

Span is the difference between the lower range (not Zero-value) and the higher range. Span range applies to those pressure gauges with a negative value (see photo above-right side gauge).

Span = 5 – (-1) = 6

Span and Full-Scale reading are the same when the lower range is equal to ZERO.

To determine the tolerance limit, we will multiply the accuracy class rating with the span or FS reading. See the example below.

• The pressure gauge has an accuracy of 1.6% of Full-Scale (FS)
  • Full-Scale reading is = 7 Bar

tolerance limit = (1.6/100) x 7 = 0.112 Bar

• Therefore, reading should be within +/- 0.112 Bar

3. Once the tolerance limits or the acceptance criteria are determined, generate the necessary pressure requirement and then compare readings. If the reading is outside of this range, then more adjustments are needed.

If you have performed one or all of the adjustments above and still the pressure gauge is out of tolerance, then it is already damaged and replacement or a deeper repair is the only option.

To determine the pressure gauge tolerances based on the dial face and accuracy class, you may refer to the table below based on BS EN 837-1.

Some Useful Tools During Adjustment

1. 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 and you can pull the pinion with it. 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 on my Facebook Page: How to use the Strap Wrench

Note: I get commissions for purchases made through links in this post.

Conclusion

In this article, I have shared the following:
1. 5 ways on how to adjust a pressure gauge,
2. The things to consider before making an adjustment or repair
3. how to identify the error in the pressure gauge and execute the adjustment.
4. How to verify a pressure gauge.
5. Some useful tools to use to perform adjustments

You can save and re-use a pressure gauge by performing regular calibration and adjustment, following or choosing the right setup suited for a specific pressure gauge as explained in this article.

If you have other methods to adjust or repair a pressure gauge, please do not hesitate to comment.

Do you need a complete pressure gauge calibration procedure? Check out this ready-made pressure gauge calibration procedure package, which consists of a procedure, measurement uncertainty calculator, a datasheet, and a calibration certificate in one package, visit this link >> buymeacoffee.com/edsponce/e/312258

To learn how to calibrate a pressure gauge, visit this post:  pressure-gauge-calibration-what you need to know

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Edwin