Industrial Calibration Method for Analog and Digital Pressure Gauges

calibrated pressure gauge showing offset value

Introduction

Pressure gauges are among the most common instruments used in industrial plants. They are installed on pipelines, compressors, boilers, and pressure vessels to monitor system pressure during operation. Because these instruments are mechanical devices, their readings can gradually shift due to wear, vibration, temperature changes, or when encountering over-pressure events.

Pressure gauge calibration is performed to verify that the gauge reading is still accurate. During calibration, the gauge reading is compared with a reference pressure standard with known accuracy. The difference between the reference pressure and the gauge reading represents the measurement error of the instrument.

Regular calibration is important because it helps ensure:

reliable pressure measurement
safe operation of pressure systems
compliance with quality and calibration requirements

This article outlines a practical method employed in calibration laboratories for calibrating pressure gauges, including both analog and digital pressure gauges.

What is Pressure?

Pressure is everywhere and all around us. We can feel the air pressure exerted on our bodies. We feel the pressure during long travel usually from a low to high elevation or vice versa, and we feel a pop in our ears. We can also observe pressure on a pack of snack foods.

Pressure is the action of pressing. In definition, a pressure is the Force acting per unit Area.

In a formula, Pressure=F/A

Pressure Unit of Conversion

How to Measure Pressure?

Pressure is measured with using a measuring instrument called a pressure gauge. It can be in analog or digital display. An analog pressure gauge operates through the movement of mechanical parts- the needle pointer and a bourdon tube, or in a digital gauge where a sensing element is a transducer, a device that converts a signal from one form to another.

Pressure is always measured with respect to a reference, these are:

Absolute zero
Atmospheric or ambient pressure
Relative (vacuum or gauge pressure)

And measured in four ways:

Absolute pressure -This is the total system pressure (gauge + atmospheric pressure)
Gauge pressure – is the pressure acted upon by the system pressure above the atmospheric pressure
Vacuum– a negative pressure acting between atmospheric pressure and absolute pressure.
Differential Pressure – the difference between two independent-acting pressures

Visual presentation of basic pressure terms and reference pressure

Some common units are Psi, Pa, Bar, mmHg, and mmH20.

Why measure pressure?

Calibration of pressure gauges is one of the basic requirements when installed. Every industry that I visited like food processing or manufacturing monitor pressure readings for several reasons, the most common of which are the following:

For safety -accurate control through proper measurement of pressure prevents pipes and other pressurized containers from bursting.
Monitoring process efficiency involves applying proper pressure to processes, along with other variables, to produce the required output.
Cost saving– high pressure corresponds to high energy consumption, therefore controlling pressure to the exact usage also controls energy.

What is Pressure Gauge Calibration

Pressure gauge calibration is the process of applying known pressure values to a gauge and comparing the indication of the gauge with a reference standard.

During calibration, pressure is generated using a controlled pressure source. The pressure is measured using a calibrated reference instrument. The reading of the pressure gauge under test is then compared with the reference value.

The difference between these two readings represents the error of the pressure gauge.

In most calibration laboratories, the process normally involves:

applying a known pressure
recording the reference pressure
recording the gauge reading
calculating the measurement error

If the error exceeds the allowable tolerance, the instrument may require adjustment or repair.

It is also useful to understand the difference between the following terms:

Calibration
Determining the measurement error of the instrument by comparison with a reference standard. Answers the question “How accurate is the pressure gauge?”

Verification
Checking that the instrument error is still within the acceptable tolerance. Answers the question, “Is it a Pass or a Fail?”

Adjustment
Correcting the instrument so that the error is reduced.

Many calibration procedures record readings as-found before adjustment and as-left after adjustment.

Check out my other post on the link below, where I explained in more detail about calibration terms and their differences >> Differences between Calibration, Verification and Validation in Measurement Process

Types of Pressure Gauges Covered

The procedure described in this guide can be applied to most industrial pressure gauges, commercial gauges, and process gauges.

Industrial and process gauges are designed for harsh environments, heavy-duty, and usually have a higher accuracy requirement compared to commercial gauges.

Analog Pressure Gauges

Analog gauges use mechanical sensing elements such as Bourdon tubes or diaphragms. Pressure causes the sensing element to move, which then moves the pointer on the dial.

a typical exploded view of analog pressure gauge showing internal parts

As per AS 1349-1986, below are the accuracy ratings based on the size of the dial face.

Pressure Gauges with dial face diameter above 63mm:
± 1%FS – from 10% to 90% of full scale range
± 1.5%FS – below 10% and above 90% of full scale range

Pressure Gauges with dial face diameter of 63mm and below is ±3% FS.

Test Gauges is ±0.25%FS

FS> Full Scale

These gauges are commonly found in:

hydraulic systems
compressors
steam systems
process pipelines

Digital Pressure Gauges

a digital gauge used as a reference standard

Digital gauges use electronic pressure sensors to measure pressure. It can function as a compound gauge or can measure vacuum depending on the model. The pressure value is displayed on a digital screen and may include additional features such as data logging.

These instruments generally provide higher resolution and accuracy compared to analog gauges and are often used as reference standards in calibration work.

Compound Gauges

Compound gauges measure both vacuum and positive pressure. These are often used in refrigeration and vacuum systems.

A compound pressure gauge used in a water system

Differential Pressure Gauges

Differential pressure gauge showing 2 inlet ports

Differential pressure gauges measure the difference between two pressure points. It has 2 inlet pressure ports, in the photo above, 1 inlet port is exposed to ambient pressure. These instruments are used in applications such as airflow monitoring, filter monitoring, and level measurement.

Separate procedures are normally used for differential pressure calibration. Check out the procedure in my other article in this link >> Differential Pressure Gauge Calibration

Equipment Required for Pressure Gauge Calibration

Pressure gauge calibration requires several pieces of equipment to generate pressure and measure it accurately.

Reference Pressure Standard

The reference standard is the instrument used to measure the true pressure value. Examples include:

electronic deadweight tester
pressure controller
calibrated pressure module
reference digital pressure gauge

The reference standard must have an accuracy significantly better than the gauge being calibrated.

Pressure Source

A pressure source is required to generate the pressure applied during calibration.

Examples include:

hand pressure pump
pneumatic pressure controller
hydraulic pressure pump

Pneumatic pressure source is used for low-pressure applications and can generate negative pressure as well, while the hydraulic hand pump is for higher-pressure calibration needs.

Typical standards attached here are the digital gauge or the pressure modules.

The pressure pumps have the capability to provide stable and controlled pressure adjustments.

Pressure Modules or Reference Gauge

Pressure modules are commonly used with multifunction process calibrators to measure reference pressure values.

Multifunction calibrators is used as display for pressure modules. Each pressure modules have different range to suit different calibration needs.

I always like to use these set, with the process calibrator and the set of pressure modules, in addition to a much higher accuracy, it is more convenient to use, specially during field work. You can calibrate almost all the industrial gauges installed on the line. For me, it is a must have calibration standard.

Calibration Fittings and Hoses

Appropriate fittings are required to connect the pressure source, reference standard, and test gauge together.

When connecting pressure gauges, the type of thread used in the pressure port is critical for sealing and safety. In practice, you will encounter two main types:

Tapered threads (seal by thread interference)- when the male and female threads are tightened, the threads wedge together, creating a seal.
1. Commonly known as NPT (National Pipe Thread)- the standard
2. Typical Gauge connection sizes are:
  - 1/4″ NPT
  - 1/2″ NPT

pressure gauge connected using 1/2 NPT (notice the teflon tape for proper sealing)

Parallel threads (seal with a gasket or sealing surface)
1. BSPP (British Standard Pipe Parallel – “G Thread”)
2. Parallel threads have a constant diameter from start to end.
3. Typically used for reference standards connectors
4. They do NOT seal through the threads. Instead, the seal happens at:
  - a gasket
  - an O-ring
  - a sealing washer
  - a flat sealing face
Also called:
- G thread
- ISO 228

One way to achieve a more accurate reading in pressure calibration is to have a proper set of fittings. Even if you have the best standards, if you fail to use proper fittings, it can affect your data and the performance of your calibration. Leakage may occur, or worse, it can damage your standard and the UUT if the wrong fittings are used.

Calibration Datasheet

Calibration results should be recorded on a datasheet or calibration record.

MDS
Sample Measurement Data Sheet

Typical information recorded includes:

applied pressure
reference pressure
gauge reading
measurement error
pass or fail result

Proper documentation ensures traceability and provides evidence of calibration.

Pre-Calibration Inspection

Before performing the calibration procedure, the pressure gauge should be inspected.

Check the following:

physical damage to the gauge
pointer position at zero pressure
condition of the pressure port
signs of leakage
dial readability
labels or seals that need to be replaced

If the gauge shows significant mechanical damage, calibration may not be reliable and the instrument may need repair.

Pressure Gauge Calibration Setup

The pressure calibration setup connects the pressure source, reference standard, and pressure gauge under test in the same pressure line.

All instruments must experience the same pressure during calibration.

Typical setup includes:

pressure source
reference pressure standard
pressure gauge under test
connection fittings

After connecting the instruments, the system should be pressurized slightly to check for leaks.

Pressure should be applied slowly to allow the readings to stabilize before recording measurements.

Before calibration is performed, we need to observe the following requirements:

Accuracy – the accuracy of the standard should be greater than the UUC or pressure gauge, usually a 4:1 ratio.
Position – Calibration should be in a vertical plane, and the level or height difference of the UUC and Standard should be the same.
Warm-up time – allow the reference standard and Unit Under Calibration (UUC) to stay in a room for 60 min.
Readability – readability is based on the resolution of the gauge, a resolution is the smallest scale division of the gauge. The resolution can be divided or read by dividing it by 2, 5, or 10, depending on the viewer’s capacity to read. For example, if the smallest scale is 5 psi, if you use 2 as the divisor, the resolution will become 2.5.
– for a digital pressure gauge, this is the smallest reading that the gauge can display.
Temperature (ambient conditions) – should be maintained within 20 +/- 2°C

Pressure Gauge Calibration Procedure

Step-by-step:

🔹 Step 1: Preparation

Select appropriate reference standard
Check environmental conditions
Inspect the gauge visually for any signs of damage or abnormalities
Record all the details about the UUC. Use a Measurement Data Sheet to record your data
Connect the pressure gauge to the pressure source and reference standard.
Preload the gauges to the highest value for 30 secs to exercise the gauge, then return to zero

🔹 Step 2: Apply Pressure Points

Apply pressure gradually to the first calibration point, then increasing (0 → full scale).
Allow the pressure to stabilize.
Slightly tapped the glass cover to minimize any frictional effect of the pointer system
Record readings at each point

🔹 Step 3: Decreasing Cycle

Reduce pressure stopping at each calibration ppint
Record readings again

🔹 Step 4: Calculate Error

Compare gauge reading vs reference
Determine deviation or measurement error

🔹 Step 5: Adjustment or Reporting

or report “as-found” condition
Adjust if possible

Calibration should normally be performed both in ascending and descending pressure to evaluate gauge hysteresis.

Calibration procedures or methods are not the same; methods are dependent on the procedures we follow or implement based on internationally recognized guides like ASTM, BS EN837-1, DKD, and many more.

For other calibration setups and procedures of other pressure measuring instruments, check out the following links:

Recommended Calibration Points

Calibration is typically performed at several points across the measurement range.

Common test points include:

0 % of full scale
25 % of full scale
50 % of full scale
75 % of full scale
100 % of full scale

Following BS EN837-1, the following test points are required:

The number of test points shall be evenly distributed over the entire scale as follows:

± classes 0.1; 0.25 and 0.6: a minimum of 10 points;
± classes 1; 1.6 and 2.5: a minimum of 5 points;
± class 4: a minimum of 4 points.

>Testing multiple points helps evaluate the linearity of the pressure gauge.

>Testing increasing and decreasing values helps evaluate hysteresis.

Take note that, depending on process requirements, some test points can be selected only or partially calibrated.

Recording Calibration Results

Calibration results are recorded in a calibration datasheet, also called a measurement data record or MDR.

Datasheets include the necessary information (like make, model, serial#, environmental conditions, etc..) to facilitate proper traceability or the repetition of calibration whenever necessary.

Example:

Test Point	Reference Pressure	Gauge Reading	Error
0%
25%
50%
75%
100%

If adjustment is performed, a second set of readings is usually recorded as as-left results or “before and after”.

Take note that this format is not fixed; the format of calibration datasheets is based on the procedure we follow or implement.

We need to have the complete details, data, and information on the gauges to prepare a comprehensive calibration certificate that aligns with the requirements of ISO 17025.

Calibration Record-Calibration Certificate Content

The measurement data record shall be completed. Calibration Certificate shall include, as a minimum:

title (Calibration Certificate),
name and the address of the lab,
unique identification number of the calibration report,
name and the address of the customer/user,
identification of the calibration method,
description of the item calibrated,
date of the receipt of calibrated item and the date of the calibration,
calibration results and the unit of measurement,
names, functions, and signatures of persons authorizing the calibration certificate,
statement to the effect that the results relate only to the item calibrated,
others as per the requirements of client or user and the certifying body.

Evaluating Calibration Results

To evaluate the performance of the gauge under calibration, you should understand the following calibration terms:

error or calibration error = the difference between the UUC and STD results after calibration
Tolerance = a range of values that is acceptable or permitted by the user from the result of the process or product measurement.
Accuracy =the closeness of UUC results to the STD (true) value.

The measured error of the pressure gauge is compared with the allowable tolerance.

If the error is within the specified tolerance, the gauge passes calibration.

If the error exceeds the tolerance, the instrument may require:

adjustment
repair
replacement

Measurement uncertainty should also be considered when evaluating calibration results.

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

Calibration Interval

Calibration frequency depends on:

usage
environment
criticality

Typical intervals:

6 months to 1 year

👉 Check out this link to read more>> “calibration interval” article

Measurement Uncertainty Budget Considerations

The methodology of the “Guide to the Expression of Uncertainty in Measurement: BIPM, JCGM 100:2008” shall be used as the basis for the expression of uncertainty of measurement.

The common contributions to the measurement uncertainty calculations for the calibration of the pressure gauge will be as follows:

Type A = analysis methods from statistics for measurement series under repeatability conditions

repeatability test

Type B = based on other scientific findings

Uncertainty Measurement of the Reference Standard
Calibration Drift of the Reference Standard
Hysteresis of the Reference Standard
Resolution of the Reference Standard
Resolution or readability of the UUC

👉Read here the importance of measurement uncertainty in calibration

Learn more about how to calculate the measurement uncertainty in this link >> Uncertainty for Analog Pressure Gauges (GUM Method)

ISO 17025 Considerations

Pressure calibration must comply with:

Traceability → reference standard must be calibrated by an authorized calibration lab
Measurement Uncertainty → results must include uncertainty
Impartiality → no bias in reporting results
Intermediate check → quality control to ensure confidence in calibration results

Common Mistakes During Calibration

Using uncalibrated reference standards
Skipping decreasing cycle
Ignoring uncertainty
Poor documentation
Not tapping the protective glass
Incorrect use of fittings

Common Problems Encountered During Calibration

Several issues can affect pressure gauge calibration.

Common problems include:

pressure leaks in fittings
unstable pressure from the pump
pointer not returning to zero
gauge hysteresis
over-pressure damage

Identifying these problems early helps avoid inaccurate calibration results.

If these issues still encountered during calibration, we need to perform the necessary pressure gauge repair or adjustment.

Common Errors In Pressure Calibration

Hysteresis (difference between up/down readings)
Zero shift
Non-linearity
Environmental effects (temperature)

Pressure Gauge Adjustment and Repair

If calibration results exceed tolerance limits, adjustments may be required.

Adjustment typically involves mechanical correction of the pointer or internal mechanism.

After adjustment, the calibration procedure should be repeated to verify the new readings.

If the gauge cannot be adjusted within tolerance, repair or replacement may be necessary.

Check out this link where I presented different techniques on how to repair and adjust a pressure gauge >> 5 Techniques For Pressure Gauge Adjustment and Repair

You may also find these related procedures useful:

These guides cover other pressure measurement instruments commonly used in industrial systems.

Frequently Asked Questions

How often should pressure gauges be calibrated?
Calibration intervals depend on the application, but many facilities perform calibration annually or based on instrument criticality.

What causes pressure gauge drift?
Drift may occur due to mechanical wear, vibration, temperature changes, or over-pressure events.

How to calculate drift in a pressure gauge?
Drift is the change in the output reading of instruments overtime at a specified period. It is simply the difference between the past result and the present result. See the example below: the readings are based on historical records, from its calibration certificate.

What tolerance is acceptable for pressure gauges?
The acceptable tolerance depends on the gauge specification and the requirements of the process being monitored. During calibration or verification, we normally use manufacturer’s specifications. Manufacturer’s specifications are mostly based on international standards, like ASTM or DKD.

What is hysteresis in a pressure gauge reading?
Hysteresis or hysteresis error is the error based on the difference between the applied increasing and decreasing pressure. Hysteresis = increasing – decreasing

example: pressure @ 10 psi increasing = 10.00; decreasing =9.95 psi
> hysteresis = 10.00-9.95 =0.05 psi

PRACTICAL IMPLEMENTATION

If you are performing in-house calibration:

You need:

uncertainty calculations
standard procedures
datasheets

👉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

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