Compliance and audits
Pressure gauge calibration: procedure, accuracy classes and intervals
Pressure gauge calibration compares the gauge with a more accurate reference, a deadweight tester or a reference gauge, at points spread across its range with the pressure first rising and then falling, and checks each error and the hysteresis against the limit for the gauge's accuracy class. Under OIML R 101 a class 1 gauge in service may be off by up to 1% of its upper range limit, and DKD-R 6-1 recommends recalibrating Bourdon tube gauges every 1 or 2 years depending on their class.
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What a pressure gauge calibration checks
A calibration compares the gauge with a reference standard whose values are known and traceable. The reference is either a deadweight tester, also called a pressure balance or piston gauge, or a reference gauge or digital calibrator that is more accurate than the gauge under test. At each test point you record the reading of the gauge and the reference value. The difference is the error.
For an analog gauge, OIML R 101 starts with a visual inspection and a preliminary check, then measures the errors and the hysteresis (clause 8). The preliminary check confirms that the pointer moves smoothly without sticking and rests on zero at ambient pressure (10.2). The errors are read at points spread over the range, first with the pressure rising and then with it falling.
Comparing the errors with the gauge's limit and recording a pass or fail is a verification against a specification. A certificate can carry both the measured errors and that statement.
The standards: ASME B40.100, EN 837-1, OIML R 101 and DKD-R 6-1
ASME B40.100-2022 is the US standard for pressure gauges and gauge attachments. It covers dial-type gauges that sense pressure with an elastic element, such as a Bourdon tube, along with diaphragm seals, snubbers and pressure limiter valves. It has a table of accuracy grades with the recommended number of test points (Table 2-2.4-1) and a section on pressure gauge testing (2-5). ASME has placed it on stabilized maintenance, so the 2022 edition stays in effect.
EN 837-1:1996 is the European standard for Bourdon tube pressure gauges in nominal sizes 40 to 250, with ranges up to 1600 bar. It sets accuracy classes, requirements and test methods. National editions such as DIN EN 837-1 and BS EN 837-1 are current.
OIML R 101:1991 is the international recommendation for ordinary indicating and recording gauges with elastic sensing elements and upper limits from 0.05 MPa to 1000 MPa. It sets accuracy classes, maximum permissible errors and a verification method. OIML R 109:1993 does the same for reference-grade gauges, which it calls standard instruments. Both are free from the OIML and listed as current.
DKD-R 6-1 is the calibration guideline of the German Calibration Service, published by PTB. It covers Bourdon tube gauges, electrical pressure gauges and pressure transmitters with an electrical output, and sets three calibration sequences by the uncertainty you aim for. Edition 03/2014, revision 3, is free from PTB.
What an accuracy class allows
The accuracy class of an ordinary gauge is a percentage of its range. Under OIML R 101 (4.1.1), a gauge in service may err by up to its class number times 1% of the upper limit of the measuring range. A new or repaired gauge has to meet 0.8 times that. For a pressure-vacuum gauge, the base is the sum of the upper limits of the pressure and vacuum ranges.
The limit is the same anywhere on the scale. A class 1 gauge reading 0 to 10 bar may be off by 0.1 bar at 1 bar as well as at 10 bar, which is 10% of the reading at 1 bar. R 101 also sets a normal limit of use (3.1.2): on gauges with an upper limit below 100 MPa, three quarters of the upper limit for steady pressure and two thirds for variable pressure.
- Hysteresis has to stay inside the same limit. R 101 does not let the hysteresis error exceed the maximum permissible error (4.1.2).
- At ambient pressure the pointer has to rest on zero within the maximum permissible error (10.2.3).
- R 101 requires the dial to show the upper limit and the accuracy class (6.1.1).
- ASME B40.100 uses accuracy grades and EN 837-1 its own classes. The limit for your gauge comes from the standard named on its datasheet, or from the maker's specification if that is tighter.
| Class | In service, % of upper limit | Example: 0 to 10 bar gauge in service | New or repaired, % of upper limit | Minimum test points |
|---|---|---|---|---|
| 0.25 | ±0.25% | ±0.025 bar | ±0.2% | 8 |
| 0.6 | ±0.6% | ±0.06 bar | ±0.48% | 8 |
| 1 | ±1% | ±0.1 bar | ±0.8% | 5 |
| 1.6 | ±1.6% | ±0.16 bar | ±1.28% | 5 |
| 2.5 | ±2.5% | ±0.25 bar | ±2% | 5 |
| 4 | ±4% | ±0.4 bar | ±3.2% | 3 |
Choosing the reference: deadweight tester or reference gauge
A deadweight tester generates pressure from known masses on a piston of known effective area. DKD-R 6-1 counts pressure balances among the references with long-term stability, and electrical pressure gauges among the less stable ones (clause 3).
The pressure a deadweight tester generates depends on local gravity, the temperature of the piston and cylinder, and air buoyancy on the masses. OIML R 110 states its maximum permissible errors at standard gravity, 9.80665 m/s², and at 22 °C ± 3 °C (4.4), so a tester used elsewhere needs the corrections its certificate or manual gives. DKD-R 6-1 asks labs using piston gauges to record the atmospheric pressure as well as the room temperature (clause 6).
A reference gauge or digital pressure calibrator is quicker to use and can cover several ranges. It needs its own current calibration with a stated uncertainty. When the reference is an indicating gauge, DKD-R 6-1 counts its resolution a second time in the uncertainty (clause 3).
How much better does the reference need to be? OIML R 101 allows the errors of the verification equipment to be at most a quarter of the maximum permissible error of the gauge under test (9.1). DKD-R 6-1 recommends that the uncertainty of the reference not exceed a third of the uncertainty you aim for on the gauge.
Set up: mounting position, height and pressure medium
Mount the gauge the way it is used. R 101 makes the attitude of use specified by the maker a reference condition (4.1.3), and the dial may carry the normal attitude of use (6.1.1). DKD-R 6-1 asks for the mounting position to be considered during calibration and stated on the certificate.
Put the reference levels of the gauge and the reference at the same height, or correct for the difference. A column of liquid adds a pressure of ρgh. With water in the line, 10 cm of height difference is close to 1 kPa, which is the whole maximum permissible error of a class 1 gauge with a range of 0 to 100 kPa. R 101 requires the error from a level difference to stay below a tenth of the maximum permissible error (9.4).
Use the right medium. R 101 calls for an inert gas up to 0.5 MPa and a non-corrosive liquid above that, unless switching between them changes the reading by less than a fifth of the maximum permissible error (4.1.3). Where the gauge and the reference use different media, a separator may not shift the reading by more than a tenth of the maximum permissible error (9.2).
Check for leaks first. R 101 suggests raising the pressure to the upper limit, isolating the gauge for 3 minutes and then watching it for 2 more. A drop of no more than 1% of the upper limit counts as leak-tight (10.2.1).
Let the gauge and the reference settle to room temperature. R 101 uses 20 °C as the reference temperature. DKD-R 6-1 allows a lab between 18 °C and 28 °C and recommends keeping the temperature within ±1 K during the calibration and recording it. Give electrical gauges their warm-up time.
Test points: how many and in what order
Spread the points evenly over the range, from zero to the upper limit. R 101 asks for at least 8 points on gauges of class 0.6 and better, 5 on classes 1 to 2.5 and 3 on classes 4 and 5 (10.3.1). Read each point with the pressure rising, hold the upper limit for 5 minutes, then read the same points with the pressure falling. Read the zero after 5 minutes back at ambient pressure.
DKD-R 6-1 ties the effort to the uncertainty you aim for (Table 1). It first exercises the gauge to its maximum, with at least 30 seconds at the top and between preloads, then sets the zero if the gauge allows it. Each pressure step waits at least 30 seconds before the reading, and Bourdon tube gauges are tapped lightly to free the pointer from friction (clause 7).
| Sequence | Target uncertainty, % of span | Minimum points, with zero | Preloads | Rising series | Falling series |
|---|---|---|---|---|---|
| A | Below 0.1 | 9 | 3 | 2 | 2 |
| B | 0.1 to 0.6 | 9 | 2 | 2 | 1 |
| C | Above 0.6 | 5 | 1 | 1 | 1 |
Hysteresis, repeatability and zero
Hysteresis is the difference between the readings at the same pressure when you approach it from below and from above. R 101 defines it that way and leaves out the two ends of the range, where only one direction exists (T.8). Work it out at each point as the falling reading minus the rising reading, and compare the largest value with the limit.
DKD-R 6-1 records the zero before and after each up-and-down cycle and reports the largest change as the zero deviation (8.6.2). It works out the hysteresis from readings corrected for that zero (8.6.5). When a sequence has two rising series, as A and B do, the difference between them gives the repeatability (8.6.3).
For reference-grade gauges, R 109 takes two readings at each point, one straight away and one after a light tap on the case. The difference shows friction in the movement and may not exceed half the maximum permissible error (6.3.7.2 and 4.2.1.3).
Digital gauges and pressure transmitters
DKD-R 6-1 treats three kinds of instrument with the same method: Bourdon tube gauges, electrical pressure gauges with a display, and transmitters whose output is a current, a voltage or a frequency. For a transmitter, the lab reads the output with its own calibrated meter, and that meter's uncertainty goes into the budget (clause 4).
Calibrate the instrument as a whole measuring chain where you can, as DKD-R 6-1 recommends (clause 7). A transmitter and its display each have an error, and the reading in the plant depends on both.
Give the instrument its warm-up time, preload it, then set the zero if it supports that before the first series. If an adjustment is needed before it can be calibrated, agree it with the owner and record the settings before and after (clause 5).
As-found readings, adjustment and a gauge out of tolerance
Take the first full set of readings before anyone adjusts the gauge. These as-found readings show how the gauge measured while it was in use. If you reset the pointer or the zero, or repair the movement, repeat the full set of points as left.
If any as-found error is outside the limit, the gauge was out of tolerance for an unknown part of the time since its last good calibration. ISO 9001 clause 7.1.5.2 asks you to determine whether earlier results were adversely affected and to act as needed. For a pressure gauge, look at what its readings were used for, such as a process setting, a test pressure on a vessel or a hose, or an alarm limit. Then judge whether the error could have changed any of those results.
How often to calibrate a pressure gauge
No standard sets one interval for all gauges. NIST does not recommend a fixed recalibration interval either: it depends on the accuracy required, the stability of the instrument and the environment it works in. For gauges under legal control, OIML R 101 leaves the period to national regulations (7.3).
DKD-R 6-1 gives recommended recalibration periods under normal conditions of use (Appendix F). It also says to recalibrate after an overload beyond the permissible limit, a repair or mishandling that could affect the result, whatever the date.
- Bourdon tube gauges of class 0.6 or better: 1 year. Classes above 0.6: 2 years.
- Electrical pressure gauges and transmitters specified at 0.5% of span or better: 1 year. Above 0.5%: 2 years.
- Piston gauges such as deadweight testers: 5 years.
What the certificate and record should show
Whether the gauge goes to a lab or stays in-house, keep a record that lets someone else repeat the calibration and reach the same decision. DKD-R 6-1 (clause 9) adds pressure-specific items to the general certificate content of ISO/IEC 17025:2017 clause 7.8.
- The gauge's ID, range and class, and the limit applied.
- The method, for example DKD-R 6-1 sequence C, and the reference used with its ID and calibration due date.
- The pressure medium, the reference level on the gauge and the mounting position during calibration.
- A table of the reference pressure and the gauge reading at each point, rising and falling, with the error and the hysteresis.
- The as-found readings, and the as-left readings after any adjustment.
- The room temperature, the date, who did the work and the next due date.
How to calibrate a pressure gauge, step by step
- Identify the gauge, its range and class, and the limit that applies from its standard or the maker's specification.
- Pick a reference with a current calibration and an error no more than a quarter of that limit.
- Inspect the gauge, check that the pointer rests on zero at ambient pressure and test the connection for leaks.
- Mount the gauge in its working position with its reference level at the height of the reference, or work out the height correction.
- Exercise the gauge to its upper limit and back, then let it settle.
- Read each point with the pressure rising, tapping the case lightly before each reading on a dial gauge.
- Hold the upper limit, then read the same points with the pressure falling and read the zero at the end.
- Work out the error at each point and the hysteresis, and compare them with the limit. Record these as-found results before any adjustment.
- If the gauge fails, adjust or repair it, repeat all points as left and assess what it measured since its last good calibration. Then set the next due date.
Where Axiospec fits
Axiospec is calibration management software. It keeps each pressure gauge with its calibration history, certificates and next due date. One calibration record holds up to 100 test points. Each point has its nominal value, its tolerance as an absolute band, a percentage of nominal or a percentage of span, whether it was read up-scale or down-scale, and the as-found and as-left readings. The certificate Axiospec produces lists those points.
On an in-house calibration, a final reading outside its tolerance sets the result to Fail, and recording a pass anyway takes a written reason. A Fail puts the gauge in quarantine, and the record asks what the gauge measured since its last pass. Axiospec does not work out the hysteresis or choose the class or the interval. Those stay with you or your lab.
The free plan covers up to 50 active instruments. A new workspace starts with a 45-day trial of Professional, with room for 1,500 active instruments and no credit card. We load your existing records for you at no charge on every plan.
Common questions
- How do you calibrate a pressure gauge?
- Connect it to a reference, a deadweight tester or a more accurate reference gauge, in its working position and at the same height. Read the gauge at points spread over its range with the pressure rising, hold the top, then read the same points with the pressure falling. Compare each error and the hysteresis with the limit for its accuracy class, and record the readings before any adjustment.
- How often should pressure gauges be calibrated?
- No standard sets one interval for all gauges. Under normal use, DKD-R 6-1 recommends 1 year for Bourdon tube gauges of class 0.6 or better and 2 years for coarser classes, plus a recalibration after an overload beyond its limit, a repair or mishandling. Adjust the interval from each gauge's as-found history.
- What does accuracy class 1.6 mean on a pressure gauge?
- The class is the permissible error as a percentage of the gauge's range. Under OIML R 101, a class 1.6 gauge in service may be off by up to 1.6% of its upper range limit anywhere on the scale, which is ±0.16 bar on a 0 to 10 bar gauge. A new or repaired gauge has to meet 80% of that.
- How many test points does a pressure gauge calibration need?
- OIML R 101 asks for at least 8 points on gauges of class 0.6 and better, 5 on classes 1 to 2.5 and 3 on classes 4 and 5, read with the pressure rising and falling. DKD-R 6-1 uses 9 points in its sequences A and B and 5 in sequence C. ASME B40.100 gives a recommended number of test points for each accuracy grade.
- What is hysteresis in a pressure gauge?
- It is the difference between the gauge's readings at the same pressure when the pressure is rising and when it is falling. OIML R 101 does not let it exceed the gauge's maximum permissible error. A calibration only shows it when the points are read in both directions.
- Can I calibrate pressure gauges in-house?
- Yes, with a reference that has its own current calibration and an error no more than a quarter of the gauge's limit, a written procedure and trained people. The usual setup is a deadweight tester, or a calibrated reference gauge with a pressure source for the range. The reference itself goes out for calibration, usually to an accredited lab.
Put it into practice
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