Calibrating pressure measuring instruments: choosing test points, reference instrument and pressure generation correctly

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Pressure measuring instruments are used every day in many industrial plants: for process monitoring, quality assurance, maintenance, commissioning or safety monitoring. To ensure that pressure gauges, pressure sensors and pressure transmitters provide reliable values, they must be checked and calibrated regularly.

During pressure calibration, a pressure measuring instrument is compared with a suitable reference. It is checked whether the indicated pressure or the output signal is still within the permissible tolerance. A clean calibration setup, a suitable reference instrument, the correct pressure generation, suitable measuring points and traceable documentation are decisive.

This article explains how pressure measuring instruments are calibrated correctly, what needs to be considered for pressure gauges, pressure sensors and pressure transmitters, and how typical errors involving the reference instrument, pressure pump, pressure controller, venting, leak test, hysteresis and measurement uncertainty can be avoided.

You can find suitable devices and solutions in our category
Pressure calibration technology.
For mobile pressure calibration, for example, the
DPI610E / DPI610E-IS portable pressure calibrator
is suitable. For modular calibration tasks with pressure, electrical signals and documentation, the
DPI620 GENII pressure calibrator / process calibrator
can be used. As a precise pressure indicator or reference instrument, the
WIKA Type CPG1500 precision digital pressure gauge
is also relevant.

Why do pressure measuring instruments need to be calibrated?

Pressure measuring instruments can change over time. Causes include overload, pressure surges, vibrations, temperature changes, mechanical stress, contamination, ageing or an unsuitable medium. As a result, a measuring instrument may slowly drift or suddenly display incorrect values.

Regular calibration shows whether the pressure measuring instrument is still operating within the permissible tolerance. It makes deviations visible and helps decide whether the instrument should continue to be used, adjusted, repaired or replaced.

Reason for calibration	Why important?	Typical example
Process safety	Incorrect pressure values can lead to incorrect decisions.	Pressure transmitter displays values that are too low.
Quality assurance	Measured values must be reproducible and traceable.	Inspection equipment monitoring in production or laboratory.
Maintenance	Drift, leaks or overload are detected at an early stage.	Regular checking of pressure gauges and pressure sensors.
Audit / documentation	Calibration results must be verifiable.	Calibration protocol for ISO or customer audits.
Plant availability	Faulty measuring points can cause downtime.	Implausible pressure values in a process plant.

Which pressure measuring instruments are calibrated?

Many types of pressure measuring instruments can be calibrated. Depending on the instrument type, the calibration setup, measuring points, evaluation and documentation differ. A simple pressure gauge is checked differently from a pressure transmitter with 4 … 20 mA output or a pressure switch with switching point and reset point.

Pressure measuring instrument	What is checked?	Special feature
Mechanical pressure gauge	Indication at defined pressure points.	Observe reading accuracy, pointer position and hysteresis.
Digital pressure gauge	Digital display over the measuring range.	Observe resolution, zero point and temperature compensation.
Pressure sensor	Pressure value or sensor signal.	Check reference, supply and signal processing.
Pressure transmitter	Pressure and output signal, e.g. 4 … 20 mA.	Check scaling, zero point and span.
Pressure switch	Switching point and reset point.	Record hysteresis and repeatability.
Differential pressure instrument	Differential pressure between two connections.	Very low ranges require suitable references.

Calibration setup: device under test, reference and pressure generation

A calibration setup basically consists of three parts: the device under test, a reference and pressure generation. The device under test is the pressure measuring instrument to be calibrated. The reference is the more accurate comparison instrument. Pressure generation provides the required test pressure.

It is important that all components match the pressure range, medium, connection, accuracy requirement and application. Even small leaks, air in the hydraulic system or incorrect adapters can affect the result.

Component	Function	Practical example
Device under test	The pressure measuring instrument to be calibrated.	Pressure gauge, pressure sensor or pressure transmitter.
Reference instrument	Comparison value with known accuracy.	Precision digital pressure gauge, pressure calibrator or pressure module.
Pressure generation	Generates the desired test pressure.	Pneumatic pump, hydraulic pump or pressure controller.
Adapters and hoses	Connect device under test, reference and pressure source.	G1/4, G1/2, NPT, quick coupling or test hose.
Documentation	Records measuring points, deviations and evaluation.	Calibration protocol or calibration software.

Selecting the correct reference instrument

The reference instrument has a major influence on the significance of the calibration. It must be more accurate than the device under test, cover the appropriate pressure range and be suitable for the medium used.

A reference with an excessively large measuring range can be unfavorable at small test points because accuracy is often related to full scale. For a pressure transmitter with 0 … 1 bar, a reference with 0 … 1,000 bar should therefore not be used unnecessarily.

Criterion	Why important?	Practical recommendation
Measuring range	Reference must safely cover the test pressure.	Select a measuring range that matches the device under test as closely as possible.
Accuracy	Reference must be better than the device under test.	Observe accuracy ratio and measurement uncertainty.
Medium	Reference must be suitable for gas or liquid.	Check media compatibility and risk of contamination.
Pressure type	Gauge pressure, absolute pressure and differential pressure differ.	Select reference according to pressure type.
Calibration status	Reference must itself be validly calibrated.	Check certificate and calibration due date.
Resolution	A display that is too coarse makes evaluation difficult.	Select resolution according to tolerance.

As a reference, for example, a
WIKA Type CPG1500 precision digital pressure gauge,
a
DPI705E precision pressure / temperature indicator
or suitable
PM700E / PM700E-IS external pressure sensors
can be used.

Pressure generation: hand pump, calibration pump or pressure controller?

Pressure generation must match the pressure range, medium and accuracy requirement. For simple field checks, a manual calibration pump is often sufficient. For repeatable, automated or highly precise calibrations, a pressure controller may be useful.

Pressure generation	Suitable for	Advantage
Pneumatic hand pump	Low to medium pressures with air or gas.	Mobile, simple, clean and quickly usable.
Hydraulic hand pump	High pressures with liquid.	High pressures possible with a compact setup.
Precision calibration hand pump	Fine pressure adjustment and comparison measurement.	Well suited for pressure gauge and sensor testing in the field.
Portable pressure calibrator	Pressure generation, measurement and evaluation in one instrument.	Practical for maintenance and on-site calibration.
Pressure controller	Automated and reproducible calibration.	High stability, fast test sequences and less operator influence.

For mobile calibrations with integrated pressure generation, for example, the
DPI610E / DPI610E-IS portable pressure calibrator
is suitable. For test setups with a separate reference, an
ICP40.2 precision calibration hand pump
or a calibration case with hand pump and reference instrument can be useful.

Gas or liquid as pressure medium?

Pressure calibration distinguishes between pneumatic and hydraulic calibration. Pneumatic calibration is performed with air or gas and is often used for low to medium pressures. Hydraulic calibration is performed with liquid and is mainly used for higher pressures.

Medium	Typical use	To be considered
Air / gas	Low to medium pressures, clean applications.	Compressibility leads to slower stabilization processes.
Oil / hydraulic fluid	High pressures and hydraulic applications.	Observe venting, cleanliness and media compatibility.
Water	Certain clean hydraulic applications.	Check corrosion and material compatibility.
Process medium	Only for special applications.	Consider contamination, safety and cleaning.

It is important that the device under test and the reference are suitable for the medium used. A sensor that has previously been exposed to oil should not be used in an oxygen-related or hygiene-relevant application without checking.

Measuring points: why 0–25–50–75–100 % makes sense

A calibration is normally not performed at only one single point. To meaningfully evaluate the measuring range, several measuring points are distributed across the range. 0 %, 25 %, 50 %, 75 % and 100 % are often used.

Measuring point	Example for 0 … 10 bar	Why important?
0 %	0 bar	Check zero point and offset.
25 %	2.5 bar	Lower measuring range is evaluated.
50 %	5 bar	Check middle of the measuring range.
75 %	7.5 bar	Upper measuring range is evaluated.
100 %	10 bar	Check full scale and span.

Depending on the application, additional measuring points can be useful, for example near a switching point, a typical operating pressure or a critical process range.

Measuring upward and downward: detecting hysteresis

For many pressure measuring instruments, it is useful to approach the measuring points once in ascending order and once in descending order. This makes it possible to detect hysteresis, friction, mechanical effects or reset errors.

With a mechanical pressure gauge, the pointer may react slightly differently during pressure build-up than during pressure reduction. Pressure sensors and transmitters can also show different values during the upward and downward run depending on design and load.

Test	What is detected?	Typical application
Upward run	Measured value with increasing pressure.	Calibration points from 0 % to 100 %.
Downward run	Measured value with decreasing pressure.	Calibration points from 100 % back to 0 %.
Hysteresis	Difference between ascending and descending measured value.	Evaluation of mechanical or sensor-related reset effects.
Repeatability	Stability when repeatedly checking the same points.	Important for critical inspection equipment.

Leak test and venting before calibration

A calibration setup must be tight and stable. Even small leaks can cause the test pressure to slowly drop and distort the measured values. Especially during hydraulic tests, careful venting is also important.

Test step	Why important?	Practical recommendation
Leak test	Ensures that the pressure remains stable.	Build up test pressure and observe pressure drop.
Venting	Air in the hydraulic system makes pressure unstable.	Carefully vent the hydraulic system before calibration.
Check adapters	Incorrect or damaged seals cause leaks.	Use suitable threads, seals and adapters.
Wait for stabilization	Pressure, temperature and signal must be stable.	Document measured value only after stabilization.
Avoid overload	Device under test or reference can be damaged.	Observe measuring range and maximum permissible pressure.

An unstable calibration setup often leads to incorrect conclusions. For this reason, before the actual calibration, it should always be checked whether the setup is tight, vented and mechanically safe.

Calibrating pressure transmitters: checking pressure value and 4 … 20 mA

For a pressure transmitter, it is not sufficient to check only the pressure at the process connection. The output signal must also be evaluated. A transmitter with 0 … 10 bar and 4 … 20 mA should output approximately 4 mA at 0 bar and approximately 20 mA at 10 bar.

Pressure point	Example measuring range 0 … 10 bar	Expected 4 … 20 mA signal
0 %	0 bar	4 mA
25 %	2.5 bar	8 mA
50 %	5 bar	12 mA
75 %	7.5 bar	16 mA
100 %	10 bar	20 mA

If the pressure value is correct but the mA signal deviates, the error may be in the scaling, electronics, supply, wiring or input card. For such tasks, a process calibrator such as the
DPI620 GENII
is particularly useful because pressure and electrical signals can be evaluated together.

Checking pressure sensors and external pressure modules

Pressure sensors and external pressure modules are often used in calibration systems, mobile measuring instruments or test setups. These references and sensors must also be checked regularly because they are themselves part of the measuring chain.

Test point	Why important?	Note
Measuring range	Sensor must match the test pressure.	Do not use unnecessarily large ranges.
Pressure type	Gauge, absolute or differential pressure differ.	Incorrect pressure type leads to incorrect evaluation.
Accuracy class	Determines suitability as reference or device under test.	Compare calibration task with accuracy.
Media compatibility	Sensor must not be damaged by the medium.	Check materials and connections.
Calibration data	Reference must be valid and traceable.	Check calibration certificate and due date.

For flexible pressure ranges, for example,
PM700E / PM700E-IS external remote pressure sensors
can be used.

Calibrating pressure gauges and digital pressure gauges

With mechanical pressure gauges, the pointer indication is compared with the reference pressure. Good readability, stable pressure generation and consideration of hysteresis and zero point are important.

With digital pressure gauges, resolution, zero point, battery status, temperature influence and unit conversion also play a role. A precision digital pressure gauge can be used both as the device under test and as a reference if its accuracy and calibration match the task.

Instrument	What to consider?	Typical error
Mechanical pressure gauge	Pointer, scale, zero point, hysteresis.	Parallax error or incorrect reading.
Digital pressure gauge	Resolution, unit, zero point, battery.	Resolution too coarse for tight tolerance.
Precision pressure gauge	Calibration status and measurement uncertainty.	Used as reference even though certificate has expired.
Pressure gauge with diaphragm seal	Filling fluid, temperature, position and diaphragm.	Influence of diaphragm seal is not considered.

As a precise digital reference or for comparison measurements, for example, the
WIKA Type CPG1500 precision digital pressure gauge
can be used.

Measurement uncertainty, tolerance and evaluation

A calibration provides measured values and deviations. Whether a pressure measuring instrument has passed depends on the permissible tolerance and the measurement uncertainty of the calibration setup. Especially with tight tolerances, the reference must be significantly better than the device under test.

Term	Meaning	Example
Setpoint	Value of the reference or set test pressure.	5.000 bar.
Actual value	Indication or signal of the device under test.	5.025 bar.
Deviation	Difference between actual value and setpoint.	+0.025 bar.
Tolerance	Permissible maximum deviation.	±0.05 bar.
Measurement uncertainty	Uncertainty range of the calibration result.	Influence of reference, resolution, stability and method.

The evaluation should not be based on gut feeling alone. For a reliable statement, tolerance, reference accuracy, resolution, stability, temperature and test procedure must be considered together.

Documentation, as-found and as-left

A calibration is only traceable if it is properly documented. The values before any possible adjustment and after completion of the work are particularly important.

Documentation point	Meaning	Why important?
As-found	Condition before any possible adjustment.	Shows how the instrument measured during operation.
As-left	Condition after calibration or adjustment.	Shows how the instrument works after completion of the work.
Measuring points	Documented setpoints and actual values.	Basis for evaluation and traceability.
Reference instrument	Comparison instrument used with serial number.	Traceability and audit capability.
Environmental conditions	Temperature, air pressure or humidity, if relevant.	Influence on measurement and measurement uncertainty.
Evaluation	Within or outside tolerance.	Decision about further use.

For many inspection instruments, the as-found value is particularly important. If an instrument was outside tolerance before adjustment, previous measurements or processes may need to be evaluated.

Typical errors in pressure calibration

Many errors in pressure calibration are not caused by the pressure measuring instrument itself, but by the test setup. Common causes include incorrect references, leaks, air in the hydraulic system, stabilization times that are too short or unsuitable adapters.

Error	Possible consequence	Practical solution
Reference too inaccurate	Calibration result is not reliable.	Select reference according to tolerance.
Incorrect measuring range	Small deviations are not reliably detected.	Select reference range as closely as possible.
Leak in setup	Pressure slowly drops.	Check seals, adapters and hoses.
Air in hydraulic system	Pressure is difficult to stabilize.	Carefully vent the system.
Reading too quickly	Measured value is not yet stable.	Wait for stabilization time.
Incorrect medium	Device under test or reference can be damaged.	Check media compatibility in advance.
As-found not recorded	Previous operating condition is not known.	Always document as-found before any adjustment.
Overpressure	Damage to device under test or reference.	Observe maximum pressure and overload limits.

Which devices are suitable for pressure calibration?

The suitable device selection depends on whether calibration is to be performed in the field, in the laboratory, mobile, stationary, manually or automatically. Pressure range, accuracy, medium, electrical signal testing and documentation requirements are also decisive.

Product / category	Suitable for	Typical application
Pressure calibration technology	Overview of pressure calibrators, pressure modules, calibration pumps, pressure controllers and accessories.	Preselection of suitable devices for pressure calibration.
DPI610E / DPI610E-IS portable pressure calibrators	Mobile pressure calibration in the field.	Generate and measure pressure and check transmitters.
DPI620 GENII pressure calibrator / process calibrator	Modular pressure, electrical and process calibration.	Calibration of pressure transmitters, process instruments and measuring chains.
DPI705E precision pressure / temperature indicator	Precise pressure measurement, leak test, maintenance and troubleshooting.	Reference indicator or mobile test instrument.
WIKA Type CPG1500 precision digital pressure gauge	Precise pressure indication and comparison measurement.	Reference pressure gauge for pressure gauge and sensor testing.
PM700E / PM700E-IS external remote pressure sensors	Flexible pressure ranges and external sensors.	Expansion of calibration systems and mobile pressure measurement.
ICP40.2 precision calibration hand pump	Manual pressure generation with fine adjustment.	Comparison measurements with separate reference instrument.
4Sight2 calibration software	Documentation and management of calibration processes.	Calibration protocols, inspection equipment management and digital records.

Practical examples from maintenance, laboratory and process industry

Example 1: Checking a pressure gauge on a hydraulic system

A pressure gauge on a hydraulic system is to be checked. The device under test is connected to a hydraulic calibration pump together with a reference pressure gauge. The measuring points are approached upward and downward. This makes it possible to detect indication deviation and hysteresis.

Example 2: Calibrating pressure transmitter with 4 … 20 mA

A pressure transmitter with 0 … 10 bar and 4 … 20 mA is checked. At 0, 2.5, 5, 7.5 and 10 bar, both the reference pressure and the output signal are measured. Deviations can therefore be assigned to the pressure sensor, transmitter scaling or current signal.

Example 3: Detecting leakage in a pressure test

During calibration, the pressure slowly drops. At first, it is suspected that the device under test is defective. After checking, however, it becomes clear that a screw connection in the test setup is leaking. A short leak test before calibration would have made this error immediately visible.

Example 4: Incorrect reference for small measuring range

A sensor with 0 … 1 bar is checked with a reference up to 1,000 bar. The resolution and accuracy of the reference are not ideal for this small range. A suitable low-pressure reference significantly improves the significance of the calibration.

Example 5: Checking pressure switch switching point

A pressure switch is supposed to switch at 6 bar. The pressure is slowly increased until the switching point is reached. The pressure is then reduced again until the reset point is recorded. This provides the switching point, reset point and hysteresis.

Checklist: calibrating pressure measuring instruments correctly

This checklist helps to better prepare and systematically perform pressure calibration.

Check question	Why important?	Practical recommendation
Which pressure range is to be checked?	Reference and pressure generation must match it.	Clarify measuring range and maximum permissible pressure.
Which pressure type is present?	Gauge, absolute and differential pressure differ.	Compare pressure type of device under test and reference.
Which tolerance applies?	Without tolerance, no evaluation is possible.	Check manufacturer specification, process requirement or QA requirement.
Is the reference suitable?	The reference determines the quality of the result.	Check accuracy, measuring range and calibration status.
Which medium is used?	Gas and liquid have different requirements.	Observe media compatibility and venting.
Is the setup leak-tight?	Leaks distort the measurement.	Perform leak test before calibration.
Have suitable measuring points been selected?	The measuring range must be covered meaningfully.	Typical 0–25–50–75–100 % upward and downward.
Is the measured value stable?	Reading too early leads to incorrect results.	Wait for stabilization time.
Is as-found documented?	The original condition is important.	Record as-found before any adjustment.
Is the documentation complete?	Calibration must be traceable later.	Record device under test, reference, measuring points, deviation and evaluation.

Conclusion: good pressure calibration starts with the right setup

Anyone who wants to calibrate a pressure measuring instrument needs more than just a test pressure. A suitable device under test, a suitable reference, stable pressure generation, leak-tight connections, meaningful measuring points and clean documentation are decisive.

For pressure gauges, indication deviation, zero point and hysteresis are the main focus. For pressure sensors and pressure transmitters, electrical signals, scaling and supply must also be taken into account. For high pressures, hydraulic calibration is useful, while pneumatic systems are often used for low and medium pressures.

For mobile pressure calibrations, suitable instruments include, for example, the
DPI610E / DPI610E-IS,
the
DPI620 GENII,
the
DPI705E,
the
WIKA Type CPG1500
or suitable
PM700E pressure sensors.
You can find a complete overview in the category
Pressure calibration technology.

FAQ: frequently asked questions about calibrating pressure measuring instruments

What does calibrating a pressure measuring instrument mean?

Calibrating a pressure measuring instrument means that the indication or output signal of a pressure measuring instrument is compared with a reference instrument. The deviation is documented and evaluated.

How do you calibrate a pressure gauge?

A pressure gauge is subjected to a defined test pressure and compared with a reference instrument. Typical measuring points are 0 %, 25 %, 50 %, 75 % and 100 % of the measuring range, often upward and downward.

How do you calibrate a pressure transmitter?

For a pressure transmitter, the applied pressure is compared with a reference. In addition, the output signal is checked, for example 4 … 20 mA. This allows pressure measurement, scaling and signal processing to be evaluated.

Which reference do I need for pressure calibration?

The reference must match the pressure range, pressure type, medium and required accuracy. It should be more accurate than the device under test and be validly calibrated itself.

Which pressure pump is suitable for calibration?

Pneumatic hand pumps are often used for low to medium pressures. Hydraulic calibration pumps are used for high pressures. Pressure controllers can be useful for automated sequences.

Is calibration performed with gas or liquid?

This depends on the pressure range and application. Pneumatic calibration with air or gas is suitable for low to medium pressures. Hydraulic calibration with liquid is suitable for high pressures.

Which measuring points are useful for pressure calibration?

Frequently, 0 %, 25 %, 50 %, 75 % and 100 % of the measuring range are checked. Depending on the application, additional points near the operating pressure, switching point or critical range can be useful.

Why is measurement performed upward and downward?

By measuring at increasing and decreasing pressure, hysteresis, friction, reset errors or mechanical effects can be detected. This is especially important for pressure gauges and pressure switches.

Why is a leak test important before calibration?

A leak in the test setup causes the pressure to drop and the measured values to become unstable. A short leak test helps detect setup errors before the actual calibration.

Why must a hydraulic calibration setup be vented?

Air in the hydraulic system is compressible and makes the pressure unstable. This makes setting the measuring points more difficult and can distort the calibration.

Which products are suitable for calibrating pressure measuring instruments?

Depending on the application, suitable products include, for example, the
DPI610E / DPI610E-IS portable pressure calibrator,
the
DPI620 GENII pressure calibrator / process calibrator,
the
DPI705E precision pressure / temperature indicator,
the
WIKA Type CPG1500 precision digital pressure gauge,
suitable
PM700E pressure sensors
or an
ICP40.2 precision calibration hand pump.