Piston gauges – also known as pressure balances or deadweight testers – are among the most accurate references available for pressure calibration. The reference pressure is not measured using an electronic pressure sensor, but is derived from the gravitational force of defined masses and the effective area of a precision piston-cylinder system.
The basic mechanical principle is straightforward. In practical use, however, even minor operating errors can influence the result. A piston that does not rotate freely, contaminated oil, trapped air, an incorrect floating position or unaccounted temperature and gravity values can reduce repeatability and increase measurement uncertainty.
For reliable results, the instrument, piston-cylinder system, mass set, pressure medium, device under test and ambient conditions must be considered as one complete calibration setup. The decisive factor is not only which masses are applied, but also whether the piston moves freely and reproducibly under stable conditions.
Suitable systems can be found in the piston gauges, pressure balances and comparison pumps category. Additional pressure references, pumps, controllers, adapters and calibration software are grouped together under pressure calibration equipment.
Contents
- How does a piston gauge generate the reference pressure?
- Preparing the installation, alignment and environment
- Why is cleanliness so important?
- Completely venting the hydraulic system
- Correctly selecting and applying mass sets
- Correctly setting the piston rotation
- Assessing the floating position and stabilisation
- Correcting for gravity, air buoyancy and temperature
- Considering the height difference to the device under test
- Normal piston fall or leakage?
- When must the pressure medium be replaced?
- Recommended operating procedure
- Typical operating errors
- Practical example: Piston rotates only briefly and falls rapidly
- Which products are suitable?
- Conclusion
- Frequently asked questions
How does a piston gauge generate the reference pressure?
The piston-cylinder system contains a precisely manufactured piston that moves inside a matching cylinder. Calibrated masses are placed on the piston or mass carrier. The pressure results from the applied force and the effective piston area.
In simplified form:
p = F / A = m × g / A
Where:
- p: generated reference pressure,
- m: effective total mass including the piston and mass carrier,
- g: local acceleration due to gravity,
- A: effective area of the piston-cylinder system.
This simplified equation is not sufficient for high-accuracy calibrations. Depending on the instrument and uncertainty requirements, air buoyancy, temperature, pressure-dependent deformation, medium density and height differences are also taken into account.
The required pressure is reached when the piston floats freely within the defined operating range and rotates with minimal friction. If the piston rests against the lower or upper stop, a valid equilibrium condition has not been achieved.
Preparing the installation, alignment and environment
A piston gauge should be installed on a stable, level and vibration-free work surface. Even a significant inclination creates lateral forces between the piston and cylinder and may impair free movement.
The following points should be checked before starting:
- level the instrument using the integrated or an external spirit level,
- use a stable laboratory bench or workbench,
- avoid vibrations from pumps, machinery and doors,
- avoid direct sunlight and heat sources,
- allow sufficient time for thermal stabilisation,
- avoid strong air currents when using pneumatic low-pressure systems,
- secure the device under test safely and without mechanical stress.
If the instrument has been moved from a cold storage area into a warm laboratory, precision calibration should not begin immediately. The piston-cylinder system, masses, pressure medium and device under test require time to reach equilibrium with the ambient temperature.
Why is cleanliness so important?
There is only a very small, defined clearance between the piston and cylinder. Dust, metal particles, fibres or contaminated pressure medium can increase friction, slow down the rotation and prevent the piston from moving freely.
Contamination entering the pressure balance from a device under test is particularly critical. A measuring instrument previously operated with process oil, water or aggressive media must not be connected to a clean reference system without suitable separation.
The following rules apply for clean operation:
- use only the pressure medium approved by the manufacturer,
- clean all connections before assembly,
- close open connections immediately using protective caps,
- handle mass sets only with clean hands or suitable gloves,
- do not touch the piston and cylinder with bare fingers,
- use lint-free cleaning materials,
- connect contaminated devices under test through a media separator or another suitable isolation system.
A piston-cylinder system must not be ground, polished or lapped without authorisation. Scratches, corrosion or stiff movement require professional inspection or repair.
Completely venting the hydraulic system
Trapped air is a frequent cause of slow stabilisation and unstable calibration points. Hydraulic oil is only slightly compressible, whereas an air bubble can be compressed significantly. The volume controller then moves without the pressure responding immediately and reproducibly.
Indications of trapped air include:
- an unusually large movement of the volume controller,
- delayed pressure generation,
- spring-like behaviour during fine adjustment,
- sudden changes in the piston floating position,
- long stabilisation times after a pressure change.
To vent the system, it is filled with pressure medium in accordance with the operating instructions. Pipes and the device under test should be arranged so that air can rise towards the intended venting point. All valves are operated in the specified sequence until no further air bubbles escape.
Devices under test with large internal volumes or unfavourably positioned pressure channels may require repeated venting.
Correctly selecting and applying mass sets
The masses belong to a specific piston-cylinder system and frequently to a specific serial number or calibration configuration. Mass sets from different instruments must not be mixed without a metrological assessment.
The following should be checked before applying the masses:
- correct pressure unit and piston range,
- allowance for the piston and mass carrier,
- correct order and combination of the masses,
- agreement with the calibration certificate or calculation software,
- absence of contamination, corrosion or mechanical damage,
- sufficient load capacity for the required pressure.
The masses are applied carefully, centrally and without impact loading. Incorrectly seated or tilted masses can generate lateral forces. Masses must not be placed or removed abruptly while the piston is floating freely.
The system must be depressurised in a controlled manner before the mass set is removed completely.
Correctly setting the piston rotation
Rotation reduces the influence of static friction between the piston and cylinder. The piston is set into gentle rotation in accordance with the operating instructions. Smooth, free rotation is an important indication of a clean and correctly aligned setup.
The rotation should be moderate. Rotating the piston as quickly as possible does not automatically improve the measurement. Excessive speed can unnecessarily disturb the pressure medium, extend the stabilisation period or cause the mass set to become unstable.
If the piston stops abruptly after only a few seconds, possible causes include:
- the piston gauge is not level,
- the piston-cylinder system is contaminated,
- the pressure medium is unsuitable or too viscous,
- the temperature is too low,
- lateral loading caused by incorrectly positioned masses,
- mechanical damage to the piston or cylinder.
The piston must not be rotated using pliers or an unsuitable tool. The intended rotating device or mass carrier must be used.
Assessing the floating position and stabilisation
The piston has a defined stroke range. For measurement, it is brought into the intended floating or operating position using the volume controller or pressure-generation system.
A position within the middle of the range is generally preferred. However, the exact marking and permissible height are specific to the instrument.
A measured value should only be recorded when:
- the piston is floating freely,
- neither stop is being contacted,
- the rotation is uniform,
- the floating position changes only slowly and reproducibly,
- the device under test and reference are thermally stable,
- no immediate readjustment is required.
A piston falling slowly may be normal due to the design. A very small quantity of pressure medium flows through the defined clearance between the piston and cylinder. The decisive factor is whether the fall rate remains within the manufacturer’s specifications and behaves reproducibly.
Correcting for gravity, air buoyancy and temperature
Local acceleration due to gravity
The gravitational force depends on the local acceleration due to gravity. This differs from the standard value depending on the geographical location and elevation. If the mass set has already been adjusted for the installation location, the same correction must not be applied again.
The information in the calibration certificate is therefore authoritative:
- reference value for acceleration due to gravity,
- actual installation location,
- correction factor or corrected mass values,
- associated measurement uncertainty.
Air buoyancy
Masses placed in air experience a small buoyancy force. For low measurement uncertainties, the effective mass is corrected using the air density and mass density. The air density depends on factors including atmospheric pressure, temperature and humidity.
Whether a separate buoyancy correction is required depends on the accuracy class, calibration certificate and evaluation method. Many calculation programs perform this correction automatically.
Temperature of the piston-cylinder system
The dimensions and therefore the effective piston area change with temperature. The measured temperature is related to the reference temperature stated in the calibration certificate.
The temperature also affects the viscosity of the hydraulic pressure medium. Cold oil can result in slow rotation and longer stabilisation times. Strongly heated oil may become less viscous and increase the piston fall rate.
Considering the height difference to the device under test
If the measuring diaphragm of the device under test is not at the same height as the reference level of the piston gauge, the liquid column creates an additional hydrostatic pressure.
In simplified form:
Δp = ρ × g × Δh
Where:
- ρ: density of the pressure medium,
- g: local acceleration due to gravity,
- Δh: height difference between the two reference levels.
In hydraulic systems, even a moderate height difference can be relevant for precision calibrations. The reference height of the piston and the measuring diaphragm or pressure connection of the device under test should therefore be documented.
Normal piston fall or leakage?
A slowly falling piston does not automatically mean that the external test setup is leaking. A small internal leakage flow within the piston-cylinder clearance is part of the operating principle.
The following may indicate an additional leak or fault:
- a considerably higher fall rate than usual,
- the piston cannot be maintained in the floating position,
- continuous and substantial readjustment is required,
- visible pressure-medium leakage at fittings,
- air bubbles or foaming oil,
- different behaviour during repeated calibration points.
To isolate the cause, the device under test can be replaced with a suitable blanking plug. If the behaviour remains unchanged, the cause is probably located in the reference setup. If stability improves significantly, the device under test, adapter or its connection should be investigated.
When must the pressure medium be replaced?
The replacement interval is not determined solely by a fixed calendar period. Usage, connected devices under test, the environment and the condition of the medium are decisive.
Replacement or inspection is advisable when:
- the oil is visibly dark or cloudy,
- particles or deposits are visible,
- water or another medium has entered the system,
- the piston-cylinder system becomes unusually stiff,
- the viscosity appears to have changed significantly,
- a contaminated or medium-filled device under test has been connected,
- replacement is specified in the manufacturer’s maintenance schedule.
Only the approved pressure medium may be used during replacement. Mixing different oils can change the viscosity, material compatibility and metrological behaviour.
Recommended operating procedure
- Place the piston gauge on a stable work surface and level it.
- Check the instrument, mass set and piston-cylinder system for cleanliness.
- Check the approved pressure medium and ensure that the fill level is sufficient.
- Connect the device under test using suitable adapters.
- Completely fill and vent the hydraulic system.
- Allow the reference, masses and device under test to reach thermal equilibrium.
- Determine the required masses using the certificate or software.
- Apply the masses carefully and centrally.
- Generate pressure slowly to just below the target value.
- Set the piston into moderate rotation.
- Use the fine adjustment to establish the required floating position.
- Wait for stabilisation and record the reference and device-under-test values.
- Approach further calibration points in ascending and descending order.
- After completion, release the pressure completely in a controlled manner.
- Only then remove the masses and disconnect the fittings.
Typical operating errors
| Error | Possible consequence | Suitable measure |
|---|---|---|
| Instrument not levelled | Lateral friction and poor piston rotation | Check the spirit level before every measurement series |
| Piston resting against a stop | No valid equilibrium condition | Set the required floating position |
| Piston rotated too quickly | Unstable mass set and longer stabilisation time | Use only moderate rotation |
| Air in the hydraulic system | Spring-like behaviour and slow pressure adjustment | Vent the system completely |
| Contaminated oil | Friction, wear and poor repeatability | Clean the system and replace the medium with an approved product |
| Mass sets from different systems mixed | Incorrect reference pressure | Check assignment and serial numbers |
| Local gravity not considered | Systematic pressure error | Compare the calibration certificate with the installation location |
| Temperature read immediately after setup | Drift caused by thermal equilibration | Allow sufficient stabilisation time |
| Height difference ignored | Hydrostatic pressure error | Determine and correct the reference levels |
| Device under test disconnected under pressure | Pressure-medium leakage and risk of injury | Fully depressurise the system before disassembly |
Practical example: Piston rotates only briefly and falls rapidly
With a hydraulic pressure balance, a test pressure of 100 bar can be generated, but the piston stops after only a short period. At the same time, it rapidly falls from the middle floating position, making constant readjustment necessary.
All external fittings are checked first. No visible pressure-medium leakage is found. The behaviour also remains unchanged when the device under test is replaced with a blanking plug.
Further inspection reveals that the oil is cloudy and contains small particles. An older pressure gauge filled with process oil had previously been connected without a media separator. Air is also trapped in a high section of the connecting line.
The system is cleaned in accordance with the manufacturer’s instructions, refilled with approved oil and completely vented. After thermal equilibration, the instrument is levelled again.
The piston can now be rotated uniformly and remains within the specified operating range for considerably longer. The calibration points are reproducible and require significantly less readjustment.
This example shows that poor piston rotation does not automatically indicate a worn piston. The installation, cleanliness, pressure medium and venting should be checked first.
Which products are suitable?
Piston gauges, pressure balances and comparison pumps
The piston gauges, pressure balances and comparison pumps category contains pneumatic and hydraulic systems for different pressure ranges and accuracy requirements.
Depending on the application, a complete solution includes:
- piston-cylinder system,
- mass set with calibration certificate,
- pressure generation and volume fine adjustment,
- adapters and connections for the device under test,
- approved pressure medium,
- ambient-condition measuring equipment,
- calculation and calibration software.
WIKA CPB3800
The WIKA CPB3800 pressure balance is a compact hydraulic piston gauge for industrial and calibration laboratories as well as suitable on-site applications.
It is particularly suitable for users requiring a self-contained mechanical reference system with integrated hydraulic pressure generation.
WIKA CPB5800
The WIKA CPB5800 is designed for particularly demanding hydraulic pressure calibrations. Different measuring ranges and accuracy levels are available depending on the piston-cylinder system and mass set.
WIKA CPB3500
The WIKA CPB3500 pneumatic pressure balance is suitable for gas-based pressure calibrations where a clean, oil-free test setup and lower pressure ranges are required.
CPU6000 CalibratorUnit
The CPU6000 CalibratorUnit assists with the acquisition and processing of ambient, piston and mass data. In combination with suitable software, correction values can be calculated and calibration results documented.
Conclusion: The accuracy of a pressure balance also depends on correct operation
A piston gauge generates pressure directly from mass, acceleration due to gravity and the effective piston area. It therefore provides a highly stable and traceable pressure reference. However, the achievable quality depends significantly on the way the instrument is operated.
The piston must rotate freely and float within the specified operating range. Contaminated medium, air bubbles, incorrect alignment, unsuitable masses or readings taken too early can significantly reduce repeatability.
For low measurement uncertainties, the local acceleration due to gravity, air buoyancy, temperature and height difference must also be considered. The information in the calibration certificate and operating instructions for the specific system is authoritative.
A clean, level and thermally stable setup is therefore just as important as a high-quality piston-cylinder system. Only when both requirements are met can the piston gauge reliably perform its function as a highly accurate pressure reference.
Frequently asked questions about operating piston gauges
Why must the piston rotate?
Rotation reduces the influence of static friction between the piston and cylinder. This allows a more stable equilibrium condition to be established.
How quickly should the piston rotate?
It should rotate uniformly and at a moderate speed. The highest possible speed is not required. The exact operating procedure depends on the instructions for the respective system.
Why does the piston not rotate freely?
Possible causes include incorrect alignment, contamination, unsuitable or cold oil, tilted masses or a damaged piston-cylinder system.
Must the piston float at an exact height?
Yes. The measurement is performed within the operating range defined by the manufacturer, frequently within the middle of the stroke. The piston must not contact either the upper or lower stop.
Is a slowly falling piston normal?
A slight downward movement may be normal due to the designed leakage flow within the piston-cylinder clearance. An unusually rapid or irregular movement must be investigated.
How often must the oil be replaced?
This depends on usage and the risk of contamination. Cloudiness, particles, foreign media, changed viscosity or unusual piston behaviour are reasons to inspect or replace the oil.
Why is the local acceleration due to gravity important?
The gravitational force of the masses depends on the local gravity value. If a mass set is used at a different location, a systematic pressure error may occur unless the appropriate correction is applied.
Must air buoyancy always be corrected?
This depends on the accuracy requirements and evaluation method. In high-accuracy applications, the effect of air density on the effective mass is taken into account.
Why is the piston temperature relevant?
The effective piston area changes as a result of thermal expansion. The temperature also influences the viscosity of the hydraulic pressure medium and therefore the rotation and piston fall rate.
May a contaminated piston be polished by the user?
No. Unauthorised grinding, polishing or lapping changes the precision geometry. Cleaning and repair must be carried out in accordance with the manufacturer’s instructions.