Pressure Calibration with a Deadweight Tester¶
A deadweight tester generates a known pressure by balancing calibrated masses on a piston of accurately known effective area, so that pressure is derived from mass, local gravitational acceleration, and area rather than from an electrical sensor. It is a primary pressure standard, and it is what other pressure instruments are calibrated against.
How a deadweight tester works¶
The instrument is a vertically mounted, precision-lapped piston in a matched cylinder. Fluid pressure below the piston pushes it upward; calibrated masses stacked on the piston carrier push it down. When the piston floats freely between its end stops, the two forces balance, and the pressure in the system equals the total downward force divided by the effective area of the piston-cylinder assembly.
Pressure equals the total mass on the piston, multiplied by local gravitational acceleration, divided by the effective area of the piston-cylinder assembly.
Every term on the right is measured independently and traceably. That is the whole argument for the technique: no part of the pressure value depends on a transducer that could drift.
Three details separate a real measurement from the idealised equation:
- The piston is rotated during measurement. Rotation converts static friction between piston and cylinder into a much smaller and more consistent dynamic friction, and it is what makes the balance reproducible rather than sticky.
- Effective area is not the geometric area. Fluid penetrates the annular gap between piston and cylinder, and the effective area lies between the piston and cylinder diameters. It also changes slightly with pressure and temperature, which is why the corrections exist.
- The fluid head matters. Pressure at the device under test differs from pressure at the piston reference level by the weight of the oil column between them. At high accuracy this is corrected, not ignored.
Applying local gravity, temperature, buoyancy, and head corrections is what Fluke calls full correction, and the published uncertainty applies only when it is done.
When to use a deadweight tester¶
- Calibrating analogue pressure gauges against a traceable reference
- Verifying pressure transducers, transmitters, and pressure switches
- Establishing or checking a laboratory's own working pressure standards
- Periodic verification where an audit trail to a primary standard is required
- Any measurement where the reference must not itself depend on an electronic sensor
What a deadweight tester cannot do¶
- It is not fast. Each point requires loading masses, generating pressure, floating the piston, and letting it settle. A multi-point calibration is a deliberate, manual procedure.
- It generates discrete pressures. Available points are set by the mass set. Arbitrary intermediate pressures require trimming, or a different instrument.
- It does not measure dynamic pressure. Transients, pulsation, and fast events are outside the method entirely.
- It is oil-wetted. Any device that cannot tolerate hydraulic oil, or that must stay clean for oxygen or medical service, cannot be connected to it.
- The uncertainty is conditional. Without full correction, the published 0.015 % of reading does not apply.
- Range is bounded below. The P3114 starts at 200 psi. Low-pressure and vacuum calibration needs a different instrument.
The P3114 at MPaCT Lab, Flagstaff, Arizona¶
The MPaCT Lab at Northern Arizona University operates a Fluke Calibration P3114 hydraulic oil deadweight tester in Flagstaff, Arizona, covering 200 to 10,000 psi. It is part of Fluke's P3000 series, and it supplies the traceable reference against which pressure instrumentation used elsewhere in the lab is verified.
Flagstaff's elevation of roughly 2,100 metres makes the local gravity correction a real term rather than a formality, and it is applied as part of every calibration performed here.
The instrument is available to NAU researchers, external academic users, and industry partners, on a fee-for-service basis or as a trained hands-on user.
| Specification | Value |
|---|---|
| Pressure range | 200 to 10,000 psi (20 to 700 bar; 2 to 70 MPa) |
| Measurement uncertainty | ± the greater of 0.015 % of reading or 0.00075 % of range, full correction |
| Method | Hydraulic piston-cylinder deadweight, piston rotated in use |
| Operating fluid | Shell Spindle Oil 22 |
| Series | Fluke Calibration P3000 hydraulic |
| Typical use | Calibration of gauges, transducers, transmitters, and pressure switches |
Specifications are as published by Fluke Calibration for the P3100 series.
Full P3114 specifications and booking →
What to submit¶
- The device. With its port thread and size identified. Adapters are not always available on short notice.
- The range and the points. Which pressures you need checked, and in which units.
- Oil compatibility. Confirm the device may be wetted with hydraulic oil. If it must stay oil-free, say so before submission.
- The acceptance criterion. The tolerance the device is expected to meet, and whether you need as-found and as-left data.
- Documentation needs. Whether a calibration record with traceability statements is required.
Frequently asked questions¶
What is a deadweight tester?¶
A deadweight tester generates a known pressure by balancing calibrated masses on a vertical piston of accurately known effective area. Pressure equals force divided by area, and the force is the weight of the masses, so the generated pressure follows from mass, local gravity, and area alone. Nothing is sensed electronically. This is why it is classed as a primary standard, and why other pressure instruments are calibrated against it rather than the reverse.
Why does a deadweight tester need local gravity?¶
Because the pressure comes from weight, not mass. Weight is mass multiplied by local gravitational acceleration, and that acceleration varies with latitude and elevation by enough to matter at this level of uncertainty. A tester calibrated at one location and used at another without correcting for the difference will carry a systematic error. Flagstaff sits at about 2,100 metres, which makes the correction non-negligible.
What accuracy can a deadweight tester achieve?¶
For the P3114, ± the greater of 0.015 % of reading or 0.00075 % of range, when full correction is applied. Full correction means accounting for local gravity, temperature effects on the piston-cylinder area, air buoyancy on the masses, and the head of fluid between the piston reference level and the device under test. Omitting those corrections gives a larger and poorly characterised uncertainty.
Can a deadweight tester calibrate a digital pressure transmitter?¶
Yes. The tester generates the reference pressure and the transmitter reports what it measures; the difference at each point is the error. Any device that can be connected to a hydraulic port and read at a stable pressure can be calibrated this way, including analogue gauges, transducers, transmitters, and pressure switches. The practical constraints are the port fitting, the pressure range, and compatibility with hydraulic oil.
Request time on this instrument¶
MPaCT Lab - Building 98E, South Engineering Lab
561 E Pine Knoll Dr, Flagstaff, AZ 86001
Phone: 928-523-2343 · Email: mpct.nano@nau.edu