What a pressure calibrator is
A pressure calibrator generates or measures a known pressure so that another instrument can be checked against it. It is the reference in the comparison, not the instrument under test, and the whole value of the exercise depends on the calibrator being more accurate than the device being checked by a known margin.
Two figures decide the choice. The first is the pressure range, which has to cover the device under test without the calibrator working at the very bottom of its own span. The second is the accuracy statement, and how it is expressed matters as much as its size: a figure quoted against full scale represents the same absolute uncertainty at any point in the range, while one quoted against the reading becomes smaller as the pressure falls. On a device tested across its range, that difference changes the verdict.
Many duties also have an electrical side. A pressure transmitter produces an output proportional to pressure, so checking it means applying a pressure and reading the resulting signal - and often supplying the loop while measuring the current as well. Instruments that combine the two functions cover that in one pass; instruments that do not need a second device alongside them.
How a pressure calibrator measures
The reference inside the calibrator is a pressure sensor of known quality, and the calibration chain runs from a national standard through successive comparisons to that sensor. The certificate supplied with the instrument records what the sensor was found to be at the time of comparison, so the uncertainty of the calibrator is an input into the uncertainty of every check made with it.
How the pressure is generated is a separate question from how it is measured. A hand pump built into the instrument, an electric pump, or an external supply all produce the pressure; the calibrator reads it. The medium matters at the same time: pneumatic generation suits lower pressures and clean dry gas, while hydraulic generation is used at higher pressures, where compressed gas would store too much energy to be safe.
The comparison is only as good as the ratio between the two uncertainties. Where the calibrator is only slightly more accurate than the device being checked, an apparent pass or fail at the tolerance limit cannot be relied on, because the measurement uncertainty overlaps the tolerance itself. That ratio is a criterion in most quality systems, and it sets a floor on the calibrator long before convenience does.
Configuration choices
Most of the specification is fixed by these decisions rather than by the accuracy class. Each one constrains the next, which is why they are settled in order.
| Configuration | Options to fix | What it actually decides |
| Pressure range and number of ranges | A single span, or several selectable ranges | Whether the device under test sits comfortably inside the range or at the bottom of it, where the relative uncertainty is largest. |
| Accuracy statement and its form | A figure quoted against full scale, or against the reading | The absolute uncertainty at the point being tested. The two forms diverge most at low pressures, which is where the choice usually matters. |
| Internal or external pressure reference | A sensor built into the instrument, or an interchangeable module | Whether one instrument can cover several ranges, and whether the reference can be exchanged or recalibrated on its own. |
| Pressure generation | A hand pump, an electric pump, or an external supply | How the pressure is produced and how much effort the test takes. An electric pump reduces operator effort over a long sequence. |
| Pressure medium | Pneumatic for lower pressures, hydraulic for higher ones | The energy stored in the system and the safety of the work, as well as whether the device under test is a gas or a liquid pressure instrument. |
| Electrical measurement and sourcing | The ability to read and generate current, voltage and resistance signals | Whether a transmitter can be checked end to end in one pass, and whether the loop can be energised from the calibrator. |
| Loop supply | Whether the instrument can power a two-wire transmitter | Whether the device under test has to be powered separately while it is being checked. |
| Documentation and logging | Whether readings can be stored and certificates generated | Whether the calibration record is produced automatically or assembled by hand, which on a large instrument list is the difference between a day and a week. |
How to choose a pressure calibrator
| Factor | Why it decides the answer | How to settle it before enquiring |
|---|
| Accuracy ratio against the device under test | The calibrator has to be more accurate than the tolerance by a defined margin for the result to mean anything | Give the tolerance on the device, and the ratio the quality system requires. |
| Pressure range of the devices | A device tested across its range should not sit at the very bottom of the calibrator’s span | Give the range and the test points, including the lowest one. |
| Whether vacuum is needed | Not every generation method reaches a vacuum, and some devices are tested below atmospheric | State whether vacuum or negative gauge pressure forms part of the test. |
| The electrical side of the test | Transmitters, switches and indicators each require something different from the calibrator | Describe what is being checked: a gauge, a transmitter with an output, a switch, or a loop. |
| Pressure medium and safety | The stored energy at the test pressure decides whether pneumatic generation is acceptable | State the pressure to be reached, and whether the device contains gas or liquid. |
| Where the work is done | A workshop bench and a plant location impose different requirements on size, power and portability | State whether calibration is a bench activity or is performed in the field. |
| Certificate requirement | A traceable certificate for the calibrator, and a record for each device checked | State what has to be issued, and to what standard. |
If any of these are still open, send what is known. A quotation can be issued against a duty description and refined once the gaps are filled.
The nearest alternatives
These are the types an enquiry for a pressure calibrator usually turns out to be, and the point at which the choice is genuinely decided.
| Nearest alternative | What separates it from a pressure calibrator |
| Multifunction calibrators | Cover pressure alongside temperature and electrical signals in one instrument, which suits a team working through a mixed instrument list. The trade is usually in the pressure specification at a given cost, so where pressure is the dominant duty a dedicated instrument may represent better value. |
| Process calibrators | Built around the loop rather than around pressure: sourcing and reading the standard process signals, and often supplying the loop. Where the task is checking a transmitter’s output rather than its pressure element, the process calibrator is the instrument and pressure may not be needed at all. |
| Temperature calibrators | Provide a controlled temperature as the reference, which is a different physical quantity and a different generation problem. Pressure and temperature are frequently both needed on one instrument list, and neither instrument replaces the other. |
| Electrical calibrators | Produce and measure electrical quantities against a reference standard, and are what a loop check is built on. A pressure calibrator with electrical functions overlaps that ground partially; where the electrical work stands alone, a dedicated instrument is the better choice. |
| Calibration pumps | Generate pressure without providing the reference measurement, and are used together with a separate gauge or calibrator. Where a reference instrument is already held, a pump alone is enough; where it is not, a combined instrument is the practical route. |
| Pressure test pumps | Produce a pressure for a proof or leak test rather than for a calibration. The purpose is to demonstrate that a system holds pressure rather than to compare a reading against a reference, so the accuracy demand is quite different. |
The commonest error here is choosing on range alone. A calibrator that covers the pressure but is barely more accurate than the device under test cannot produce a result that stands up, however convenient it is to use.
What to include in the enquiry
The first reply can be a quotation rather than a set of questions if the enquiry carries these. Where a figure is not known, send the duty description and we will confirm what it implies.
- The device under test: type, range, output and tolerance
- The accuracy ratio required by the quality system
- Test points, including the lowest pressure to be checked
- Whether vacuum or negative gauge pressure is involved
- Whether an electrical output has to be measured, sourced or supplied
- Maximum pressure to be reached, and whether generation is pneumatic or hydraulic
- Whether the work is on a bench or in the field
- What certificates or records have to be issued
- Quantity, destination port and required delivery date
