What this category covers
Pressure is the most widely installed measurement in a plant, and the one where a specifying mistake survives longest unnoticed. A gauge that reads correctly on the bench but sits at a temperature the mechanism was not compensated for, or a transmitter whose diaphragm seal clogs on a crystallising medium, will fail in service while every number on the datasheet remains true.
The category covers the whole chain from the mechanical gauge on a pump discharge to a differential transmitter across an orifice plate, and includes the diaphragm seals, manifolds, syphons and cooling elements that adapt a standard instrument to an awkward process.
The first question is never the range. It is whether the pressure is gauge, absolute or differential, because that single answer determines what the instrument compares the process against and therefore what it can physically measure.
Product types in pressure measurement
| Product type | How it works | Usually specified for |
| Pressure gauge | Deflection of a Bourdon tube, capsule or diaphragm read against a mechanical scale | Local indication at a pump, filter, regulator or vessel where no signal is needed |
| Digital pressure gauge | The same sensing element with an electronic display and often a peak-hold function | Calibration checks, field verification and anywhere a precise local reading is required |
| Differential pressure gauge | Two pressure connections acting on either side of a single element | Filter and strainer monitoring, flow across a primary element, and level in a closed tank |
| Vacuum gauge | Element referenced to atmosphere, scaled below zero | Pump suction, condenser and vacuum process duties |
| Pressure transmitter | Sensing element with electronics producing a conditioned output | Control, recording and any duty where the value has to travel to a control room |
| Differential pressure transmitter | Two process connections, output proportional to the difference | Compensated flow, level in pressurised vessels, and filter differential |
| Absolute pressure transmitter | Element referenced to a sealed vacuum | Barometric, altitude and any duty where the reference must not be atmosphere |
| Pressure switch | Snap action at a set point, used as a discrete signal | Interlocks, alarms, pump cut-in and cut-out, and safety functions |
| Diaphragm seal | Isolating diaphragm and fill fluid transmitting pressure to the instrument | Corrosive, viscous, crystallising, hygienic or high-temperature media that must not reach the sensor |
Manifolds, syphons, cooling towers, snubbers and capillaries are accessories that make a standard instrument fit a specific installation.
How to choose a pressure instrument
There is rarely a single correct instrument for a duty, but there is always a
shortlist that can be justified and a set of answers that eliminates the rest. These
are the questions that decide it.
- Pressure type first: gauge, absolute, differential or compound. Everything downstream depends on this answer.
- Working range against span: the operating point should sit in the upper part of the span. An instrument run near the bottom of its range has the worst relative error, and one run above it has a short life.
- Overpressure and surge: pumps, water hammer and relief events routinely expose a transmitter to pressures far above the working value, and the stated proof pressure is a selection criterion rather than a footnote.
- Wetted materials: 316L stainless steel covers most duties, but aggressive media require PTFE facing, Hastelloy, Monel, titanium or a fully isolating diaphragm seal.
- Process temperature: high-temperature service needs a syphon, cooling element or capillary to keep the fill fluid and electronics inside their limits.
- Process connection and sealing: thread, flange and facing standard, plus whether the joint has to withstand thermal cycling without weeping.
- Hygienic requirements: sanitary connections, crevice-free wetted surfaces and materials accepted for food or pharmaceutical contact.
- Environment: ambient temperature, condensation, vibration and washdown determine whether a sealed reference or a remote seal arrangement is needed.
- Area classification, which usually narrows the available range before performance is considered at all.
- Signal and integration: 4-20 mA two-wire, HART, Modbus or a switch contact, and the loop supply available.
Working through the selection in order
- Decide whether the measurement is gauge, absolute, differential or compound, and write it down before looking at any instrument.
- Establish the normal, maximum and surge pressure. The maximum decides the rating; the normal decides the span.
- Ask whether the medium can reach the sensing element at all. If it cannot - because it corrodes, crystallises, freezes or is food - a seal is mandatory and the instrument is chosen second.
- Fix the process temperature at the instrument, not at the pipe. A high line temperature with a long impulse line may need nothing; a short one may need a syphon.
- Set the accuracy required by the process, and check whether it applies to the reading, the span or the full scale.
- Decide how the value is used: local indication, remote signal, control, recording or a discrete alarm.
- Apply the environment and the area classification.
- Check the installation: can the instrument be isolated, and does it need a manifold or a block-and-bleed arrangement for maintenance?
Comparing the technologies
| Type | Measures against | Where it is the right answer | Where it misleads |
| Gauge pressure | Local atmospheric pressure | Almost all process indication, because the process is also referenced to atmosphere | High-altitude or barometric work, where the atmospheric reference itself moves |
| Absolute pressure | A sealed vacuum | Vacuum systems, distillation and any duty where the absolute value is the figure of record | Routine process indication, where it costs more and answers a question nobody asked |
| Differential pressure | The difference between two pressures | Flow across a primary element, level in a closed tank, filter loading | Static pressure is high, where both legs must be rated for the full line pressure |
| Compound | Atmosphere, with a span crossing zero | Duties that alternate between vacuum and positive pressure | Applications that never cross zero, where the extra span wastes resolution |
| Sealed gauge | A trapped reference | Outdoor and washdown installations where the reference must not breathe | Where the reference cavity is not compensated for ambient temperature |
Parameters to confirm before ordering
What is left after the technology is chosen is a list of values that only the
installation can supply. These are the ones that change the model, the price or the
lead time:
- Pressure type: gauge, absolute, differential or compound.
- Normal, maximum and any surge or overpressure value, in the units the process uses.
- Medium, and whether it is corrosive, viscous, crystallising, abrasive or hygienic.
- Process temperature at the instrument, and the ambient temperature around it.
- Process connection: thread or flange standard, size, facing and rating.
- Wetted-material requirement or project material standard.
- Required accuracy, and the basis on which it is stated.
- Output: 4-20 mA, HART, Modbus, switch contact or local indication only.
- Whether a seal, capillary, syphon, cooling element or manifold is needed.
- Area classification and the protection concept required, where the area is classified.
Figures are quoted, not assumed
Specifications vary by model and application, so no performance figure is stated on
this page. Where a parameter has not been documented for a particular model, the
product page says so rather than filling the gap with a plausible number. Send the
values above and we will return the ones the selected model actually holds.
Where these instruments are used
