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PRESSURE MEASUREMENT

Pressure instruments: transmitters, switches and gauges — how they work and which one to choose

Pressure is measured almost everywhere in a plant — on pumps, compressors, filters, tanks, steam lines and hydraulic systems — and the right instrument depends on what the reading is for: a signal for the control system, the difference between two points, an alarm at a limit, or simply a dial to read on the spot. This page covers the four types of instrument we supply — pressure transmitters, differential pressure transmitters, pressure switches and pressure gauges — how each one works, where it fits and where it does not, the sensing elements inside them, diaphragm seals, and how to choose.

The four types of pressure instrument

For each one: the principle in a sketch, where it fits and where it does not, and how many instruments of that type are in our catalogue.

Pressure transmitters

The process pressure pushes on a thin diaphragm; a sensing element behind it turns the tiny deflection into an electrical signal, and the electronics send it as 4–20 mA — often with HART or a fieldbus — over the two wires that also power the instrument.

Where it fits
Continuous measurement for control, recording and alarms in the control system: pumps, compressors, lines, vessels, and hydrostatic level in open tanks and wells.
Where it does not
Points with no power or wiring where a local reading is enough, and level in a pressurised vessel, which needs a differential measurement.
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Differential pressure transmitters

Two pressures act on the two sides of the same sensing cell, and the transmitter sends the difference between them. From that difference it can work out the level in a closed tank, the condition of a filter, or the flow through an orifice plate or other primary element.

Where it fits
Level in pressurised or closed vessels, pressure drop across filters and heat exchangers, flow with a primary element, and very low pressures such as furnace draft.
Where it does not
A simple gauge pressure at one point, where a pressure transmitter is cheaper and has only one connection to install and keep clear.
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Pressure switches

A switch that opens or closes an electrical contact when the pressure passes a set point. Mechanical versions use a diaphragm, a bellows, a piston or a disc that moves against a spring; electronic versions measure the pressure and switch an output.

Where it fits
Alarms, interlocks and emergency shutdown on high or low pressure — independently of the control system, with no programming.
Where it does not
Wherever you need to see, record or control with the pressure value itself: a switch only says above or below the set point.
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Pressure gauges

The pressure enters a curved Bourdon tube — or, for low pressures, a diaphragm or capsule — and makes it move; a small movement turns that motion into the rotation of a pointer on a dial.

Where it fits
Local indication on pumps, compressors, filters, steam and air lines, with no power and no wiring — and as a check next to a transmitter.
Where it does not
Remote monitoring and recording: a gauge sends no signal to the control system, and someone has to walk to it to read it.
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At a glance

The typical properties of the four types in one table, and their typical pressure range in one chart.

Pressure instruments compared: what they give, typical range, typical accuracy, output, power and main limitation
TechnologyGivesTypical rangeTypical accuracyOutputPowerMain limitation
Pressure transmittersContinuous value25 mbar…2000 bar±0.075–0.2% of span4–20 mA, HART, Profibus PALoop power, typically 12–36 V DCNeeds power, and a display or control system to read it
Differential pressure transmittersContinuous value10 mbar…70 bar *±0.065–0.1% of span4–20 mA, HART, Profibus PALoop power, typically 12–36 V DCTwo process connections to install and keep clear
Pressure switchesOn/off at a set point10 mbar…400 barSet-point repeatability, typically within 1%Contact (SPDT/DPDT)None for mechanical; electronic versions need powerAlarm or trip only — no continuous value
Pressure gaugesLocal reading10 mbar…4000 barAccuracy class 1.0 or 1.6 (EN 837), typicalPointer on a dial; contacts as an add-onNoneRead only on the spot — no signal to the control system

Typical ranges, not a model commitment, on a logarithmic scale. For differential pressure transmitters (*), the range is that of the difference; the static pressure of the line can be much higher. We confirm the figures of each instrument for your application.

Typical pressure range by type

Typical pressure range of each type, in bar, on a logarithmic axis. For differential pressure transmitters, the range of the difference.
  • Pressure transmitters25 mbar…2000 bar
  • Differential pressure transmitters10 mbar…70 bar *
  • Pressure switches10 mbar…400 bar
  • Pressure gauges10 mbar…4000 bar

Application guide: which instrument for which job

Each type of instrument exists to solve a particular problem. Start from yours — the medium, the pressure, what the reading is for — and see which type we would suggest, which one to avoid, and why.

Application guide for pressure measurement: for each application, the instrument we suggest and why, and the one to avoid and why
ApplicationWe suggestWhyAvoidWhy not
Corrosive medium: acids, alkalis, chlorides, seawaterDiaphragms in Hastelloy C, tantalum or gold-plated steel, as Klay offers, or a ceramic sensor, as on the Klay 3000 and CER series — or any instrument behind a diaphragm seal in a resistant material. — Any type can be used if every wetted part resists the medium; the materials matter more than the type of instrument.
Hygienic process: food, dairy, beverages, pharmaceuticalsA flush diaphragm on a hygienic connection leaves no pocket for product or bacteria and can be cleaned in place — the Klay 4000-SAN, for example, carries EHEDG and 3-A. — Avoid any instrument with a threaded port or a dead space where product can stay; a gauge or switch needs a hygienic diaphragm seal.
Viscous, crystallising or solids-laden medium: pulp, slurries, sludge, polymersA flush diaphragm has no opening to clog — Klay builds flush-diaphragm connections for pulp and paper.A standard gauge has a narrow port and a Bourdon tube that block; if you need local indication here, fit the gauge with a diaphragm seal.
Level in a closed or pressurised tank, or pressure drop across a filterOne instrument measures the difference directly: the pressure at the bottom minus the gas pressure above the liquid, or the pressure before minus after the filter.A single transmitter at the bottom of a closed tank reads the liquid plus the gas pressure; subtracting two transmitters adds the errors of both.
Local indication with no power: pumps, compressors, filters, utility linesA gauge needs no power and no wiring, and shows the pressure on the spot.A transmitter needs loop power, and a display or a control system to read it.
Only an alarm or a trip at a set pressure: high or low pressure protection, interlocksThe contact switches directly, with no PLC and no programming, and works independently of the control system; versions with SIL 2 certification exist, such as the 12 Series. — If you also need the value, use a transmitter with alarms in the control system — or a combined instrument such as the One Series, which is a switch and a transmitter in one.
Very high pressure: hydraulics, high-pressure gas, injectionTransmitters such as the D22 and D23 reach 1,000 bar and the TX300 30,000 psi; the GS gauge has ranges up to 4,000 bar.It measures a difference over a limited span — up to 70 bar on the D35 — not a high gauge pressure.
Very low pressure or vacuum: furnace draft, ventilation, clean roomsSmall spans in millibar, measured against the atmosphere or another room — the D34 is made for draft and furnace measurements, and the DP-4000 starts at 10 mbar.A Bourdon tube is not sensitive enough at a few millibar; that needs a diaphragm or capsule gauge, or a transmitter.
Hazardous areas (ATEX)Intrinsically safe transmitters, such as the Klay 2000, 4000 and 8000 with ATEX and IECEx Ex ia, explosion-proof ones such as the TX300, and flameproof switches such as the Delta Mobrey 301. — Any type can be used with the right certification and installation; we check it for your zone.

Sensing elements: what turns pressure into a reading

Inside every pressure instrument there is an element that the pressure moves or deforms. Electronic instruments — transmitters and electronic switches — read that deformation electrically; mechanical instruments — gauges and most switches — use the movement itself.

The element decides how small a pressure the instrument can resolve, how much overpressure it survives, which media it can touch and how it ages. The catalogue page of each model states its element where the manufacturer does, and we confirm it for your application.

  • Piezoresistive silicon: a small silicon chip whose resistance changes as it bends, usually behind a thin metal diaphragm with a fill fluid between them — the design of the Delta Mobrey D22. Very sensitive, for small and large ranges.
  • Ceramic: a diaphragm of high-purity alumina that the medium touches directly, read capacitively or with resistors printed on it. Resists corrosion and abrasion — used on the Klay 3000, 3000-SAN, CER-2000 and CER-8000 and the Delta Mobrey DSGE.
  • Metal diaphragm with strain gauges: a stainless steel diaphragm with a thin-film or bonded resistor bridge on its back. Robust, often welded, suits high pressures.
  • Mechanical elements: the Bourdon tube of gauges, and the diaphragm, bellows, piston or snap-acting disc of switches — the J40 uses a sealed metal bellows, the 12 Series a Belleville disc and the Delta Mobrey 301 a stainless steel diaphragm.

Diaphragm seals: keeping the instrument away from the process

A diaphragm seal puts a flexible metal diaphragm between the process and the instrument. The space behind the diaphragm and the instrument is filled with a fill fluid, which carries the pressure to the sensing element; the instrument itself never touches the medium. The seal mounts directly on the instrument or connects to it through a capillary.

Seals are the answer for corrosive, viscous, crystallising, solids-laden, hygienic or very hot media, and for keeping a gauge, a switch or a transmitter clear of what would clog or attack it. The Delta Mobrey D35 differential pressure transmitter, for example, is built with dual diaphragm seals.

  • The fill fluid expands with temperature: a seal adds a small temperature effect, and a long capillary slows the response.
  • On differential pressure, keep both capillaries the same length and at the same temperature, so that their effects cancel out.
  • Choose the diaphragm material for the medium (stainless steel, Hastelloy, tantalum, or a coating) and the fill fluid for the process temperature — and a food-grade fill for hygienic processes.
  • A quick coupling between seal and instrument, such as the Krohne REconnect, lets you remove the instrument for maintenance without losing the fill fluid or letting air in.

How to choose a pressure instrument

The right instrument follows from the application. These are the criteria we look at:

  1. What the reading is for

    A signal for control and recording needs a transmitter; a local check needs a gauge; an alarm or a trip can be a switch.

  2. Gauge, absolute or differential

    Against the atmosphere (gauge), against vacuum (absolute) or between two points (differential) — the reference decides the instrument.

  3. Range and overpressure

    Choose a range so that the normal pressure sits in the middle part of it, and check the overpressure the instrument survives, including pressure spikes.

  4. Medium and temperature

    Corrosive, viscous, hygienic or hot media decide the wetted materials, a flush diaphragm, or a diaphragm seal.

  5. Accuracy and signal

    Accuracy is stated as a percentage of the span; for long cables and a control system, 4–20 mA with HART or a fieldbus.

  6. Environment

    Hazardous areas, vibration, pulsation, outdoor installation and marine use determine the certification, the construction and the accessories.

Pressure instruments: frequently asked questions

Need help choosing a pressure instrument?

Tell us about the medium and the operating conditions, and our engineers will recommend the right type and the specific instrument.