What does an accuracy of “0.075% of span” mean?

The error is a percentage of the adjusted range, not of the reading. A Klay 4000 set to 0–10 bar with 0.075% accuracy is within ±0.0075 bar across that range; set to a narrower range, the error in bar becomes smaller too.

It is how far the range can be narrowed from the maximum one the sensor allows. With a 20:1 turndown, as on the Klay 4000, a 0–100 bar transmitter can be set to 0–5 bar; the accuracy is then stated on the narrower span.

Gauge or absolute pressure?

Gauge pressure, against the atmosphere, suits most process measurements. Absolute pressure, against vacuum, is needed when the atmospheric pressure would distort the reading — vacuum processes, gas density and barometric applications.

Can a pressure transmitter measure level?

Yes, in an open tank: the pressure at the bottom grows with the height of the liquid — about 98 mbar per metre of water. Submersible probes such as the Delta Mobrey DSGE do exactly that. In a closed or pressurised tank, use a differential pressure transmitter instead.

PRESSURE MEASUREMENT TECHNOLOGIES

Pressure transmitters: process pressure as a 4–20 mA signal

A pressure transmitter measures the pressure at one point and sends it continuously to the control system — the basis of pump control, compressor protection, line and vessel monitoring, and hydrostatic level. Inside, a diaphragm and a sensing element turn the pressure into an electrical signal; the electronics send it as 4–20 mA, often with HART or a fieldbus. Our catalogue includes the Klay Instruments 2000, 3000, 4000 and 8000 series with their hygienic (-SAN) and valve (-VALVE) versions and the CER-2000 and CER-8000, the Delta Mobrey 387, D22 and D23 and the DSGE level probes, and the United Electric TX300.

See the pressure transmitters

How a pressure transmitter works

The process pressure acts on a thin diaphragm and deflects it by a tiny amount. A sensing element turns that deflection into an electrical quantity: in a typical design, a silicon chip whose resistance changes as it bends, sitting behind an isolating metal diaphragm with a fill fluid that carries the pressure to it — the construction of the Delta Mobrey D22. Other transmitters use a ceramic sensor that the medium touches directly, such as the Klay 3000 and CER series and the Delta Mobrey DSGE probes, or a metal diaphragm with strain gauges.

Most transmitters measure gauge pressure, against the atmosphere; absolute versions measure against a sealed vacuum and do not change with the weather. The United Electric TX300, for example, is available with gauge and absolute ranges.

The electronics compensate the signal for temperature and send it as a 4–20 mA current over the same two wires that power the transmitter. The 4 mA at the bottom of the range is a “live zero”: a broken wire reads 0 mA and is detected at once. With HART, the control room can also read the range, diagnostics and the process temperature; the Klay 4000 also offers Profibus PA.

The range can be adjusted within limits — the turndown: up to 20:1 on the Klay 4000, 10:1 on the 2000, 4:1 on the 8000. Accuracy is stated as a percentage of the adjusted span: 0.075% on the Klay 4000, 0.1% on the 2000, 0.2% on the 8000.

The process pressure, pushing on the diaphragm
The fill fluid, which carries the pressure to the sensing element
The sensing element (orange chip) and, above it, the electronics
The 4–20 mA loop to the control system, over the same two wires that power the transmitter
The signal rises in a straight line with the pressure, from 4 mA at the bottom of the range

Where a pressure transmitter fits — and where it does not

A strong choice for

Pump and compressor control and protection, line and vessel pressure, with the value in the control system
Hydrostatic level in open tanks, wells and boreholes — the Delta Mobrey DSGE probes are made for it
Hygienic processes: the -SAN versions have flush diaphragms and hygienic connections, and the 4000-SAN carries EHEDG and 3-A
Pulp, slurries and viscous media, with a flush diaphragm that has no port to clog
Hazardous areas: intrinsically safe Klay 2000, 4000 and 8000 (ATEX and IECEx Ex ia), or the explosion-proof TX300
Marine and offshore: marine approvals on the Klay 2000, 8000 and CER series, and the all-stainless Delta Mobrey 387

Think twice when

You need level in a closed or pressurised tank, or the pressure drop across a filter: that is a differential pressure transmitter
There is no power or wiring and a local reading is enough: a pressure gauge is simpler
You only need a trip at a limit, independent of the control system: a pressure switch does it with no PLC
The line has pressure spikes far above the range, such as water hammer, and nothing protects the diaphragm

How it compares with the other technologies

Typical pressure range of each technology, in bar, on a logarithmic axis. The technology of this article is highlighted.

Pressure transmitters (the technology of this article)25 mbar…2000 bar
Differential pressure transmitters10 mbar…70 bar *
Pressure switches10 mbar…400 bar
Pressure gauges10 mbar…4000 bar

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.

All technologies

Installing a pressure transmitter

Where and how you take the pressure from the process matters as much as the transmitter itself. Keep the impulse line short and sloped, so that it does not trap gas in a liquid line or liquid in a gas line, and fit an isolation valve or a manifold, so that the transmitter can be removed and its zero checked without stopping the process.

Then:

On liquidsTake the pressure from the side or the lower half of the pipe and mount the transmitter below the tapping, so that the impulse line stays full of liquid and gas bubbles rise back into the pipe.

On gasesTake the pressure from the top of the pipe and mount the transmitter above it, so that condensate drains back into the pipe instead of collecting in the line.

Flush diaphragm for difficult mediaFor viscous, crystallising or hygienic media, mount a transmitter with a flush diaphragm level with the inner wall: no narrow port to clog, and no pocket where product can stay.

On steam, keep the hot steam away from the diaphragm with a water-filled line or a siphon.
Where the pressure pulsates or water hammer occurs, fit a snubber and choose a range with enough overpressure margin.
Choose the range so that the normal pressure sits in the middle part of it, not at the very end.
Use screened cable, and in hazardous areas follow the installation conditions of the certificate.

Pressure transmitters by industry

Water & WastewaterPump stations, network pressure and booster control, and hydrostatic level in tanks, wells and boreholes.

Oil & GasWellheads, separators and compressors in hazardous areas, with intrinsically safe or explosion-proof transmitters.

Food & BeverageTanks, lines and fillers, with flush diaphragms and hygienic connections that can be cleaned in place.

ChemicalCorrosive media, with Hastelloy C, tantalum or gold-plated diaphragms or ceramic sensors; the D23 gold-plated diaphragm guards against hydrogen permeation.

MarineEngine, ballast and cargo systems on board, with marine class approvals and stainless construction.

Pressure transmitters in our catalogue

The pressure transmitters in our catalogue, each with its own page and technical data.

DELTA MOBREYAnalogue Level Probes - DSGE SeriesHydrostatic level measurement probes for tanks, wells and piezometers with 4–20 mA output. View product

KLAY INSTRUMENTS8000-SANHygienic level transmitter with flush diaphragm, 0.04–80 bar range, ±0.2% accuracy, and field-adjustable calibration. View product

KLAY INSTRUMENTSHydrobarPrecise hydrostatic level transmitter for water, wastewater, and sludge measurement with 0.2% accuracy and multiple connection options. View product

KLAY INSTRUMENTS2000Klay Series 2000: robust flush-diaphragm pressure transmitter, 0.1–100 bar, ATEX/IECEx certified, 4–20 mA output with optional HART. View product

KLAY INSTRUMENTS2000-SANPrecision microprocessor-based pressure transmitter with hygienic connections for demanding process environments. View product

KLAY INSTRUMENTS2000-VALVEIntegrated pressure transmitter with microprocessor control, 0.1…10 bar range, adjustable zero/span, IP66 rated for harsh environments. View product

KLAY INSTRUMENTS3000Intelligent ceramic pressure transmitter with 0.2% accuracy, HART-ready, and robust IP66 protection for demanding industrial environments. View product

KLAY INSTRUMENTS3000-SANIntelligent ceramic pressure transmitter for hygienic applications with 0.1-50 bar range and HART capability. View product

Pressure transmitters: frequently asked questions

Ask about a pressure transmitter

Tell us the medium, the pressure range and type (gauge or absolute), the process temperature, the process connection, the accuracy, the output and the area classification — and our engineers will suggest the right transmitter, with price and delivery time.