Pressure Transmitters

Overview

A pressure transmitter turns the pressure of a gas, liquid or vapor into a 4–20 mA, voltage or digital signal a control system can read. Picking one is less about the headline range and more about four choices: gauge, absolute or differential; the sensing element; the wetted material for your medium; and the accuracy you actually need. The two-wire 4–20 mA scheme has a live zero, so 4 mA is true zero pressure and 0 mA reads as a broken wire, which makes a fault obvious instead of silent. For the cold and hot ends of the range, see the cryogenic pressure transducer and high-temperature transmitter, or the whole pressure instruments page.

Technical specifications

Parameter	Specification
Pressure type	Gauge, absolute and sealed gauge; differential on the DP class
Range	−100 kPa to 100 MPa (vacuum to 14,000 psi)
Accuracy	±0.3% or ±0.5% FS composite error, including non-linearity, hysteresis and repeatability (SI-300); smart DP class per datasheet
Sensing element	Diffused-silicon piezoresistive (capacitive DP and ceramic variants in the series)
Output	4–20 mA; 0.5–4.5 / 1–5 / 0–10 V (HART, RS-485 on smart models)
Supply	12 / 24 VDC
Wetted material	316L isolation diaphragm; stainless steel housing
Process connection	M20×1.5 standard; NPT / G threads and flanges to order
Overpressure	150% FS
Hazardous area	Intrinsically safe Ex ia IIC T4 option
Working / medium temperature	−20 to 85 °C (storage −40 to 125 °C)
Temperature drift	Zero and sensitivity ±1.5% FS over −20 to 85 °C
Long-term stability	±0.2% FS per year
Response time	≤1 ms (to 90% FS)
Shock and vibration	Vibration 20 g (20–5,000 Hz); impact 100 g, 11 ms
EMC	Surge 2 kV (IEC 61000-4-5); ESD 8 kV contact / 15 kV air (IEC 61000-4-2)
Insulation	100 MΩ at 250 VDC
Ingress protection	IP65
Certification	CE

Pressure types: gauge, absolute and differential

Match the reference to what you are measuring. Gauge reads against local atmosphere and suits most line-pressure and open-tank work. Absolute reads against vacuum, for sealed-vessel or vapor-pressure measurement where atmospheric swing would corrupt the reading. Differential reads the gap between two points and is the basis of orifice flow, closed-tank level, and filter or catalyst-bed monitoring; the smart differential class offers a higher accuracy grade for custody transfer and low-DP applications (class per datasheet).

Features

Sensing elements

The element sets accuracy, overpressure tolerance and how the unit ages. Most general service runs on a piezoresistive cell; high-accuracy differential work uses a capacitive cell; rugged ultra-high-pressure or high-cycle service uses a sputtered thin-film or ceramic diaphragm.

Sensing element	Typical accuracy	Strength	Best fit
Piezoresistive (diffused silicon)	±0.3 / ±0.5% FS	Wide range, good value	General gauge / absolute service
Capacitive (incl. remote seal)	higher-accuracy class	High accuracy, stable	Differential, custody, low DP
Sputtered thin-film	±0.25 % FS	Rugged, high-cycle	Hydraulic, ultra-high pressure
Ceramic	±0.25 % FS	Corrosion resistant	Caustic and abrasive media

Wetted materials

Pick the wetted material from the medium first, then confirm the pressure rating. For welded stainless wetted parts specify 316L, not 316: the low-carbon grade keeps carbon below 0.03 % so chromium carbides do not form at the weld, which prevents weld decay and intergranular corrosion. High-strength 17-4PH suits ultra-high pressure and high-cycle hydraulics, but it pits in chloride and seawater, so it is the wrong pick for brine even though it is strong.

Medium	Wetted material	Note
General process, clean water, steam	316L stainless	Default; specify the L for welded parts
Ultra-high pressure, hydraulics	17-4PH diaphragm	Strong; avoid chloride / seawater
Caustic, pH 14 slurry, abrasive	PTFE connection + ceramic	Field-proven on caustic battery slurry
High-temp acid, chlorides	Hastelloy / tantalum	Confirm against the specific acid
LOX, cryogenic	Monel 400 / 316L	See cryogenic pressure page

Models and options

Model class	Type	Range	Accuracy	Output / wetted
SI-300	Piezoresistive gauge / absolute	−100 kPa to 100 MPa (vacuum to 14,000 psi)	±0.3 / ±0.5% FS	4–20 mA / HART; 316L
SI-2088	Compact OEM, wide-temp	0.1–10 bar abs and up	±0.25 % FS	4–20 mA / 0.5–4.5 V; −196 to +280 °C media
SI-702	High / ultra-high pressure	up to 1500 MPa (ultra-high)	Per datasheet	4–20 mA; 17-4PH diaphragm
DP class (SMT3151DP)	Capacitive differential	low DP to high static	higher-accuracy class	4–20 mA / HART; 316L
Corrosion class	Ceramic / PTFE	per service	±0.25 % FS	PTFE connection + ceramic

Selection example: accuracy across a turned-down range

Say a process normally runs at 5 bar and peaks at 20 bar. A transmitter sized 0–100 bar at Per datasheet carries ±0.1 bar of fixed error, which is ±2 % at the 5 bar working point. Resize it to 0–25 bar at the same Per datasheet and the fixed error drops to ±0.025 bar, about ±0.5 % at 5 bar, a fourfold improvement with no change in device class. Size the span to the working range, not to the maximum the pipe could ever see. For a billed or balanced point, step up to the higher-accuracy class differential class.

Applications

For air and gas pressures below 1 bar, in the 0-2.5 to 0-30 kPa band, pick the dedicated low pressure transducer rather than scaling down an SI-300.

Related products

Browse all pressure instruments →

FAQ

What does a pressure transmitter do?

It measures process pressure and converts it into a standard electrical signal, typically 4–20 mA (with HART) or a digital bus, that a control system reads. The SI-300 senses pressure on a diaphragm and its electronics output a signal proportional to that pressure, so a PLC or display can use it directly.

What is the difference between a pressure sensor and a pressure transmitter?

A sensor produces a raw signal (such as a millivolt output) proportional to pressure; a transmitter is that sensor plus the electronics that condition, amplify, and scale the signal into a standard 4–20 mA or digital output, ready to wire to a controller over a long cable without losing accuracy.

What is the difference between a pressure regulator and a pressure transmitter?

A regulator is a mechanical valve that controls and holds pressure at a setpoint; a transmitter only measures pressure and reports it, without changing it. Use a transmitter to monitor the pressure and a regulator to control it.

What is the purpose of a transmitter?

To turn a physical measurement into a standard, transmittable signal. A pressure transmitter lets a remote control system read the pressure accurately over distance, with the 4–20 mA current loop staying immune to cable voltage drop and electrical noise.