Gauge Pressure Transmitters

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Gauge pressure reads against local atmosphere through a vented reference, which is what most plant measurements actually want: tank head, pump discharge, line pressure. The capacitive cell measures by pole-distance change: process pressure deflects a metal membrane, the capacitance between the membrane and a fixed electrode shifts, and the measuring circuit turns that shift into the output. Capacitance cells take little input energy, respond fast and ride out harsh environments, which is why this architecture earned its installed base. Our piezoresistive lines cover the same duties in a compact body: the SI-300 for process points and the SI-390 for OEM design-ins.

Span options

Span group (SI units)	Equivalent
0–0.1 to 0–3.5 kPa	0–10 to 0–350 mmH2O
0–0.8 to 0–8 kPa	0–80 to 0–800 mmH2O
0–4 to 0–40 kPa	0–400 to 0–4,000 mmH2O
0–20 to 0–200 kPa	0–2,000 to 0–20,000 mmH2O
0–70 to 0–700 kPa	0–0.7 to 0–7 kgf/cm2
0–210 to 0–2,100 kPa	0–2.1 to 0–21 kgf/cm2
0–700 to 0–7,000 kPa	0–7 to 0–70 kgf/cm2
0–2.1 to 0–21 MPa	0–21 to 0–210 kgf/cm2
0–4.1 to 0–41 MPa	0–41 to 0–410 kgf/cm2

Each group is continuously rangeable within its limits, so a 0–20 to 0–200 kPa cell can be calibrated to any span in between. Pick the group whose upper limit puts your working pressure at 60 to 80% of the calibrated span.

Technical specifications

Parameter	Specification
Reference accuracy	±0.075% or ±0.1% of calibrated span, varies by material and span
Long-term stability	±0.1% of URL over 10 years
Sensing element	High-reliability micro-capacitance silicon cell
Fill fluid	Silicone oil standard; fluorine oil for oxygen and chlorine service
Wetted materials	SUS316 (diaphragm SUS316L); Hastelloy C; Tantalum; Monel, gold plating and ceramic by custom build
Power supply	17.9 to 42 VDC
Output signal	4–20 mA (SFN or HART communication); Modbus RS-485
Display	Versatile indicator with push-button programming; built-in loop calibration
Process connection	Rc 1/2 or 1/2 NPT internal thread; remote seals can be fitted
Ingress protection	IEC IP66 / IP67
Hazardous area	Flameproof and intrinsically safe versions available; confirm certification documents for your destination market at order
Serviceability	Field repair with common spare parts; HART 5.2 multi-drop

Specifications at reference conditions unless stated. Confirm the exact figure for your span, material and options on the order sheet.

Replacing a 1151-pattern fleet

Plants with decades of capacitive transmitters on the books face the same fork: hunt surplus stock and refurbished units, re-engineer every loop to a modern coplanar platform, or fit a new-build unit on the same architecture. The SI-3151GP exists for the third path. The push-button display, the loop calibration facility and the spare-parts serviceability follow the conventions your instrument techs already know; send us the model code from the nameplate of the unit you are replacing and we will match span, materials, connection and options line by line.

The buying pattern is familiar from our inquiry file: a German plant came to us to second-source legacy imported instruments their vendor had moved on from, flow meters in that case, and the logic is identical for pressure fleets. Nobody wants to re-validate forty loops because one vendor retired a product line.

Models and ordering

Every SI-3151GP is assembled to its order code. Span group and wetted material drive the build; everything else is option codes.

Ordering example: SI-3151GP, span group 0–20 to 0–200 kPa calibrated to 0–200 kPa, SUS316L diaphragm, silicone fill, 4–20 mA HART, Rc 1/2, display fitted, for a pump discharge running at 140 kPa. The working point sits at 70% of calibrated span.

Applications

Related products

FAQ

What is a gauge pressure transmitter?

A transmitter that measures pressure relative to local atmospheric pressure, with the reference side of the sensing element vented to ambient. Readings above atmosphere are positive, below it negative, and the output shifts with weather and altitude because the reference does. That is the right behavior for most plant measurements, where the equipment around the instrument also lives at local atmosphere.

Gauge or absolute: which do I order?

Gauge when the process only fills against atmosphere and your other instruments read gauge too. Absolute when the measurement must not move with the weather: vacuum processes, vapor pressure work, anything referenced to a sealed or evacuated state. The two are different builds with different references; one cannot stand in for the other. Absolute pressure equals gauge pressure plus local atmospheric pressure.

Can it replace my old 1151-pattern transmitter?

That is the design intent: same capacitive architecture, conventional dual-compartment body, field-repairable electronics, spans from 0.1 kPa to 41 MPa, HART communication. Send the complete model code from your existing nameplate and we confirm span, wetted materials, process connection and options against it before quoting, so the swap is verified on paper before anything ships.

Request a quote

Tell us the span, the medium and wetted material, the communication option and, for replacements, the old model code. We respond with a configured model, lead time and price, usually within one working day.