Selecting a Pressure Transmitter: Range, Accuracy and Process Connection

A pressure transmitter looks like a simple purchase — until the ordering code asks you thirty questions. Here is the short path to a correct specification, in the order we ask customers.

1. Gauge, absolute or differential?

Gauge (relative to atmosphere) covers most utility and process duty: pump discharge, pipeline and filter pressure, hydrostatic level in open tanks. Absolute is required where atmospheric variation would corrupt the measurement — vacuum processes, distillation, altitude-sensitive applications. Differential measures across filters and orifice plates, and level in pressurized vessels; it needs a high-static design such as the ABB 266HSH when the line pressure is high.

2. Set the range around the operating point

Aim for the normal operating pressure to sit at 50–70 % of the calibrated span. A transmitter running at 5 % of span wastes accuracy; one at 95 % has no headroom for excursions. Check the overpressure limit against water hammer and pump dead-head conditions, not just normal duty — the ceramic Cerabar PMC71B survives overload that would permanently shift a thin metal diaphragm.

3. Accuracy: buy what the loop needs

• 0.5 % of span is fine for monitoring and alarms.
• 0.1–0.25 % suits control loops and reporting.
• 0.04–0.075 % class — SITRANS P320 at 0.065 %, Cerabar Platinum at 0.05 % — is for custody transfer, energy balances and calibration references.

Read the full error budget: temperature effect, static-pressure effect (for DP) and long-term drift matter more than the headline number once the transmitter is in a hot pipe rack.

4. Wetted parts and process connection

316L stainless handles most clean services. Go to Alloy C (Hastelloy), tantalum or gold-plated diaphragms for chlorides, hydrogen and aggressive chemistry; ceramic cells cover most of these at lower cost up to 40 bar. For hygienic processes choose Tri-Clamp or DIN 11851 connections with flush diaphragms and FDA-listed fill fluids. For hot or viscous media specify a diaphragm seal and capillary — and accept the slower response and temperature effect that come with it.

5. Output, approvals and safety

Confirm the output your system expects — 4–20 mA HART is still the default; PROFINET with Ethernet-APL and IO-Link are now available on current-generation instruments. Specify the hazardous-area certificate (ATEX/IECEx zone and protection concept: intrinsically safe Ex ia or flameproof Ex d) and the SIL level where the point feeds a safety instrumented function. Our guide to ATEX and IECEx is available in Vietnamese.

6. Commissioning and verification

Current instruments commission from a phone over Bluetooth and verify themselves in place (Heartbeat Technology on Endress+Hauser, remote safety commissioning on SITRANS P). Specify these if your plant has a calibration programme — see setting calibration intervals you can defend.

Worked example

A water treatment plant needs filter-outlet pressure, normally 2.5 bar, maximum 6 bar, 4–20 mA HART, outdoor, no hazardous area. Specification: gauge transmitter, 0–10 bar cell calibrated 0–6 bar, 0.1 % class, 316L, G1/2 connection, IP66/68 housing, HART 7 — for example a Cerabar PMC51B or SITRANS P320. Add a block valve and a cable gland rated for the housing, and specify the calibration certificate on the order.

Send us your P&ID tag list — we return a fully specified transmitter schedule with ordering codes and alternatives from at least two manufacturers. Request a quote.

Quick answers

Ceramic or metal diaphragm?

Ceramic (Cerabar PMC) for abrasive, corrosive or vacuum service and overload resistance up to 40 bar; metal (PMP, SITRANS P, ABB 266) for higher pressures, diaphragm seals and hydrogen service.

What does 'turndown' mean?

The ratio between the sensor's maximum span and the span you calibrate. A 16 bar sensor set to 0–1.6 bar has a turndown of 10:1. Accuracy specifications usually apply up to a stated turndown (often 5:1 or 10:1) and degrade beyond it.