Pressure · 2.2

Pressure transmitters: choosing the range, damping and overpressure rating

26 October 2026 · 9 min read · IEC 60770-1, NAMUR NE43

In short

Pick the range so that the normal value sits at about 50–75 % of span, with the maximum and the trip points inside the range.
Pick the sensor so that its overpressure limit is above the design pressure and the hydrotest pressure.
Damping is a first-order filter with time constant τ. On a rising ramp, the reading lags by rate × τ.
Keep damping low on trip and fast-control loops, and use it only to calm a noisy indication.

Pressure transmitters are the workhorse of any plant, and most are specified from the same few datasheet lines: range, sensor limit, overpressure, damping and failure mode. Each has a rule, and getting them wrong shows up later as nuisance trips, slow trips or a damaged sensor after the hydrotest.

Where this matters

Trips: a wrong range or heavy damping delays the trip.
Hydrotest and commissioning: a transmitter left connected during the hydrotest sees 1.3–1.5 times the design pressure.
Pump and compressor discharge: pulsation makes readings noisy and tempts people to add damping.
Spares: standardising sensor sizes reduces spares, but only within sensible turndown.

How it works

Range against process data

Line up the process values and place them on the range:

% of span = (P − LRV) / (URV − LRV) × 100

The usual targets are normal at 50–75 %, maximum operating below about 90 %, and trip and alarm points inside the range with margin, never at the very end.

Sensor limit and overpressure

The sensor's URL must be at or above the URV, with turndown kept sensible (article 1.4). The overpressure limit is the pressure the transmitter survives without a shift. It must be above the design pressure, and also above the test pressure if the transmitter stays connected during the hydrotest. Otherwise, isolate it or remove it.

Damping

Damping is a first-order lag with time constant τ:

step: reading reaches 63 % in τ, 95 % in 3τ ramp at rate R: lag = R × τ

A 2 s damping sounds harmless, but on a pressure rising at 1 bar/s the reading trails the real pressure by 2 bar.

Failure mode

Set the failure direction to upscale (≥ 21 mA) or downscale (≤ 3.6 mA) per NAMUR NE43, so the DCS or SIS reacts the safe way. For a high-pressure trip, upscale failure trips the plant, and the reverse fails to trip. The choice comes from the safety review.

What the standards and practice say

NAMUR NE43 sets 3.8–20.5 mA as the valid measuring range, and ≤ 3.6 mA or ≥ 21 mA as failure.
IEC 60770-1 covers how response time and damping are stated. The transmitter's own response time adds to any damping you set.
Pressure equipment codes set the hydrotest pressure, often 1.3 to 1.5 times design. Check the project's test procedure.
Many companies' practices limit damping on safety and fast-control loops to a small value, often 0.5 s or less. Follow the project rule.

Worked example: a pump discharge transmitter

Input	Value
Normal / maximum operating	12 / 15 barg
High-pressure trip	18 barg
Design pressure / hydrotest (1.5 ×)	20 / 30 barg
Pressure rise rate on blocked outlet	about 1 bar/s

Step 1, range. Try 0–25 barg. Normal 12/25 = 48 %, maximum 15/25 = 60 %, trip 18/25 = 72 %. All are inside the range with margin. (0–20 barg would put the trip at 90 %, too close to the end.)

Step 2, sensor. The URL must be at least 25 bar, and the overpressure limit at least 30 barg if the transmitter stays in during the hydrotest. A common 0–70 bar sensor gives a turndown of 70/25 = 2.8 : 1. That's fine.

Step 3, damping.

Damping τ	Lag on a 1 bar/s rise	Real pressure when the reading hits 18 barg
0.2 s	0.2 bar	≈ 18.2 barg
2 s	2 bar	≈ 20 barg (at design pressure)

Step 4, failure. This is a high trip, so set upscale failure, and alarm on the failure current in the SIS.

What this example shows

Range from the trip, not just the normal value. The trip point decides the minimum URV.
The hydrotest is part of the specification. Either the sensor survives it, or the procedure isolates the transmitter.
Damping hides margin. 2 s of damping took up the whole gap between the trip and the design pressure.

When it fails: the fixes in order

Noisy reading: fix the cause first (pulsation, a poor tapping or a gas pocket). Then use a snubber. Add damping last, and only on indication.
Nuisance trips: check that the range puts the trip well inside it, and use a voting arrangement (2oo3) rather than heavy damping.
Shifted after the hydrotest: recalibrate, then change the procedure or the sensor rating.
Reading pinned at 20.5 mA: the range is too small for the process. Re-range from fresh process data.

Common mistakes

Ranging so that the trip point is at 95–100 % of span.
Leaving the default damping (often 0.4–1 s) unexamined, or raising it on a trip loop.
Forgetting the hydrotest pressure when choosing the overpressure limit.
Copying the failure direction from another project.
Choosing URV equal to design pressure by habit, even when the trip is far lower.
Not recording the damping setting on the datasheet.

Checklist

Normal, maximum and trip values are known and placed on the range.
Normal is at 50–75 % of span, and the trip is inside the range with margin.
URL ≥ URV, with sensible turndown.
The overpressure limit is above the design and hydrotest pressure, or the transmitter is isolated for the test.
Damping is stated, and kept small on trip and fast-control loops.
The failure direction (NE43 upscale or downscale) comes from the safety review.
Pulsating service has a snubber or a suitable mounting.
The range is the same on the datasheet, the transmitter and the DCS (article 1.2).

Quick reference

Range targets: normal 50–75 %, maximum < 90 %, trip inside the range with margin.
Damping: 63 % in τ, 95 % in 3τ. Ramp lag = rate × τ.
NE43: valid 3.8–20.5 mA, failure ≤ 3.6 or ≥ 21 mA.
Overpressure limit > design pressure, and > hydrotest if connected.