Pressure · 2.3
DP transmitters and manifolds: static pressure, 3-valve and 5-valve sequences
26 October 2026 · 9 min read · IEC 60770-1, ISO 2186
In short
- A DP transmitter measures a small difference on top of a large static pressure. It must be rated for the static pressure, not just the DP range.
- The manifold isolates, equalises and vents the transmitter. A wrong valve sequence can put full line pressure on one side of the sensor.
- Safe rule: the equaliser is open whenever only one block valve is open.
- Static pressure shifts the zero, so zero the transmitter at line pressure with the equaliser open.
DP transmitters measure flow across orifice plates, level in vessels and pressure loss across filters. They are the most demanding pressure instrument to handle in the field, because their range may be a few hundred millibar while each side sits at tens of bar.
Where this matters
- Flow by orifice (article 4.2): the DP range is often 250–500 mbar at static pressures of 10–100 bar.
- Level (article 5.1): DP sets the level, and the static pressure is the vessel pressure.
- Filters and strainers: DP shows fouling.
- Commissioning and maintenance: most damage to DP cells happens when valves are opened in the wrong order.
How it works
The sensing cell has a high side and a low side. The cell's maximum working pressure (MWP, or static pressure rating) must exceed the design pressure. Its one-sided overpressure limit is what it survives if full pressure reaches one side only. Good cells have a mechanical backstop for this, but repeated events still shift the zero.
Manifolds
| Type | Valves | Use |
|---|---|---|
| 3-valve | High block, low block, equaliser | Most DP services |
| 5-valve | As 3-valve, plus two vent or test valves (or a double equaliser with a vent) | Gas and hazardous fluids, venting and in-situ testing without breaking connections |
Safe sequences (3-valve)
Putting in service: 1) equaliser open, both blocks closed; 2) open the high block; 3) close the equaliser; 4) open the low block.
Taking out of service: 1) close the low block; 2) open the equaliser; 3) close the high block; 4) vent through the drain or test valves.
Zero check in service: close the low block, open the equaliser, read and zero at line pressure, close the equaliser, and reopen the low block.
The sequences ensure that the cell never sees line pressure on one side with the other side at atmosphere. In gas service, vent to a safe location. A 5-valve manifold lets you do that without loosening fittings.
Static pressure effect
A cell deforms slightly under static pressure, and that shifts the zero and the span. Vendors quote it, typically as a % of URL per 100 bar for zero. The zero part can be trimmed at line pressure. The span part is usually smaller and is corrected in the factory.
What the standards and practice say
- IEC 60770-1 defines static pressure effect tests. ISO 2186 covers pressure signal lines between primary and secondary elements for flow measurement.
- Always specify the static pressure rating on the DP datasheet, at least the design pressure.
- Gas service usually uses 5-valve manifolds. Lethal or toxic services may require double block and bleed.
- Close-coupled (direct-mounted) manifolds on the transmitter reduce leak points and impulse-line problems (article 2.6).
Worked example: a filter DP transmitter
| Input | Value |
|---|---|
| Line pressure / design | 40 / 50 barg |
| DP range | 0–500 mbar |
| Sensor URL | 2.5 bar |
| Static effect on zero (example spec) | ±0.1 % of URL per 100 bar |
Step 1, rating. The static pressure rating must be at least 50 barg. A standard 100 or 160 bar class cell is suitable.
Step 2, static zero shift at 40 bar. 0.1 % × 2.5 bar × 40/100 = 0.001 bar = 1 mbar, which is 0.2 % of the 500 mbar span. Zeroing at line pressure removes most of it.
Step 3, what a wrong sequence does. Opening the low block first, with the equaliser closed and the high side vented, puts 40 bar on one side of a cell ranged 0.5 bar. That is 80 times the span, or 16 times the URL. The cell depends entirely on its one-sided overpressure rating.
| Situation | Pressure seen by the cell | Result |
|---|---|---|
| Correct sequence | ≤ 0.5 bar DP | Normal |
| Zero not trimmed at line pressure | +1 mbar offset | 0.2 % error |
| Wrong sequence | 40 bar one side | Zero shift, possible damage |
What this example shows
- Static rating and DP range are separate specification lines. Both go on the datasheet.
- Zero at line pressure is a cheap correction of the static effect.
- Procedure protects the cell. Put the sequence on the hook-up drawing or a tag at the manifold.
When it fails: the fixes in order
- Zero offset: zero at line pressure with the equaliser open.
- A shift after maintenance: suspect a one-sided overpressure. Recalibrate, and retrain on the sequence.
- A leaking equaliser reads low: a passing equaliser bleeds DP away. Test it by closing the blocks and watching the reading.
- An erratic reading: look at the impulse lines next (article 2.6).
Common mistakes
- Specifying the DP range but not the static pressure rating.
- Opening the low block first, or closing the equaliser before a block is open.
- Zeroing at atmosphere and then using the transmitter at 40 bar.
- Using a 3-valve manifold in toxic gas service with no way to vent safely.
- Swapping the high and low connections, which gives a reading that falls as flow rises.
- Ignoring a passing equaliser valve.
Checklist
- The DP range, the sensor URL and the static pressure rating are all stated.
- The static rating is at or above the design pressure.
- The one-sided overpressure rating is known.
- The manifold type (3- or 5-valve) suits the fluid and its hazard.
- The vents go to a safe location for gas or toxic service.
- The high and low sides are marked on the transmitter and the hook-up.
- The putting-in-service sequence is on the hook-up or at the manifold.
- The zero is trimmed at line pressure during commissioning.
Quick reference
- In service: equaliser open → high block open → equaliser closed → low block open.
- Out of service: low block closed → equaliser open → high block closed → vent.
- Rule: the equaliser is open whenever only one block is open.
- Static zero shift = spec % × URL × (Pstatic/reference). Trim it at line pressure.
Further reading
- IEC 60770-1, Transmitters for use in industrial-process control systems: methods for performance evaluation
- ISO 2186, Fluid flow in closed conduits: connections for pressure signal transmissions between primary and secondary elements
- In this series: 1.5 Gauge, absolute and differential pressure · 2.2 Pressure transmitters · next, 2.4 Diaphragm seals
The static effect figure is an illustrative specification in the usual vendor form. Use the actual product data and the site's valve operating procedures. IEC and ISO are named for identification only.