A flow measurement using an orifice plate operates with a differential pressure range of, for example, 0 ... 100 mbar. The differential pressure transmitter is correctly calibrated, yet after maintenance the measuring point shows a significant zero offset. One of the two impulse lines contains a small high point where a gas bubble has collected. Can such a seemingly minor installation deviation really cause an error of several millibars?
Yes. In liquid applications, the impulse lines themselves are part of the hydraulic measuring system. Both lines must transfer the pressure from the high- and low-pressure sides of the primary element to the differential pressure transmitter with as little distortion as possible.
Both lines should therefore be completely filled with liquid. Gas bubbles, different liquid column heights, different temperatures or a partially drained line on one side change the hydrostatic pressure acting on one side of the transmitter.
Especially with small differential pressure ranges, this influence can be greater than the actual differential pressure being measured.
For liquid applications, the differential pressure transmitter is therefore usually installed at the same level as the pressure taps or below them. The impulse lines are routed without high points so that trapped gas can rise back toward the process and both measuring sides have hydraulic conditions that are as similar as possible.
What is an impulse line?
In a classic differential-pressure flow measurement, a primary element such as:
- orifice plate,
- flow nozzle,
- Venturi tube or
- averaging Pitot tube
generates a differential pressure that depends on the flow rate.
Upstream of the primary element, the static pressure is higher:
p+
downstream of the primary element, the pressure is lower:
p-
.
The differential pressure is:
Δp = p+ - p-
These two pressures must be transferred to the differential pressure transmitter.
This is the purpose of the two impulse lines:
High-pressure tap → high-pressure line → H side of transmitter
Low-pressure tap → low-pressure line → L side of transmitter
The lines therefore do not simply transfer some arbitrary pressure.
Together with the pressure taps, valves and transmitter, they form the complete hydraulic measuring system.
Why must impulse lines for liquids be completely filled?
In a liquid application, both impulse lines should be completely filled with the process liquid or with a designated sealing liquid.
This creates a defined liquid column on both sides of the transmitter.
A completely filled line transfers pressure virtually incompressibly.
If gas is present in one line, however, several characteristics change at the same time.
The gas bubble has:
- a much lower density than the liquid,
- significantly higher compressibility,
- different dynamic behaviour.
This can influence both the static zero point and the response time of the measurement.
Why is the transmitter installed below the pressure taps?
In liquid applications, the differential pressure transmitter is normally installed beside or below the pressure taps.
The key advantage is the natural effect of the density difference:
Liquid sinks
Gas rises
.
If the transmitter is located below the process line and the impulse lines rise continuously from the transmitter toward the process line, gas bubbles can rise toward the pressure taps.
From there, they can return to the process.
This prevents permanent gas pockets from forming in the measuring lines.
For liquid measurements, manufacturers therefore generally recommend installing the transmitter at the same level as the pressure taps or below them. :contentReference[oaicite:1]{index=1}
How should the lines be sloped?
What matters most is not an aesthetically pleasing piping layout, but a clearly defined venting direction.
From the differential pressure transmitter, both liquid-filled impulse lines should ideally:
rise continuously toward the process tap
.
This allows gas bubbles to migrate upward by themselves.
The following should be avoided:
- local high points,
- unnecessary loops,
- horizontal sections with changing slope,
- pockets in which gas can become trapped.
Manufacturers sometimes specify minimum slopes for impulse lines. The actual installation, however, must always take into account the project requirements, valves used, process medium and manufacturer instructions.
For liquid applications, Emerson, for example, specifies a continuous upward slope from the transmitter to the process tap and also recommends avoiding high points in liquid-filled lines. :contentReference[oaicite:2]{index=2}
Why are high points problematic?
A high point is a section where an impulse line first rises and then falls again.
Gas bubbles tend to collect at this highest point.
A simplified example:
Process ───╮
╰─ High point ─╮
╰─ Transmitter
Once a gas bubble becomes trapped there, it often cannot escape by itself.
Even if the line was completely vented during commissioning, gas can form again later, for example due to:
- outgassing of dissolved gases,
- temperature changes,
- pressure reduction,
- maintenance work,
- incomplete refilling.
Correct line geometry is therefore far more reliable than assuming the line only needs to be vented properly once.
How do gas bubbles cause measurement errors?
A vertical liquid column generates hydrostatic pressure.
In simplified form:
p = ρ × g × h
Where:
ρ= density of the liquid,g= gravitational acceleration,h= height of the liquid column.
If a gas bubble replaces part of this liquid column, the density in that section is much lower.
This changes the hydrostatic pressure on that side of the differential pressure transmitter.
The other impulse line remains completely filled with liquid.
The transmitter therefore detects a differential pressure even though the actual process differential pressure may be unchanged.
In addition, gas is compressible.
A gas bubble can therefore damp or delay the transmission of rapid pressure changes.
How do different liquid column heights affect the measurement?
Even without gas bubbles, different heights of the two liquid columns can cause a zero-point offset.
If both process taps are at the same height and both lines are hydraulically identical up to the transmitter, the same additional liquid column acts on both sides.
In simplified form:
High-pressure side: p+ + ρgh
Low-pressure side: p- + ρgh
When the difference is calculated, the additional hydrostatic component cancels out:
Δp = (p+ + ρgh) - (p- + ρgh)
and therefore:
Δp = p+ - p-
However, this only works if both liquid columns are comparable.
Differences can arise from:
- different heights,
- gas bubbles,
- different liquid densities,
- different temperatures,
- partially drained lines.
Example: 10 cm water column compared with a 100 mbar measuring span
A simple example shows how significant the elevation effect can be.
For water, approximately:
ρ ≈ 1000 kg/m³
At a height difference of:
0.10 m
the resulting pressure difference is:
Δp = 1000 × 9.81 × 0.10 Pa
or approximately:
981 Pa ≈ 9.8 mbar
For a differential pressure transmitter configured for a measuring span of:
0 ... 100 mbar
this corresponds to almost:
10 % of the total measuring span
.
Even just a few centimetres of effective difference between liquid columns can therefore cause a significant error with small differential pressure ranges.
Why should the high- and low-pressure lines be as symmetrical as possible?
In differential pressure measurement, not only each individual line matters, but especially how closely the two lines match.
The high- and low-pressure lines should therefore be as similar as possible in terms of:
- length,
- internal diameter,
- slope,
- material,
- valves,
- thermal environment.
Good symmetry improves:
- hydrostatic equality,
- dynamic behaviour,
- temperature equality,
- measurement reproducibility.
Especially with very small differential pressures, asymmetrical line conditions can account for a significant proportion of the total measurement error.
What role does the temperature of the impulse lines play?
The density of a liquid depends on temperature.
If the high- and low-pressure lines are heated differently, the liquid columns will have slightly different densities.
With large elevation differences between the process and transmitter, this can generate an additional differential pressure.
Typical causes include:
- one line is exposed to sunlight while the other is in shade,
- only one line is insulated,
- one line runs close to a hot process pipe,
- different heat tracing.
For precise differential pressure measurement, both impulse lines should therefore be exposed to thermal conditions that are as similar as possible.
This is particularly relevant for longer vertical line sections.
Where should the pressure taps be located?
For a horizontal liquid line, pressure taps are often arranged on the side of the pipe.
A tap directly at the lowest point of the line can be unfavourable because:
- sediment,
- rust,
- solids,
- deposits
can accumulate there.
These substances can then enter the small impulse lines and cause blockages.
For liquid flow measurements, manufacturers therefore typically recommend side-mounted pressure taps and transmitter installation at the same level or below them. :contentReference[oaicite:3]{index=3}
The exact position must, however, always match the type of primary element and its installation instructions.
How are the lines properly vented?
Before commissioning, both measuring sides must be completely filled with liquid and any trapped gas removed.
Many differential pressure transmitters or valve manifolds therefore provide suitable venting and draining facilities.
A sensible general procedure includes:
- Bring the measuring point into a safe operating condition.
- Set the valve manifold according to the operating instructions.
- Fill the impulse lines with process medium in a controlled manner.
- Completely vent gas from both sides.
- Close the vent points leak-tight again.
- Apply the same static pressure to both sides.
- Check the zero point or mounting-position error.
The exact valve sequence depends on the system, valve manifold, medium and safety requirements.
It should therefore not be replaced by a generic standard sequence.
What is the purpose of a three- or five-valve manifold?
A differential pressure transmitter is often combined with a three- or five-valve manifold.
A typical three-valve manifold has:
- a shut-off valve on the high-pressure side,
- a shut-off valve on the low-pressure side,
- an equalizing valve between the two measuring sides.
A five-valve manifold typically adds two additional vent or test valves.
This allows, among other things:
- isolating the transmitter from the process,
- equalizing both sides,
- checking the zero point,
- venting the measuring sides,
- connecting test equipment.
However, the valve manifold does not correct incorrectly routed impulse lines.
A gas pocket at a high point can form again after commissioning even if the measuring point was previously vented correctly.
How is a liquid measuring point commissioned?
Before the first application of pressure, the complete hydraulic installation should be checked.
In particular, verify:
- high-pressure line actually connected to the H side,
- low-pressure line actually connected to the L side,
- no local high points,
- no mechanically kinked lines,
- shut-off valves in the correct position,
- valve manifold correctly installed,
- lines completely filled,
- no visible leaks.
Both measuring sides are then filled and vented in a controlled manner.
If both sides are exposed to the same static pressure, the differential pressure should generally be close to:
0 mbar
.
Any remaining zero offset caused purely by mounting position can be corrected on suitable transmitters using a position adjustment.
A hydraulically induced error caused by gas bubbles or different liquid columns should not simply be electronically “zeroed out”.
When should the zero point be checked?
A zero-point check is particularly useful:
- after initial installation,
- after work on the impulse lines,
- after venting,
- after replacing the transmitter,
- after unusual process events,
- if the flow offset suddenly changes.
For a meaningful check, both transmitter sides must be hydraulically brought to the same pressure.
If the transmitter then indicates a significant differential pressure, first check:
- valve positions,
- venting,
- line filling,
- transmitter position.
What happens if an impulse line is partially blocked?
An impulse line can become partially or completely blocked by, for example:
- deposits,
- corrosion products,
- solids,
- crystallization,
- frozen medium.
A complete blockage does not necessarily result in an obviously incorrect static measured value immediately.
The trapped liquid volume can initially retain an earlier pressure condition.
Typical indications therefore include:
- slow response to process changes,
- different response of the high- and low-pressure sides,
- unusually damped signal,
- persistent offset after load changes.
Depending on the device version, modern differential pressure transmitters can provide additional diagnostic information to help assess the measuring point.
What applies to viscous or contaminated liquids?
Impulse lines are particularly advantageous for clean liquids with sufficiently low viscosity.
More problematic are media that are:
- highly viscous,
- crystallizing,
- solidifying,
- heavily loaded with solids,
- adhesive.
Such media can block small impulse lines.
Depending on the application, possible countermeasures include:
- larger line cross-sections,
- suitable flushing concepts,
- heat tracing,
- sealing liquid,
- diaphragm seal systems.
An impulse line that is permanently at risk of blockage should not simply be compensated for by cleaning it more and more frequently.
In that case, the suitability of the entire measurement concept for the process medium should be reviewed.
When are diaphragm seals preferable to impulse lines?
For difficult process media, directly mounted diaphragm seals or diaphragm seals connected via capillaries can be an alternative to conventional medium-filled impulse lines.
This can be useful for:
- highly viscous liquids,
- crystallizing media,
- aggressive substances,
- very hot processes,
- hygienic applications.
A diaphragm seal system also has its own influencing factors.
These include, for example:
- capillary length,
- temperature,
- fill fluid,
- diaphragm stiffness,
- response time.
The choice between impulse lines and diaphragm seals should therefore be based on the complete process requirements.
Practical example: zero-point error caused by a gas bubble
A differential pressure measurement monitors the flow through a water line.
Transmitter measuring range:
0 ... 100 mbar
The transmitter is installed approximately:
1.5 m
below the process line.
After maintenance, the measurement suddenly indicates:
+7 mbar
while the system is at standstill, even though practically the same process pressure is present at both pressure taps.
An electrical zero-point check of the transmitter shows no abnormality.
Inspection of the impulse lines reveals a local high point on the low-pressure side.
A gas bubble is trapped there.
The line is completely vented and then secured so that it rises continuously from the transmitter to the pressure tap.
After pressure equalization, the differential pressure is again close to:
0 mbar
.
The initially suspected transmitter fault was therefore a hydraulic installation error in the impulse line.
Systematically diagnosing measurement errors
- Check process conditions for plausibility.
- Check the H and L connections.
- Check the valve positions on the manifold.
- Apply the same static pressure to both sides.
- Check the transmitter zero point.
- Inspect impulse lines for high points.
- Completely vent both lines.
- Check for different liquid column heights.
- Compare line temperatures.
- Check lines for kinks or blockages.
- Check shut-off valves for full opening.
- Check pressure taps for deposits.
- Observe the measured value under several process conditions.
- Only then suspect a transmitter fault.
Systematically planning impulse lines
- Determine the process medium and process conditions.
- Define the expected differential pressure and static line pressure.
- Define the pressure tap positions according to the primary element.
- For liquids, position the transmitter at the same level as the taps or below them wherever possible.
- Plan both lines with a continuous upward slope toward the process.
- Consistently avoid high points.
- Design the high- and low-pressure lines as symmetrically as possible.
- Avoid unnecessarily long line runs.
- Select a suitable tube or hose diameter.
- Aim for the same thermal conditions on both lines.
- Provide suitable venting facilities.
- Select a suitable three- or five-valve manifold.
- Assess the risk of freezing and blockage.
- For problematic media, consider diaphragm seals as an alternative.
- Document the measuring-point geometry for future maintenance.
Common mistakes
- Installing the transmitter significantly above the liquid pressure taps: Gas can accumulate in the impulse lines.
- Allowing high points in the lines: Gas bubbles can become permanently trapped there.
- Venting only one line properly: Different liquid columns cause a zero-point error.
- Underestimating hydrostatic pressure components: Just 10 cm of water column corresponds to approximately 9.8 mbar.
- Immediately correcting a zero-point error electronically: This merely hides a hydraulic installation error.
- Routing the high- and low-pressure lines very differently: Temperature and dynamic influences become asymmetrical.
- Insulating one line but not the other: Different liquid densities can generate additional differential pressure.
- Locating pressure taps at the lowest point of a contaminated line: Sediment can enter the impulse lines.
- Using excessively small line diameters with contaminated media: The risk of blockage increases.
- Treating a gas bubble only as a dynamic problem: In vertical line sections, the missing liquid column alone can already generate a static offset.
- Considering the valve manifold a solution for poor line routing: It enables operation and testing but does not correct incorrect hydraulics.
- Suspecting a transmitter fault without checking the impulse lines first: Many apparent Δp errors originate in the process connection.
SITRANS P320 for differential pressure and flow measurement
A specific process transmitter for such applications is the Siemens SITRANS P320.
The series is designed for:
- gauge pressure,
- absolute pressure,
- differential pressure,
- level,
- volumetric flow,
- mass flow.
For differential pressure applications, the P320 can measure the differential pressure generated across a primary element and process it according to the required measurement task.
Key features include:
- high measurement accuracy,
- HART communication,
- diagnostic functions according to NAMUR NE107,
- local parameterization using push-buttons,
- versions for different differential pressure ranges and static process pressures,
- different materials for wetted parts,
- Ex and SIL versions.
For the differential pressure version, Siemens provides suitable venting options at the process connection for liquid applications.
However, the transmitter itself can only measure the differential pressure actually present at its two connections.
A high-quality measuring cell cannot compensate for an impulse line that is incorrectly routed, only partially filled or not properly vented.
Further information can be found under Siemens SITRANS P320 process transmitter and under process transmitters / differential pressure transmitters at ICS Schneider.
Conclusion
In differential pressure measurement with liquids, the impulse lines are part of the actual measuring chain.
Both lines must transfer the process pressure to the high- and low-pressure sides of the transmitter under hydraulic conditions that are as similar as possible.
Gas bubbles are particularly problematic. They replace part of the liquid column with a medium of much lower density and at the same time alter the dynamic behaviour of the pressure transmission.
For liquid applications, the transmitter is therefore usually installed beside or below the pressure taps. From the transmitter, the impulse lines should rise continuously toward the process so that gas can migrate back into the process line by itself.
Elevation differences must also not be underestimated. With water, just 10 cm difference between the liquid columns corresponds to approximately 9.8 mbar. With small differential pressure measuring spans, this can represent a significant proportion of the total range.
The high- and low-pressure lines should therefore be as symmetrical as possible in terms of height, length, temperature and routing.
Before commissioning, both lines must be completely filled and vented. A remaining offset should not be corrected electronically too quickly before gas bubbles, valve positions, elevation differences and blockages have been ruled out.
For reliable differential pressure measurement with liquids, the following therefore applies: install the transmitter below the pressure taps wherever possible, route both impulse lines continuously upward toward the process without high points, fill and vent them completely, minimize hydrostatic height differences and keep both measuring sides as symmetrical as possible.
FAQ: Impulse lines for liquids
Where should a differential pressure transmitter be installed for liquids?
Typically at the same level as the pressure taps or below them. This allows gas bubbles to migrate back to the process through the upward-sloping impulse lines.
Why must an impulse line rise in liquid applications?
The continuous upward slope from the transmitter to the process line allows trapped gas to migrate back toward the process by itself because of its lower density.
Why are high points in a liquid-filled impulse line problematic?
Gas bubbles can collect there. They change the hydrostatic liquid column and can cause both a static zero-point error and delayed measurement response.
How significant is an elevation difference?
The influence is calculated from Δp = ρ × g × Δh. With water, 10 cm height difference corresponds to approximately 9.8 mbar.
Do the liquid columns in the two impulse lines not cancel each other out?
Yes, provided both sides have the same liquid density and the same effective height. Differences between the two lines, however, create an additional differential pressure.
Can a small gas bubble really cause a relevant measurement error?
Yes. It replaces part of the liquid column with much lighter gas and can therefore cause a significant offset, especially with small differential pressure ranges.
Why do measurements sometimes respond slowly when gas bubbles are present?
Gas is compressible. Pressure changes are therefore transmitted differently than in a line that is completely filled with liquid.
Should the high- and low-pressure lines be the same length?
As symmetrical a line arrangement as possible is advantageous. In addition to length, height, diameter, temperature conditions, valves and filling condition of both lines should be kept comparable.
Why is the temperature of the impulse lines important?
Liquid density depends on temperature. Liquid columns at different temperatures can therefore generate an additional differential pressure when larger elevation differences are present.
Where are the pressure taps located for liquid applications?
On horizontal lines, they are often positioned at the side so that sediment from the bottom of the pipe is less likely to enter the impulse lines. The exact position depends on the primary element used.
Can I simply correct a zero-point error at the transmitter?
A pure mounting-position error of the transmitter can be adjusted if necessary. An error caused by gas bubbles, different liquid columns or blocked impulse lines should first be eliminated hydraulically.
When are diaphragm seals preferable to impulse lines?
They can be advantageous for highly viscous, crystallizing, adhesive, aggressive or very hot media where conventional impulse lines are prone to blockage or require extensive maintenance.
Which transmitter is suitable for differential pressure flow measurements?
One specific example is the Siemens SITRANS P320. It is designed for differential pressure, flow and level measurement and offers HART communication as well as diagnostic functions according to NAMUR NE107.