If a differential pressure transmitter delivers implausible values, the cause is not always the measuring instrument itself. Especially in differential pressure measurements, many errors are caused by the connection, the manifold, blocked or incorrectly filled impulse lines, reversed high/low sides, gas bubbles, condensate, temperature differences or an incorrectly performed zero adjustment.
This makes troubleshooting demanding. A differential pressure transmitter only measures the pressure difference between two connections. However, if an impulse line is blocked, a valve in the manifold is in the wrong position or both sides were not adjusted under identical conditions, the measured value can deviate significantly from the actual process condition.
This article explains the typical causes of incorrect differential pressure values, how to check the manifold, impulse lines and zero point, and what must be considered especially in filter monitoring, differential pressure flow measurement, level measurement and process plants.
Table of contents
- Why differential pressure values can be implausible
- How a differential pressure transmitter basically measures
- Correctly assigning the high and low side
- Checking the manifold: 3-valve and 5-valve manifold
- Performing zero adjustment correctly
- Checking differential pressure lines and impulse lines
- Blocked impulse line as a cause of error
- Gas bubbles, condensate and incorrect filling
- Taking installation position and height differences into account
- Temperature differences in the lines
- Application: Filter monitoring
- Application: Flow measurement via orifice plate or differential pressure element
- Application: Level measurement with differential pressure
- Table: Measurement pattern, possible cause and test step
- Practical example: Differential pressure does not rise despite a contaminated filter
- Conclusion: Always check the entire measuring path when values are implausible
- FAQ: Frequently asked questions about incorrect differential pressure values
Why differential pressure values can be implausible
An implausible differential pressure value does not automatically mean that the transmitter is defective. Often, the measured value is incorrect simply because the pressure difference at the device does not correspond to the actual pressure difference in the process. This can happen if an impulse line is blocked, a valve remains closed or the high and low sides have been reversed.
A zero point error can also lead to incorrect values. If the transmitter was not adjusted correctly or if the same pressure was not present on both sides during zero adjustment, an offset remains. This offset has a particularly significant effect at low differential pressures.
For differential pressure measurements, the entire measuring path must therefore always be considered: process connection, tapping points, impulse lines, manifold, transmitter, installation position, medium, temperature and parameterisation. The transmitter is only one part of the system.
Typical symptoms include a permanently too high or too low differential pressure, a negative measured value, a very sluggish signal, a jump after valve operation, an apparently stable value despite process changes or strong fluctuations without any obvious process cause.
How a differential pressure transmitter basically measures
A differential pressure transmitter compares two pressures with each other. One side is often referred to as the high side or plus side, the other as the low side or minus side. The displayed differential pressure is calculated as follows:
Differential pressure = pressure on the high side minus pressure on the low side
If the same pressure is present on both sides, the differential pressure should ideally be zero. If the pressure on the high side is greater than the pressure on the low side, a positive differential pressure results. If the assignment is reversed, the value can appear negative or implausible.
In filter monitoring, the pressure before and after the filter is compared. In flow measurements via an orifice plate or differential pressure element, the pressure difference before and after the restriction is evaluated. In level measurements, the hydrostatic pressure of a liquid column is compared with a reference pressure.
The measuring principle is simple. In practice, however, the implementation is sensitive to connection errors, line condition, filling and installation situation.
Correctly assigning the high and low side
One of the most common causes of incorrect differential pressure values is a reversed high/low assignment. If the process lines are connected incorrectly, the transmitter displays a negative value or a value with the wrong direction.
In filter monitoring, the high side must normally be connected upstream of the filter, i.e. on the side with the higher pressure. The low side is connected downstream of the filter. When the filter becomes contaminated, the pressure loss across the filter increases and the differential pressure rises.
In flow measurement via an orifice plate, the high side is upstream of the orifice plate and the low side is downstream. If this is reversed, the signal can become negative or the flow calculation may operate incorrectly.
In level measurement, the correct assignment depends on whether the tank is open or closed, whether a dry or filled reference line is used and how the transmitter is installed. Here, the design must be checked particularly carefully.
Checking the manifold: 3-valve and 5-valve manifold
A manifold simplifies installation, commissioning, zero adjustment and maintenance of a differential pressure transmitter. At the same time, it is a common source of error if valves are in the wrong position or do not close completely.
A 3-valve manifold typically consists of two shut-off valves and one equalising valve. The shut-off valves isolate the high and low side from the process. The equalising valve connects both sides with each other so that the same pressure is applied to both transmitter sides. This is particularly important for zero adjustment.
A 5-valve manifold additionally has vent or drain valves. These allow the lines to be vented, drained or checked. This is particularly helpful with liquids, condensate, gas bubbles or maintenance work.
If the equalising valve accidentally remains open, the transmitter measures almost no differential pressure, even though a pressure difference exists in the process. If a shut-off valve remains closed, one side of the transmitter can be isolated from the process. The measured value then remains fixed or reacts sluggishly or incorrectly.
| Valve position | Possible effect | Test note |
|---|---|---|
| Equalising valve open | Differential pressure is equalised, measured value near zero | Close equalising valve after zero adjustment |
| High side shut off | Transmitter does not measure the current process pressure on the high side | Check shut-off valve position |
| Low side shut off | Transmitter does not measure the current process pressure on the low side | Check shut-off valve position |
| Vent open or leaking | Measured value may deviate or fluctuate | Check vent and drain valves |
| Valve does not close completely | Internal leakage or slow pressure change possible | Check manifold for tightness |
Performing zero adjustment correctly
Zero adjustment is an important step during commissioning and maintenance of a differential pressure transmitter. The transmitter is adjusted so that it displays zero when the same pressure is applied to both sides.
A typical error is performing the zero adjustment even though the same pressure is not actually present on the high and low side. In this case, an incorrect condition is stored as the zero point. This later leads to a permanent offset in the measured value.
For correct zero adjustment, the process is shut off depending on the system, the transmitter is pressure-equalised and the equalising valve is opened. Both measuring chambers must see the same pressure. Only then may the zero adjustment be performed. After adjustment, the equalising valve must be closed again and the process side opened correctly.
With small measuring ranges, the zero point is particularly critical. Even small height differences, medium columns or temperature differences can cause a measurable offset. The zero adjustment should therefore always be performed under defined and documented conditions.
Checking differential pressure lines and impulse lines
Differential pressure lines or impulse lines transmit the process pressure from the tapping points to the differential pressure transmitter. If these lines are not clear, not correctly filled or not routed correctly, the process pressure does not arrive correctly at the transmitter.
With gases, liquid accumulation in the lines should be avoided if it can falsify the pressure. With liquids, gas bubbles must be avoided because they can dampen or falsify the signal. With steam, condensate pots are often used so that both sides have comparable conditions.
The lines should be designed as similarly as possible. Different line lengths, different temperatures or different filling conditions can cause measurement errors. Especially at low differential pressures, such effects can be significant.
During inspection, the lines should be checked for continuity, tightness, blockage, kinks, incorrect slope, condensate, gas bubbles and correct isolation.
Blocked impulse line as a cause of error
A blocked impulse line is a common cause of implausible or sluggish differential pressure values. Deposits, dirt, crystallisation, condensate, frozen medium or particles can block the pressure channel.
If a line is completely blocked, the transmitter can no longer follow the current process pressure on this side. The measured value then remains fixed or changes only very slowly. If the line is partially blocked, the signal reacts sluggishly or is damped.
In filter monitoring, a blocked impulse line can cause a contaminated filter not to be detected. In flow measurements, the calculated flow can be incorrect. In level measurements, an incorrect level can be displayed.
For inspection, lines can be vented, flushed or compared on the pressure side. Medium, pressure, temperature and safety requirements must be taken into account. With hazardous media, such work may only be carried out using suitable procedures and protective measures.
Gas bubbles, condensate and incorrect filling
Gas bubbles and condensate are typical sources of error in differential pressure measurements. The decisive factor is whether gas or liquid is being measured in the respective application and what line filling is intended.
In liquid measurements, gas bubbles in the impulse lines cannot transmit pressure rigidly. The signal can therefore be damped, delayed or shifted. Especially at low differential pressures, even a small gas bubble can have a significant influence.
In gas measurements, liquid in the lines can lead to an additional hydrostatic pressure component. This falsifies the differential pressure. If condensate accumulates on only one side, a one-sided pressure shift occurs.
In steam and condensate applications, symmetrical filling is particularly important. Both lines must have comparable condensate columns so that no false difference is created by different filling heights.
Taking installation position and height differences into account
The installation position of the differential pressure transmitter and the height position of the impulse lines can influence the measured value. If liquid columns are present in the lines, they generate additional hydrostatic pressure components.
With large height differences between tapping points and transmitter, it must be checked whether a zero offset must be taken into account. This is especially true for level measurements, measurements with filled impulse lines or applications with seal fluids.
The position of the transmitter itself can also be relevant. Depending on the design and measuring range, installation position and mechanical mounting can influence the zero point. The zero adjustment should therefore be performed after final installation in the final mounting position.
If a transmitter has been removed, rotated or installed at a different location, it should be checked whether a new zero adjustment is required.
Temperature differences in the lines
Temperature differences between the high and low line can cause errors in differential pressure measurements. This is especially true for long impulse lines, liquid fillings, steam, condensate or applications with small measuring ranges.
If one line is warmer than the other, density, volume or condensation behaviour can differ. This can create an additional pressure component that does not correspond to the actual process differential pressure.
In outdoor installations, sunlight, wind, ambient temperature or insulation can play a role. In process plants, one line may be closer to a hot component than the other. Unevenly insulated lines can also cause measurement errors.
For stable measurements, the high and low lines should have thermal conditions that are as comparable as possible. In critical applications, insulation, trace heating or modified line routing may be required.
Application: Filter monitoring
In filter monitoring, the pressure before and after the filter is measured. The more contaminated the filter becomes, the greater the pressure loss across the filter. The differential pressure transmitter displays this increasing pressure loss.
Implausible values here are often caused by reversed high/low connections, blocked impulse lines or incorrect valve positions in the manifold. If the differential pressure does not rise despite a contaminated filter, a line may be blocked or the equalising valve in the manifold may be open.
If the differential pressure is permanently too high, the filter may actually be contaminated. However, a low-pressure line may also be blocked, a valve may be closed or a tapping point may be clogged.
For reliable evaluation, the measured value should be compared with the actual system condition. Filter condition, flow, valve position and process pressure must be considered together.
Application: Flow measurement via orifice plate or differential pressure element
In flow measurement via an orifice plate, nozzle or another differential pressure element, the differential pressure is used as a measure of flow. The differential pressure increases with increasing flow. However, the calculation is sensitive to connection and line errors.
If the high and low sides are reversed, the differential pressure can become negative. If a line is blocked, the flow indication reacts sluggishly or incorrectly. If gas bubbles or condensate are present, the measured value can fluctuate or be shifted.
The tapping points at the differential pressure element must also be correctly designed and clear. A contaminated pressure tapping can falsify the measured value. Such errors have a particularly strong effect at low differential pressures.
During troubleshooting, it should be checked whether the process flow is plausible in relation to the displayed pressure difference. In addition, manifold, lines, filling and zero point should be checked.
Application: Level measurement with differential pressure
In hydrostatic level measurement with differential pressure, the pressure of a liquid column is evaluated. In open tanks, the hydrostatic pressure at the lower connection is often measured. In closed tanks, the gas space pressure must also be taken into account. A differential pressure transmitter is used for this purpose.
Implausible values are often caused by incorrect density assumptions, incorrect installation height, incorrect zero point, filled or dry reference lines, condensate or gas bubbles. Temperature changes can also influence the density of the medium and therefore the measured value.
In closed tanks, the low side is particularly important. If the reference line does not have the intended condition, the level can be displayed significantly incorrectly. A dry reference line behaves differently from a filled reference line.
For correct level measurement, installation height, density, pressure range, reference line, zero point and scaling must be designed and checked carefully.
Table: Measurement pattern, possible cause and test step
| Measurement pattern | Possible cause | Next test step |
|---|---|---|
| Differential pressure is negative | High and low side reversed or process direction assumed incorrectly | Check connection and process diagram |
| Differential pressure remains at zero | Equalising valve open, both sides connected or no process differential pressure present | Check manifold and process condition |
| Measured value reacts very sluggishly | Partially blocked impulse line, damping too high or gas bubbles | Check lines, filter and damping |
| Measured value remains fixed | Impulse line blocked or shut-off valve closed | Check valve position and line continuity |
| Measured value jumps after valve operation | Pressure was trapped on one side or manifold was operated incorrectly | Check valve sequence and zero adjustment |
| Zero point shifted | Zero adjustment performed incorrectly, height difference or medium column | Equalise both sides and repeat zero adjustment |
| Measured value fluctuates strongly | Pulsation, gas bubbles, condensate, process fluctuation or EMC | Check process, lines, damping and signal |
| Filter differential pressure does not rise | Impulse line blocked, equalising valve open or incorrect connection | Check high/low assignment, manifold and lines |
| Flow indication implausible | Zero point error, incorrect orifice plate connections, blocked line or incorrect filling | Check differential pressure element, lines and transmitter scaling |
Practical example: Differential pressure does not rise despite a contaminated filter
In a process plant, a differential pressure transmitter is used for filter monitoring. According to the maintenance schedule, the filter is already heavily loaded, but the displayed differential pressure remains unusually low. Initially, it is suspected that the transmitter is defective.
During inspection, it becomes clear that the transmitter responds correctly during zero adjustment. The output signal is also basically plausible. The manifold is then checked. The equalising valve is closed and the shut-off valves are in the correct position.
In the next step, the impulse lines are checked. It turns out that the high line upstream of the filter is partially blocked. As a result, the actual upstream pressure reaches the transmitter only with delay and damping. The differential pressure therefore does not rise according to the real filter condition.
After cleaning the impulse line and checking again, the transmitter shows a significantly higher differential pressure. The filter is then replaced. The measurement subsequently confirms a low differential pressure again.
This example shows: When differential pressure values are implausible, not only the transmitter should be checked. The manifold, impulse lines and process connections are at least equally important.
Conclusion: Always check the entire measuring path when values are implausible
A differential pressure transmitter only delivers reliable values if the entire measuring path works correctly. This includes high/low assignment, manifold, impulse lines, filling, venting, installation position, temperature conditions and zero adjustment.
Many implausible measured values are not caused by a defective transmitter, but by incorrectly positioned valves, blocked lines, gas bubbles, condensate, reversed connections or an incorrect zero point. Especially at low differential pressures, even small deviations can have a major effect on the measured value.
Systematic troubleshooting therefore begins at the process connection and does not end at the measuring instrument. Anyone who checks manifold, lines and zero point properly will find the cause much faster and avoid unnecessary replacement of functioning transmitters.
FAQ: Frequently asked questions about incorrect differential pressure values
Why does my differential pressure transmitter measure incorrectly?
Common causes include reversed high/low connections, incorrect valve positions in the manifold, blocked impulse lines, gas bubbles, condensate, incorrect zero adjustment or an unsuitable installation position.
What happens if the high and low side are reversed?
The differential pressure may be displayed as negative or the measuring direction is wrong. In flow, filter or level measurements, this leads to implausible results.
Why does the differential pressure transmitter display zero?
Possible causes include an open equalising valve, incorrectly positioned shut-off valves, no actual process differential pressure or a connection between the high and low side.
How do you perform zero adjustment on a differential pressure transmitter?
For zero adjustment, both sides of the transmitter must see the same pressure. Usually, the process is shut off and the equalising valve is opened. Only then is the zero point set. Afterwards, the equalising valve must be closed again.
Can a blocked impulse line falsify the measured value?
Yes. If an impulse line is blocked, the current process pressure does not arrive correctly at the transmitter. The measured value can remain fixed, react sluggishly or be completely wrong.
Why does the differential pressure transmitter react very slowly?
A sluggish reaction can be caused by partially blocked lines, excessive damping, gas bubbles, condensate or very long impulse lines.
What role does the manifold play?
The manifold enables shut-off, equalisation, venting and zero adjustment. Incorrectly positioned or leaking valves can significantly falsify the measured value.
What is the difference between a 3-valve and a 5-valve manifold?
A 3-valve manifold typically has two shut-off valves and one equalising valve. A 5-valve manifold additionally has vent or drain valves for maintenance and testing.
How do gas bubbles affect differential pressure measurement?
Gas bubbles in liquid lines can prevent correct pressure transmission. This can dampen, delay or shift the signal.
How does condensate affect the measurement?
Condensate can create additional hydrostatic pressure components. If condensate accumulates on only one side, a false differential pressure is created.
Why is the installation position important?
Height differences and liquid columns can affect the zero point. The zero adjustment should therefore be performed in the final installation position.
What should be checked first if the differential pressure is implausible?
First, high/low assignment, valve position in the manifold, zero adjustment and continuity of the impulse lines should be checked. Only after that should a transmitter defect be suspected.