For diaphragm seals, the smallest possible process connection is often requested. It requires little installation space, can be integrated more easily into existing pipelines and reduces the design effort required for vessels, machinery and pipework. However, a small connection often also results in a small effective diaphragm area.
This creates a technical conflict: a small diaphragm is comparatively stiff and displaces less system fill fluid at the same pressure than a larger diaphragm. At high pressure ranges, this is often uncritical or even advantageous. At low measuring ranges, with large temperature changes or long capillary lines, however, the combination may result in increased temperature error, slower response or a technically unfeasible design.
The diaphragm size must therefore not be selected independently of the pressure measuring instrument. The complete diaphragm seal system comprising the diaphragm, system fill fluid, mounting arrangement, capillary and pressure gauge, pressure switch or pressure transmitter is decisive. Only after a technical calculation can it be determined which minimum measuring range can be achieved with the required accuracy and response time.
Table of Contents
- How does a diaphragm seal system work?
- Why does the diaphragm diameter affect the measurement?
- Why do low measuring ranges become critical?
- A small process connection does not always mean a small diaphragm
- Temperature influence and zero-point shift
- What role does the system fill fluid play?
- Diaphragm size and response time
- Direct mounting, cooling element or capillary line?
- Pressure gauge or pressure transmitter?
- How is the overall accuracy assessed?
- High nominal pressure and a low measuring range are not the same
- Typical configuration and ordering errors
- Practical example with a hot, viscous medium
- Correctly configuring a diaphragm seal system
- Which measuring instruments / products are suitable?
- Conclusion
- Frequently asked questions
How does a diaphragm seal system work?
A diaphragm seal separates the actual pressure measuring instrument from the process medium. The medium acts on an elastic metallic diaphragm. Behind the diaphragm is a hydraulic system completely filled with a suitable fluid.
When the diaphragm deforms under the process pressure, it displaces a small volume of fluid. This volume displacement is transmitted through the system fill fluid to the connected measuring instrument. The instrument may, for example, be a pressure gauge, pressure switch or pressure transmitter.
Depending on the design, the complete system consists of:
- diaphragm seal with process connection and diaphragm,
- system fill fluid,
- direct connection, cooling element or capillary line,
- connected pressure measuring instrument,
- where applicable, protection, flushing or mounting components.
The system filling must be free of bubbles and remain permanently leak-tight. Even small gas bubbles would change the transmission behaviour because gases are significantly more compressible than liquids.
An assembled diaphragm seal system must therefore not be separated like a simple threaded adapter. Opening the system may allow fill fluid to escape or air to enter. Professional refilling and recalibration are then required.
Why does the diaphragm diameter affect the measurement?
The effective diaphragm area determines how strongly the diaphragm can deform at a given pressure. With an otherwise comparable design, a larger diaphragm has lower stiffness and can displace a greater volume.
A small diaphragm, by contrast, is stiffer. A higher pressure is required to generate the same deflection and volume displacement. This explains why compact diaphragm seals are particularly suitable for medium and high pressure ranges, while low pressure ranges often require a larger diaphragm.
| Characteristic | Small diaphragm | Large diaphragm |
|---|---|---|
| Mechanical stiffness | Comparatively high | Comparatively low |
| Volume displacement at the same pressure | Low | Greater |
| Suitability for low measuring ranges | Often limited | Usually more favourable |
| Temperature influence on the system | May be greater in relation to the measuring span | Usually more favourable due to lower diaphragm stiffness |
| Possible nominal pressure | Very high depending on the design | More dependent on design and material |
| Space requirement | Low | Greater |
In addition to the diameter, diaphragm thickness, material, contour and manufacturing process also affect stiffness. The visible outside diameter of the diaphragm seal is therefore not automatically identical to the effective diaphragm diameter.
The diaphragm must also not be made arbitrarily thin. It must withstand the process pressure, possible pressure spikes, mechanical stress, corrosion and, where applicable, abrasion.
Why do low measuring ranges become critical?
The connected pressure measuring instrument requires a certain volume displacement to cover its full measuring range. This required volume is often referred to as the displacement or control volume.
At a high pressure range, sufficient pressure is available to deflect the small diaphragm and displace the required volume of fluid. At a low measuring range, the resulting diaphragm movement is significantly smaller.
If the diaphragm is too stiff, the system may exhibit the following problems:
- The intended full-scale value is not reached with the required accuracy.
- The characteristic curve is influenced by the diaphragm spring characteristic.
- The temperature error becomes too large in relation to the small measuring span.
- The zero point becomes sensitive to temperature changes.
- The required response time cannot be achieved.
- The requested combination is not approved by the manufacturer.
A diaphragm seal with a permissible nominal pressure of, for example, 400 bar is therefore not automatically suitable for a measuring range of 0 to 400 mbar. The nominal pressure describes the mechanical load capacity. The measuring range, by contrast, describes the pressure span that the complete measuring system is intended to measure reliably.
A universally applicable minimum measuring range cannot be derived from the process connection alone. It also depends on the diaphragm diameter, measuring instrument, fill fluid, temperature range and mounting arrangement.
A small process connection does not always mean a small diaphragm
The process thread and effective diaphragm diameter are two different dimensions. With a diaphragm seal featuring a recessed diaphragm, the actual threaded connection may be smaller than the outside diameter of the diaphragm seal body.
This makes it possible, for example, to combine a G ½ or ½ NPT process connection with different diaphragm diameters. The pressure reaches the diaphragm through an opening in the lower body, while the diaphragm is located in a larger diaphragm seal body.
With a flush design, by contrast, the diaphragm is positioned directly at the process opening. If the entire diaphragm seal must also be very compact, the available diaphragm area is correspondingly limited.
The designs therefore differ fundamentally:
| Design | Advantage | Possible limitation |
|---|---|---|
| Recessed diaphragm | Larger diaphragm area possible despite a threaded connection | The internal cavity may be unsuitable for viscous or crystallising media |
| Small flush diaphragm | Compact and without a narrow pressure channel | Low measuring ranges may be limited |
| Large flush diaphragm | Favourable for low pressures and difficult media | Larger process connection and greater space requirement |
| Flanged diaphragm seal | Large diaphragm and numerous material options | Greater installation effort and higher cost |
| In-line diaphragm seal | Low-dead-space measurement within the unobstructed pipe cross-section | Installation in the process pipeline is required |
For viscous, pasty or crystallising media, it is not sufficient merely to select the largest possible recessed diaphragm. If the pressure channel upstream of the diaphragm can become blocked, a genuinely flush or in-line design is often more important.
Temperature influence and zero-point shift
The system fill fluid changes its volume with temperature. When the diaphragm seal system heats up, the fluid expands. Because the system is closed, this volume change acts on the diaphragm and on the measuring element of the connected instrument.
The resulting force can produce a zero-point shift even without any change in process pressure. During cooling, the effect occurs in the opposite direction.
The extent of the temperature error depends, among other things, on:
- effective diaphragm diameter and diaphragm stiffness,
- total fill volume of the system,
- thermal expansion coefficient of the fill fluid,
- process and ambient temperature,
- temperature distribution along a capillary line,
- measuring span of the connected instrument.
Due to its lower stiffness, a large diaphragm can absorb the volume expansion with a smaller additional pressure force. With a small, stiff diaphragm, the same volume change can generate a significantly larger pressure error.
This is particularly critical with small measuring spans. A temperature-related zero-point shift of a few millibar would be virtually irrelevant over a measuring range of 0 to 400 bar, but could be unacceptable over a range of 0 to 100 mbar.
The temperature error of a diaphragm seal system must therefore be assessed first in absolute pressure units and then in relation to the intended measuring span.
What role does the system fill fluid play?
The fill fluid hydraulically transmits the pressure from the diaphragm to the measuring instrument. Its selection affects the temperature range, response time, suitability for vacuum and process safety.
Important properties include:
- viscosity and its temperature dependence,
- thermal expansion coefficient,
- freezing and boiling behaviour,
- vapour pressure in vacuum applications,
- compatibility with the process medium,
- approval for food or pharmaceutical applications,
- suitability for oxygen, chlorine or other reactive media.
A low-viscosity fill fluid supports rapid pressure transmission. However, as the temperature falls, the viscosity of many fluids increases significantly. The system then responds more slowly even though the specified lower operating temperature of the fluid has not yet been exceeded.
The temperature approval of the fill fluid alone therefore does not guarantee an adequate response time. The dynamic behaviour of the complete combination must be considered separately.
If the diaphragm becomes damaged, the fill fluid may enter the process. For food, pharmaceutical products, oxygen or other critical media, an explicitly suitable fluid must therefore be selected.
Diaphragm size and response time
A small design is often automatically associated with fast measurement. With a diaphragm seal system, however, the relationship is not so straightforward.
The response time is determined by the complete hydraulic transmission chain:
- volume displacement of the connected measuring instrument,
- stiffness and movement of the diaphragm seal diaphragm,
- viscosity of the fill fluid,
- length and internal diameter of a capillary,
- temperature of the fill fluid,
- absence of gas and quality of the system filling.
A pressure gauge with a comparatively large displacement volume requires more fluid displacement than a compact electronic pressure sensor. A small diaphragm may therefore still function with a pressure transmitter while the same diaphragm is unsuitable for a low-range Bourdon tube pressure gauge.
Long or very narrow capillary lines increase the hydraulic flow resistance. The pressure change therefore reaches the measuring instrument with a delay. At low temperatures, the higher viscosity of the fill fluid further increases this effect.
For rapid pressure changes, short machine cycles or the measurement of pressure spikes, a directly mounted system with a low fill volume should therefore be used wherever possible. A diaphragm seal system is not automatically suitable for highly dynamic pressure profiles.
Direct mounting, cooling element or capillary line?
Direct mounting
With direct mounting, the measuring instrument is positioned immediately on the diaphragm seal. This keeps the fill volume and hydraulic transmission path small. This design generally provides the most favourable response time and a lower additional temperature influence.
Direct mounting is only possible, however, if the process and ambient temperatures remain within the permissible limits of the measuring instrument and vibrations at the measuring point are acceptable.
Cooling element
A cooling element increases the distance between the process and measuring instrument and can reduce the temperature at the instrument connection. It remains relatively compact but increases the fill volume and response time compared with direct mounting.
Capillary line
A capillary enables the diaphragm seal and measuring instrument to be installed separately. It is used when high temperatures, strong vibrations, poor readability or limited installation space prevent mounting directly at the process.
As the capillary length increases, the following also increase:
- fill volume,
- temperature influence,
- response time,
- sensitivity to ambient-temperature fluctuations,
- hydrostatic influence caused by differences in height.
The capillary should therefore be selected only as long as technically necessary. The intended height difference between the diaphragm seal and measuring instrument must also be known at the configuration stage.
Pressure gauge or pressure transmitter?
The required diaphragm size depends significantly on the connected measuring instrument. The pressure measuring instrument is not an arbitrarily interchangeable component of the system.
| Measuring instrument | Typical characteristic within the diaphragm seal system |
|---|---|
| Bourdon tube pressure gauge | Requires a comparatively large volume displacement depending on nominal size and measuring range |
| Diaphragm pressure gauge | Suitable for low pressure ranges, but must also be configured as a complete system |
| Electronic pressure transmitter | Often has a small displacement volume and therefore allows more favourable combinations |
| Process transmitter | High accuracy and turndown are possible, but the temperature effect of the diaphragm seal remains relevant |
| Pressure switch | The switching and reset points may be affected by the additional system error |
The measuring range of the basic instrument must not be considered in isolation. A highly accurate transmitter loses its accuracy advantage if the additional temperature error of the diaphragm seal system significantly exceeds the inherent instrument accuracy.
For very low measuring ranges, a pressure transmitter with a directly integrated flush measuring cell may be technically more suitable than a separate measuring instrument fitted with a compact diaphragm seal. This applies particularly where the medium, temperature and materials permit such a solution.
How is the overall accuracy assessed?
The overall measurement deviation does not consist solely of the accuracy stated in the data sheet of the pressure gauge or transmitter. The effects of the diaphragm seal system and the specific installation must also be taken into account.
Factors to consider include:
- basic accuracy of the measuring instrument,
- additional characteristic deviation of the diaphragm seal system,
- temperature influence on zero point and span,
- influence of the mounting position,
- hydrostatic pressure of the capillary filling,
- long-term stability,
- repeatability and hysteresis,
- process dynamics and pulsation.
With a capillary-connected system, the difference in height alone may create a relevant zero-point shift. If the measuring instrument is installed above the diaphragm seal, the fluid column acts differently than when the instrument is mounted below the measuring point.
After installation, the zero point should therefore be checked under the actual installation conditions and corrected where necessary. A zero-point adjustment does not, however, automatically eliminate temperature-dependent changes during operation.
For demanding measuring points, the complete diaphragm seal system should be calibrated as one assembly. Separate calibration of the diaphragm seal and measuring instrument does not fully represent the hydraulic behaviour of the assembled combination.
High nominal pressure and a low measuring range are not the same
Due to their compact and stiff design, small diaphragms can be designed for very high nominal pressures. This is particularly advantageous for small threaded connections and high-pressure applications.
However, the permissible nominal pressure does not indicate how low the measuring range may be. A diaphragm seal may mechanically withstand 600 or 1,000 bar and still be unsuitable for accurate measurement from 0 to 250 mbar.
The following must be distinguished:
- Measuring range: The range within which the pressure value is to be measured.
- Nominal pressure: The mechanical pressure rating of the diaphragm seal and process connection.
- Overload limit: The load that the complete measuring system may withstand for a short period.
- Burst pressure: The pressure at which mechanical failure must be expected.
The lowest permissible limit within the complete combination is always decisive. A high-pressure-resistant diaphragm seal does not automatically increase the overload resistance of the connected measuring instrument.
Typical configuration and ordering errors
The smallest available process connection is specified
Only afterwards is an attempt made to achieve a low measuring range. However, the available diaphragm area may not be sufficient for the required volume displacement.
Only the nominal pressure is considered
High mechanical load capacity is confused with suitability for any desired measuring range.
The process temperature is specified without the ambient temperature
The process, ambient and, where applicable, capillary temperatures must be known for the system calculation.
A long capillary is ordered as a precaution
Unnecessary capillary length increases the fill volume and adversely affects the response time and temperature behaviour.
The measuring instrument is replaced at a later stage
A different pressure gauge or transmitter may have a different displacement volume. The original system configuration is then no longer necessarily valid.
The fill fluid is selected solely according to the maximum temperature
Viscosity, expansion, suitability for vacuum and compatibility with the medium are not considered.
The diaphragm seal is ordered separately
The diaphragm seal and measuring instrument are only screwed together on site. Without professional evacuation, filling and calibration, this does not create a functional diaphragm seal system.
A low transmitter range is achieved using turndown
The electronic transmitter can be ranged down, but the absolute temperature error of the diaphragm seal remains and becomes larger in relation to the reduced measuring span.
Practical example: Pressure measurement on a hot, viscous medium
In a production plant, the pressure of a viscous medium at 160 °C is to be measured. The normal operating pressure is between 0.3 and 1.5 bar. A measuring range of 0 to 2.5 bar and a flush G ½ process connection are required.
Due to space restrictions, a very compact diaphragm seal with a small diaphragm and a three-metre capillary line is initially requested. The measuring instrument is to be installed outside the hot area.
The combination is critical for several reasons:
- The measuring range is comparatively low for a very small diaphragm.
- The process temperature causes significant expansion of the fill fluid.
- The long capillary increases the fill volume.
- The viscosity-dependent response time may increase significantly in a cold environment.
- The absolute temperature error has a strong influence on a measuring span of only 2.5 bar.
As a solution, a design with a larger effective diaphragm is assessed. The capillary is also reduced to the length actually required, and a fill fluid suitable for the temperature and medium is selected.
If the existing G ½ connection does not permit a sufficiently large flush diaphragm, the process connection must be enlarged or a different measuring-point design used. Another possible solution is a suitable flush pressure transmitter with an integrated measuring cell, provided its temperature limits can be observed.
The complete system is then calculated for the process and ambient temperatures and calibrated as an assembly. Only this configuration enables a reliable statement to be made regarding measurement error and response time.
Correctly configuring a diaphragm seal system
At least the following information is required for the technical configuration:
- minimum, normal and maximum process pressure,
- required measuring range and permissible measurement deviation,
- medium and chemical composition,
- viscosity, solids and possible crystallisation,
- minimum and maximum process temperature,
- minimum and maximum ambient temperature,
- process connection and available installation space,
- required diaphragm and diaphragm seal materials,
- pressure gauge, pressure switch or pressure transmitter,
- direct mounting, cooling element or capillary,
- required capillary length and difference in height,
- expected rate of pressure change,
- overpressure, pulsation and possible pressure spikes,
- hygiene, Ex or safety requirements.
Based on this information, the required diaphragm diameter and the combination that can be approved by the manufacturer are determined. Selecting individual catalogue items alone is not sufficient for critical measuring ranges and temperatures.
Which measuring instruments / products are suitable?
Diaphragm seals
The diaphragm seals category includes different diaphragm, in-line, flanged, threaded and hygienic designs.
Depending on the application, small flush diaphragm seals, larger diaphragm seals, in-line designs or capillary-connected systems may be used. Diaphragm material, fill fluid, measuring range and mounting arrangement must always be specified together.
WIKA Type 990.36 compact diaphragm seal
The WIKA Type 990.36 is a compact diaphragm seal with a threaded connection and flush diaphragm.
It is particularly suitable for viscous, crystallising or contaminated media and, depending on the version and process connection, can also be used at high pressures. Versions with a protective plate for abrasive media are optionally available.
Due to the small diaphragm, the combination with the intended measuring range and measuring instrument must be technically assessed. Its compact design does not mean that arbitrarily low pressure ranges can be achieved.
WIKA Type 990.34 with different diaphragm diameters
The WIKA Type 990.34 is a welded diaphragm seal with a threaded connection and recessed diaphragm. Different effective diaphragm diameters are possible depending on the pressure rating, material, measuring instrument and measuring range.
The design is suitable for aggressive, hot, corrosive or contaminated media. Direct mounting, a cooling element or a flexible capillary line are possible.
This type illustrates that the process thread and diaphragm diameter must be considered separately. Even with a compact threaded connection, the diaphragm seal body may contain a larger effective diaphragm.
Analogue pressure sensors and pressure transmitters
The analogue pressure sensors / pressure transmitters category includes sensors with different measuring cells, process connections and output signals.
Where the medium and temperature conditions are suitable, a transmitter with a directly integrated flush measuring cell may be an alternative to a separate diaphragm seal system. This eliminates additional capillaries and part of the fill volume.
IMP331 with flush G ½ connection
The IMP331 is available with a flush G ½ connection from low pressure ranges upwards. It is suitable for media that are compatible with the stainless-steel and sealing materials used.
Such an integrated solution can be advantageous where a compact process connection and low measuring range are required, provided the process temperature remains within the permissible sensor limits.
With a 4–20 mA pressure transmitter, the electrical output signal can be checked during commissioning using a suitable multimeter or the Druck UPS4E current-loop calibrator. However, this electrical test does not replace pressure calibration of the complete measuring system.
Conclusion: A small diaphragm requires particularly careful system configuration
Small diaphragm seal diaphragms enable compact process connections and high pressure ratings. At the same time, they are stiffer and displace less fluid volume at the same pressure than larger diaphragms.
This can limit technical feasibility at low measuring ranges. Combinations involving a small diaphragm, small measuring span, high process temperature and long capillary line are particularly critical.
The temperature influence is caused primarily by the change in volume of the system fill fluid. Large diaphragms, a low fill volume and a suitable fill fluid reduce the resulting zero-point error.
The response time is also not determined by the diaphragm alone. The displacement volume of the measuring instrument, capillary length, capillary cross-section, fill fluid and temperature must be assessed together.
A diaphragm seal should therefore not be selected separately based on thread size. The measuring instrument, diaphragm seal, fill fluid and mounting arrangement form a matched system that must be calculated, manufactured and calibrated using the actual process data.
Frequently asked questions about diaphragm seal diaphragm size
Why does a low measuring range often require a larger diaphragm?
A larger diaphragm is less stiff and displaces a greater fluid volume even at low pressure. This enables it to operate the connected measuring instrument more reliably across its full measuring range.
Can a small diaphragm seal also measure 0 to 100 mbar?
This is not generally impossible, but it depends greatly on the diaphragm diameter, measuring instrument, temperature, fill fluid and mounting arrangement. The combination must be calculated and approved by the manufacturer.
Does a small diaphragm cause a greater temperature error?
Often, yes. Due to the greater diaphragm stiffness, the temperature-related volume change of the fill fluid can generate a greater additional pressure and therefore a larger zero-point shift.
Does a small diaphragm seal automatically respond faster?
No. The response time depends on the complete system. The capillary length, capillary cross-section, fill volume, viscosity and displacement volume of the measuring instrument are particularly important.
Is a G ½ connection always associated with a small diaphragm?
No. With a recessed diaphragm, the diaphragm seal body may be significantly larger than the process thread. With a very compact flush design, by contrast, the diaphragm area is more strongly limited by the overall dimensions.
Can a longer capillary be ordered as a precaution?
This is technically possible but usually not advisable. Additional length increases the fill volume, temperature error and response time. The capillary should be selected only as long as actually required.
Can I fit a diaphragm seal to an existing transmitter at a later stage?
A functional system requires professional evacuation, filling and joint calibration. Simply screwing the components together afterwards without a system filling is not sufficient.
Which information does ICS Schneider require for the configuration?
The required information includes the measuring range, operating pressure and overpressure, medium, process and ambient temperature, process connection, materials, required measuring instrument, mounting arrangement, capillary length, difference in height, required response time and permissible overall measurement deviation.