Diaphragm seals pressure transmitter

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Diaphragm seals pressure transmitterDiaphragm seals pressure transmitter


  • Level, flow, pressure, interface, density
  • Extreme hot and cold temperatures
  • Corrosive, clogging, or viscous processes
  • Hygienic requirements
  • Special process connections

A technology for manufacturing pressure sensors with custom-made diaphragm sealsA technology for manufacturing pressure sensors with custom-made diaphragm seals 

High-tech industries such as chemical, oil and gas, pulp and paper, food and beverage put their measuring devices to the test.
Manufacturing processes require pressure sensors with high accuracy and reliability.
The reduction of manufacturing costs requires avoiding rejects and production stoppages.
The obsolescence of some measuring devices requires finding replacement instruments that can fit in place.
When your need is specific and no conventional equipment fits the particularities of your installation, we build a custom seal sensor.


Seals are used with differential, gauge or absolute pressure sensors.
They can be mounted rigidly, directly on the sensor or with a capillary tube that connects the seal to the transmitter.
The seal and the sensor are welded together without any gaskets, and then filled with an oil suitable for the application.

Measuring principle of diaphragm seals pressure sensors

Measuring principle of diaphragm seals pressure sensorsIn some chemical or hygienic applications, it is necessary to isolate the measuring instrument from the process medium pressure.
The diaphragm seal serves as a protective interface between the measuring cell and the process medium.
The diaphragm and the contacting parts are made of a material resistant to the process medium and welded to the base of the measuring cell.
A capillary tube or a connecting sleeve provides the connection between the seal membrane and the sensor measuring cell.
This space must be degassed under vacuum and then filled with a suitable oil filler and sealed.
The measured pressure exerts a force on the outer surface of the membrane.
As the diaphragm flexes inward, it attempts to compress the filling fluid inside the instrument.
This filling fluid is designed to resist compression, so the force is channeled directly to the measuring cell.
The entire operation of the diaphragm pressure sensor is based on Pascal's principle.
This principle states that a pressure exerted on a fluid is transmitted without loss through the fluid in all directions.
For this process to work, the displacement force of the diaphragm must be greater than the force required to move the sensing element in the measuring cell.
In practice, this means that the smaller the force required to move the sensing element, the easier it is to build an accurate sealing system.


Diaphragm seals pressure transmitter specific1

What are the applications of diaphragm seals pressure sensors ?

What are the applications of diaphragm seals pressure sensorsTo ensure the integrity of the manufacturing process, it is necessary to know when to install a seal pressure transducer.
Separator pressure sensors are used to measure fluids at high temperatures.
The applications of seal pressure transducers are multiple: liquid flow measurement, gas flow measurement, steam flow measurement, fluid level measurement in a tank, fluid density measurement or pressure measurement.

For food, hygienic and pharmaceutical applications, seals are available with hygienic fittings and flush diaphragms to meet sanitary regulations and requirements.

Sensors with isolating diaphragms are used in the following applications:

  • the temperature of the fluid is very high or very low,
  • the medium is corrosive and may attack the materials in contact,
  • the medium is charged and may block the connection line to the sensor,
  • the medium is charged and may block the connection line to the sensor, thus preventing the pressure from reaching the measuring cell of a conventional pressure sensor,
  • the medium is viscous or may solidify (drying or polymerization) in the sensor connection line, thus preventing the pressure from reaching the measuring cell or makes the diaphragm of a conventional pressure sensor immobile,
  • the medium may freeze when the temperature drops,
  • for hygienic and sanitary applications that require specific surface conditions to prevent the formation of bacteria,
  • for hydrogen permeation applications: when hydrogen ions (H+) are present and can pass through the membrane,
  • for process-specific connections or for remote measurement display
  • elimination of wet legs or dry legs at a tank level measurement
  • to facilitate service and maintenance.

Diaphragm seals pressure transmitter specific 2

How to select the diaphragm seal pressure transmitters ?


  • Reliable system construction
  • Welded design with no threaded connections
  • 100 percent helium leak tested
  • Backup convolutions on the diaphragm protect seal integrity

LARGE CHOICE OF PROCESS CONNECTIONDepending on the constraints of installation and use, several types of seals are available meeting most industry standards :

  • Flush mounting design from DN40 to DN100 for use in general applications
  • Seals with extensions (from 50 to 200 mm)
    for use in viscous applications with plugging issues or installed flush with inner tank wall to prevent process plugging
  • Flanged, screwed or welded neck adapters to improve performance on smaller process connections
  • Seals for sanitary applications according DIN, SMS or Tri-Clamp standards for use in hygienic applications

The mounting adapter is dedicated to either adapt the remote seal to a specific process connection or increase the sensibility of the transmitter with special process conditions.
Consult us for specific seals.


Pressure sensors with seals can have rigid assembly, with direct mounting on the cell if the temperature does not exceed 150°C.


The use of a capillary limits the effects of process temperature on the accuracy of the instrument.
A capillary length of 500 mm allows the temperature of the instrument to be reduced to room temperature.
The length of the capillary should be as short as possible, as it influences the accuracy of the measurement and the response time.
capillary assemblyCapillaries are available from 0.5 m to 15 m with PVC coated (-10 to +80° C) or stainless steel (-40 to 350° C) protection shield.
Two different capillary diameters allow for optimization of accuracy and time response.
The inner diameter is 1 mm for standard applications and 2 mm for special applications.
Temperature variations along the length of the capillary can affect the accuracy of the instrument.
Capillary length applies to both high and low side for balanced systems.
Applies to low side only for rigid assembly and to high side only for remote mount single seal systems with capillary.
A temperature compensation must be done at the factory to guarantee the accuracy.



For applications with corrosive fluids, the seals are available in different corrosion resistant materials (stainless steel, Hastelloy C, Monel, Tantalum, Titanium, Zirconium, Nickel, ..).
Please consult us to determine the most suitable material for your application.


The quality of the surface of the membrane and the parts in contact with the fluid is very important for hygienic and sanitary applications (cleaning in place CIP/SIP Service).
In order to prevent the risk of contamination by substances such as product residues or micro-organisms, it is necessary to control the surface condition.


Different average roughness values are available for seal membranes: Ra < 0.4 to 0.8 µm for smooth surfaces, Ra < 1.6 µm near welds.


The diaphragm is an elastic metallic measuring element.
It must be as large as possible to be as flexible and sensitive as possible.
It is available in different materials (stainless steel, Hastelloy C, Monel, Tantalum, ...) and can be equipped with multiple coatings (PFA, PVDF, Gold, ...) to resist chemical aggression of the measured fluids.
DIAPHRAGM THICKNESS AND MATERIALIts thickness varies according to the materials.
The membrane is welded on the seal and we check the tightness with a helium leak test.


The filling fluid used must be suitable for the temperature range of the application.
The minimum and maximum temperatures of the measured medium as well as the ambient temperature must be considered.
In addition, the filling medium must be compatible with the medium being measured, especially for media such as oxygen.
FILLING FLUIDSFor applications in the food industry, food grade filling oils are available to avoid contamination of the measured medium in the event of membrane failure.

Filling fluid of the diaphragm seals
DesignationOperating temperature (°C)
Density (25°C)
P abs > 1 barP abs < 1 bar
Silicone oil-40 à 180-40 à 1200,95
Fluorinatedil-20 à 200-20 à 1201,84
Sanitary oil-10 à 250-10 à 1200,94
Silicone oil 20 à 2001,07
Silicone oil0 à 30020 à 2001,07
Silicone oil10 à 350 20 à 2001,09

The values given are valid for the most common applications.
For special applications, please contact us and indicate the ambient and process temperature, the pressure to be measured, the static pressure and the vacuum values.
Depending on your specific operating conditions, we can use other filling fluids.

Flushing  ringFlushing  ring

Flushing ring are available as an option for diaphragm seal systems with a large choice of materials.
The flushing ring is mounted between the diaphragm seal and the process flange connection
It is a wetted part.
The ring has a one or two ports that can be used to flush and clean the surface of the diaphragm.
It can also be used as a calibration ring by applying a pressure through the ports .


These values must be added to the accuracy class of the pressure transducer ±0.065% for the differential and relative pressure range, and 0.2 % for the absolute pressure range.

However, vacuum accuracy cannot be guaranteed beyond 20 Torr (27mbar abs) in standard designs.
This is due to the fact that most filling fluids contain microscopic amounts of trapped air or gases, which tend to expand significantly when approaching absolute zero pressure.
This expansion affects the instrument's measuring cell.
Process fluid temperature: minimum -90°C, maximum +400°C, depending on the type of filling fluid used, and the membrane material.


Elimination of TEMPERATURE EFFECTThe seal pressure sensor consists of a diaphragm seal (with or without capillary) and a pressure measuring cell.
The instrument is filled with a filling fluid at a specific temperature (usually +20 ±2° C) called reference temperature.
A change in the ambient or measured fluid temperature causes a proportional change in the volume of the filling fluid.
Therefore, this has an effect on the internal pressure of the measuring system and adds an error.
To minimize this error, it is necessary to compensate for the volume change caused by temperature.
Small diameter diaphragms can only compensate for a small volume change.
It is therefore recommended to use seals with diaphragms of the largest possible diameter, depending on the process conditions.
When the process temperature is between +150° C and +250° C, a heat sink must be used between the seal and the measuring cell to avoid temperature diffusion.
Above 250° C, capillary seals must be used to protect the instrument from high process temperatures.
This reduces the temperature of the measuring cell to a value close to the ambient temperature.

In order to minimize the influence of temperature :

  • use a suitable filling fluid,
  • choose a membrane diameter as large as possible,
  • reduce the volume inside the instrument to as small as possible,
  • reduce the effect of temperature on the cell by using capillary assemblies or heat sinks.
  • isolate the sensor and capillaries from ambient temperature stresses and/or trace the capillaries
  • require factory pressure and temperature compensation


Diaphragm seals pressure transmitter documentation