Diaphragm seal pressure transmitter

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A diaphragm seal pressure transmitter is a device used to measure pressure. It incorporates a diaphragm seal, a barrier between process fluid and the measuring instrument and a pressure transmitter that converts pressure into an electrical signal. Its primary role is to protect sensitive equipment from harsh process fluids, ensuring accurate readings without direct contact.

Diaphragm seals pressure transmitter


Diaphragm seal pressure transmitter

Applications of remote seals for pressure instrumentation


A technology for manufacturing pressure sensors with custom-made diaphragm sealsHigh accuracy custom-made diaphragm seal pressure transmitter manufacturer

High-tech industries such as chemical, oil and gas, pulp and paper, food and beverage put their measuring devices to the test.
Critical process media require pressure instrument 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 diaphragm seal transmitter.

Advantages of Fuji Electric diaphragm seal pressure transmitter

  1. Reliable and robust design system construction
  2. Welded design with no threaded connections
  3. 100 percent helium leak tested
  4. Backup convolutions on the diaphragm protect seal integrity

Construction of remote seal pressure transmitter 

Diaphragm seals are used in combination with differential presssure transmitter, gauge pressure transmitter or absolute pressure transmitter.

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


Measuring principle of diaphragm seal pressure transmitter

Measuring principle of diaphragm seals pressure sensorsTo understand what is diaphragm seal, we need to dig into the pressure transmitters applications. In 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 flexible diaphragm and the wetted 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 pressure 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 fill fluid inside the instrument. The pressure is transmitted hydraulically to the pressure gauge.
This fill fluid is designed to resist compression, so the force is channeled directly to the measuring cell.
The entire operation of the diaphragm seal pressure transmitter 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

When to use diaphragm seal pressure transmitter ?

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.
Remote seal type transmitter is used to measure fluids at high temperatures.

The applications of diaphragm seal pressure transducers are multiple: liquid flow measurement, gas flow measurement, steam flow measurement using remote seal differential pressure transmitter and primary element, fluid level measurement in a tank using diaphragm seal level transmitter, fluid density measurement with remote seal type dp transmitter or advanced pressure measurement.

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

Diaphragm seal type pressure transmitter is 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 pressure transmitter,
  • 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 AND ROBUST DESIGN The choice of a diaphragm pressure transmitter depends on the process application and environmental conditions.

Factors to consider include

  • Accuracy requirements
  • Operating temperature range
  • Pressure ranges (from vacuum to pressure)
  • Type of process connection
  • required filling liquid.

As a diaphragm seal manufacturer, Fuji Electric France can manufacture common types diaphragm seal type pressure transmitter according to your process conditions.



Depending on the constraints of installation and use, several types of diaphragm seal are available meeting most industry standards :


  • Flush mounting design from DN40 to DN100 for use in general applications
  • Diaphragm seal 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, welded neck adapters, threaded diaphragm seals to improve performance on smaller process connections
  • Sanitary diaphragm seal for hygienic 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 transmitter with seal 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 specially for remote seal capillary level transmitter.
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.



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. The membrane is welded on the seal and we check that it is properly mounted with a helium leak test.
It must be as large as possible to be as flexible and sensitive as possible. Its thickness varies according to the materials.

For applications with corrosive fluids, the seal is  available in wide selection of diaphragm material : stainless steel, Hastelloy C, Monel, Tantalum, Titanium, Zirconium, Nickel and other corrosion resistant materials. can be equipped with multiple coatings (PFA, PVDF, Gold, Hydroseal...) to resist chemical aggression of the measured fluids. Please consult diaphragm seal manufacturer, like Fuji Electric, to determine the most suitable material for your application




TThe fill 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 fill 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.

Diaphragm seal fill fluid selection
Designation of fill fluidOperating 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 fill fluids.

Flushing  ring

Diaphragm seal flushing ring

Flushing ring is available as an accessories for diaphragm seal flange with a large choice of materials.
The diaphragm seal flushing ring is mounted between the diaphragm seal and the process flange connection.
It is a wetted part.
The remote seal flushing 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.

Process fluid temperature: minimum -90°C, maximum +400°C, depending on the type of fill fluid used, and the membrane material.


EFFECT OF VACCUM ON remote seal type transmitter 

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.
 Consult us for remote seal pressure transmitter with vaccum application.


Elimination of TEMPERATURE EFFECTThe remote seal pressure transmitter consists of a diaphragm seal (with or without capillary) and a pressure measuring cell.
The diaphragm seal capillary tube is filled with a fill 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 fill 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, diaphragm seal with capillary 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 fill fluid,
  • choose a membrane diameter as large as possible,
  • reduce the volume inside the diaphragm seal capillary  to as small as possible,
  • reduce the effect of temperature on the cell by using capillary assemblies or heat sinks.
  • isolate the pressure transmitter and capillaries from ambient temperature stresses and/or trace the capillaries
  • require pressure and temperature compensation at factory from the diaphragm seal manufacturer


Diaphragm seal pressure transmitter documentation