Technical information

Diaphragm seals and diaphragm seal systems
Application - Functionality - Design
WIKA data sheet IN 00.06

Definition
Diaphragm seals are used for pressure measurements when
the process medium should not come into contact with thepressurised parts of the measuring instrument.
A diaphragm seal has two primary tasks:
1. Separation of the measuring instrument from the process
medium
2. Transfer of the pressure to the measuring instrument

Functionality of a diaphragm seal
The functionality of a diaphragm seal is illustrated in thefigure on the right.
Principle
The process side of the seal is isolated by a flexible
diaphragm. The internal space between this diaphragm andthe pressure measuring instrument is completely filled witha system fill fluid. The process pressure is transmitted by theelastic diaphragm into the fluid and from there to the measuring element of the pressure measuring instrument.
In many cases, between the diaphragm seal and pressure
measuring instrument, a capillary is connected in order (forexample) to eliminate or to minimise temperature effects
from the hot fluid to the measuring instrument. The capillaryaffects the response time of the overall system.
Diaphragm seal, capillary and measuring instrument form aclosed system. The sealed filling screws on the diaphragm
seal and the measuring instrument must therefore never beopened, since the function of the system is affected following
any escape of fill fluid!
The diaphragm and the process connection are the elementsof the system which come into contact with the medium.
Therefore, the material from which they are made must meetthe relevant requirements in terms of temperature and corrosion resistance.

WIKA data sheet IN 00.06 ∙ 05/2022Pressure measuring
instrument■ Pressure gauge■ Process transmitter■ Pressure switch
System fill fluid
Mounting type

■ Direct mounting■ Capillary■ Heat sinkDiaphragm
Diaphragm seal

■ Upper bodyof diaphragm seal

■ Lower bodyof diaphragm seal

Process connection■ Threaded connection■ Flange connection■ Hygienic connection

Installation example of a diaphragm seal system

If the diaphragm is leaking, the system fill fluid can enter
the medium. For applications in the food industry, it must beapproved for contact with food. In selecting the fill fluid, thefactors of compatibility, temperature and pressure conditionsin the medium are of crucial importance. Customer-specificsolutions can be realised for the different operating conditions of the applications.
Diaphragm seal systems are capable of withstanding
extreme temperatures of -130 ... +450 °C [-202 ... + 842 °F]
and pressures of 35 mbar ... 3,600 bar [0.5 ... 52,200 psi].

Page 1 of 8

Fields of application
The use of diaphragm seals enables the user to employ a large number of pressure measuring instruments for the mostdifficult process conditions.
Examples■ In the manufacture of the product, and in the production■ The medium is corrosive and the pressure measuring

instrument itself (e.g. the interior of a Bourdon tube) cannot
be sufficiently protected against it.
■ The medium is highly viscous and fibrous, thus causing
measuring problems due to dead spaces and constrictions in the bores of the pressure measuring instrument(pressure ports, Bourdon tubes).
■ The medium has a tendency towards crystallisation orpolymerisation.
■ The medium has a very high temperature. As a result, thepressure measuring instrument is strongly heated. This
heating leads to a large temperature error in the measurement on the pressure measuring instrument. The increased
temperature can also lead to the upper limits for the thermalloading of the measuring instrument components being
exceeded.
■ The measuring location is not favourably located. For
reasons of space, the pressure measuring instrument caneither not be mounted or cannot be read or can only beread with difficulty. By installing a diaphragm seal and using
a longer capillary, the pressure measuring instrument canbe installed in a location where it can be easily viewed.

plant, hygienic requirements must be followed. For thisreason, dead spaces and undercuts in the wetted parts
must be avoided.
■ The medium is toxic or harmful to the environment. It cannot
be allowed to escape into the atmosphere or environment
through leakage. On the grounds of safety and environmental protection, the appropriate protective measures must
therefore be taken.
Thanks to its many years of experience, WIKA is able to turnchallenging tasks into solutions with a technological edge.
Advantages from using diaphragm seals■ Long service life of the measuring assembly■ Reduced installation effort■ Elimination of maintenance

Combinations for diaphragm seal systems
A diaphragm seal system is defined with the pressure measuring instrument, the mounting type and the diaphragm seal withprocess connection.
The optimal diaphragm seal designs, materials, system fill fluids and accessories are available for each application.

Pressure gauge

Pressure sensor

Direct mounting

Flange connection
WIKA data sheet IN 00.06 ∙ 05/2022

Process transmitter

Heat sinkThreaded connection

Pressure switch

CapillaryHygienic connection
Page 2 of 8Mounting types
The required mounting type for pressure measuring instruments with diaphragm seals depends, among other things,
on the operating conditions of the diaphragm seal system.
The choice is between direct mounting, flexible capillary orheat sink. This makes the diaphragm seal system adaptable to customer-specific conditions. When selecting themounting type, influences on the measuring capability of thediaphragm seal system must be taken into account. Mounting
via a capillary or a heat sink results in a longer response timethan direct mounting, for example.

Heat sink
With hot media, the heat sink ensures that the system fill fluid
cools down sufficiently to guarantee precise measurement.
Heat sink (examples)

Direct mounting
Direct mounting is achieved by welding the measuring instrument directly to the diaphragm seal via a connection adapter.
Direct mounting via axial connection adapter

Capillary
The capillary is a flexible connecting piece between themeasuring instrument and the diaphragm seal, which typically consists of a tube, a capillary protection hose and, optionally, a further jacket. Capillaries are used where the process
media are at high temperatures, as these are cooled down
through the connecting line. Furthermore, this mounting typeis suitable for decoupling strong vibrations, or if the measuring instrument cannot be mounted at the measuring location
or is easier to read at another location.
Capillary (example)

WIKA data sheet IN 00.06 ∙ 05/2022

Page 3 of 8

Process connection and design
Diaphragm seal systems are used in demanding applications in a wide range of industries. The optimal process connections
and designs are available for each application.
The decision for the right diaphragm seal depends on both the specifications as well as the installation options and requirements of each specific measuring task.
Classification of the process connections■ Flange connection■ Threaded connection■ Hygienic connection

Flange connection

Diaphragm seals with flange connection can be used forprocesses with aggressive, adhesive, corrosive, highly
viscous, environmentally hazardous or toxic media.
Diaphragm seals with flange connections are available withdimensions for all common standard flanges. The sealing
face is flush and the diaphragm is designed to be either flush
or internal.

Extended-diaphragm version

14206602.02
14219250.01

Cell-typeIn-line diaphragm seal with flange connection
These diaphragm seals are exceptionally suitable for themeasurement of flowing media. The in-line diaphragm seal
is clamped directly into the pipeline using flanges at bothends. This integration into the process line prevents disturbing turbulences, as this design has no corners, dead spaces
or other obstructions in the flow direction. Different nominal
widths allow the in-line diaphragm seals to be adapted to any
pipeline cross-section. The in-line diaphragm seals are alsoavailable in a cell-type design.
In-line diaphragm seal with flange connection

14484905.01

Cell-type
The cell-type is a specific variant of diaphragm seal withflange connection. It consists of a cylindrical plate, whose
diameter is matched to the sealing face area of corresponding standard flanges. The diaphragm is flush and matched to
the nominal width.
A blind flange is used to mount the cell-type, available for allcommon flange standards.

WIKA data sheet IN 00.06 ∙ 05/2022

14217473.01

Internal diaphragm14219130.01

Flush diaphragm

Extended-diaphragm version
Diaphragm seals with extended diaphragm are used, among
other things, on thick-walled and/or insulated product lines,vessel walls, etc. The version with extended diaphragm isavailable for flange- and cell-type designs.

Page 4 of 8

Threaded connection

Diaphragm seals with threaded connection can be used
for processes with aggressive, adhesive, corrosive, highlyviscous, environmentally hazardous or toxic media. The
connection of the upper and lower bodies of the diaphragm
seal is available in either a threaded or a welded design.
These diaphragm seals are available with female or malethread in their basic design. The wide variety of available
process connections enables many different adaptations
without any problems. The material of the upper body of thediaphragm seal and the lower body of the diaphragm seal
can be the same or be different.

14265415.01

Welded design

Hygienic connection

Diaphragm seals in hygienic design can be used for processes with gases, compressed air or vapour and also with liquid,
pasty, powdery and crystallising media. The diaphragm seals
resist the temperatures that occur and meet the requirements
for sterile connections. The flush diaphragm can be integrated hygienically into all applications thanks to the various
process connections.
SIP and CIP criteria, which are an essential requirement for
sanitary applications, are met by using WIKA diaphragmseals.

In-line diaphragm seal for hygienic connection

WIKA data sheet IN 00.06 ∙ 05/2022

14216061.01

Grooved union nut

14204961.02

Clamp connection

In-line diaphragm seal for hygienic connection
The in-line diaphragm seal is perfectly suited for use withflowing media. With the seal being completely integrated into
the process line, measurements do not cause any disturbing
turbulences, corners, dead spaces or other obstructions in
the flow direction. The in-line diaphragm seal is clamped
directly into the pipeline. With in-line diaphragm seals, themedium flows through unhindered and effects the self-cleaning of the measuring chamber. Different nominal widths allow
the in-line diaphragm seals to be adapted to any pipelinecross-section.

14216061.01

14204961.02

Threaded design

Page 5 of 8

Materials and coatings
The predominant material for diaphragm seals is stainlesssteel 316L. For the wetted parts, a wide range of special
materials and coatings are available for specific areas ofapplication. WIKA offers this variety of different materials
in order to be able to find the best possible solution for thedemands of the measuring location.
The material selection for diaphragm seals is strongly
dependent upon the operating conditions. In addition to thepressure load, the requirements for the temperature and also
the resistance against the medium must be known. Then thematerial selection for the diaphragm seal can be made. The
selection can be designed with different materials for thebase body, the sealing face and the diaphragm, as these are
not equally wetted in every design.

Material combinations and coatings
Especially when using special materials, high costs and longdelivery times can occur.
Such circumstance can be resolved by intelligent selectionof material combinations or coatings. A cost-effective basematerial is used for the load-bearing parts, for example, andonly the wetted parts are made of a special material or havea coating. The joining and connection technology play animportant role here, as different materials cannot always bewelded. Regardless of the type of connection technology,
these diaphragm seals can withstand extreme operating
conditions.

Material

Unified numbering system (UNS)

Stainless steel 904L (1.4539)

N08904

Stainless steel 321 (1.4541)

S32100

Stainless steel 316Ti (1.4571)

S31635

Stainless steel 1.4466 (urea grade)

S31050

Duplex 2205 (1.4462)

S31803

Superduplex 1.4410

S32750

Tantalum (also lining)

R05200

Hastelloy C276 (2.4819)

N01276

Hastelloy C22 (2.4602)

N06022

Inconel 600 (2.4816)

N06600Incoloy 825 (2.4858)

N08825

Inconel 625 (2.485)

N06625

Monel 400 (2.4360)

N04400

Nickel 200 (2.4066)

N02200

Nickel 201 (2.4068)

N02201

Titanium 3.7035 (class 2)

R50400Titanium 3.7235 (class 7)

R52400

Zirconium GR702

R60702

Stainless steel 316L (1.4404 or 1.4435)

S31603The maximum permissible process temperature is limited by the joining method and the system fill fluid. The maximum
process temperature can be found in the data sheet for the diaphragm seal.
Coatings

Stainless steel with ECTFE coating
Stainless steel with PFA (FDA; 21 CFR 177.1550 and 21 CFR 177.2440)
Stainless steel with PFA, anti-static (suitable for Ex applications)
Stainless steel with gold plating
Stainless steel with gold-rhodium
Stainless steel with Wikaramic®

WIKA data sheet IN 00.06 ∙ 05/2022

Page 6 of 8

System fill fluids
When selecting the system fill fluid for diaphragm seals,
factors such as medium compatibility as well as temperature and pressure conditions at the measuring location
are of critical importance in order to avoid endangering theprocess. Depending on the system fill fluid, the appropriate
minimum and maximum operating temperature range must
be observed. In addition, the change in volume of the system
fill fluid at extreme application temperatures must be taken
into account.
Highly flammable applications, such as oxygen and chlorine
applications, and the high demands within both sanitary
applications and the semiconductor industry are also crucial
in choosing the right fluid.

Designation

Identification
numberSolidification
point

Boiling/degradation
point

The properties of the system fill fluids affect the permissible
operating temperature of the diaphragm seal system. Since
the parameters of the individual system fill fluids vary, WIKA
offers a wide range to cover different applications.
FDA approval
The FDA (“Food and Drug Administration”) is a US authority under the Department of Health. It is responsible for themonitoring of foodstuffs and pharmaceuticals and serves toprotect public health in the United States.
Liquids that could find their way into the end product in theevent of a failure must be FDA-compliant.

Density at
25 °C

Kin. Viscosity at
25 °C

RemarkKN°C°C

g/cm³cSt

54.5

Universal application

Glycerine

7-35

+240

1.26

759.6

FDA 21 CFR 182.1320

Silicone oil

17-90+2000.92

4.4

Low temperatures

Halocarbon

21-60+175

1.89

10.6

Oxygen 1) and chlorine

Methylcyclopentane30-130+600.740.7

For very lowtemperatures

High-temperaturesilicone oil

32-25+400

1.06

47.1

For high temperatures

Neobee® M-2059-35+260

0.92

10.0

FDA 21 CFR 172.856,
21 CFR 174.5

DI water

64+4+851.00

0.9

For ultrapure media

Silicone oil

68-75

+250

0.93

10.3

DI water/propanol
mixture75-30+600.92

3.6

For ultrapure media

Medicinal whitemineral oil

92-15+260

0.85

45.3

FDA 21 CFR 172.878,
21 CFR 178.3620(a);USP, EP, SP

Silicone oil

2-45+3000.96

Other system fill fluids on request
Note:■ The stated lower temperature limit is a purely physical characteristic of the system fill fluid. The resulting response time has to be calculated and evaluated separately.
■ The upper temperature limit for a diaphragm seal system is further restricted by the operating pressure and the diaphragm. To determine the upper temperature limit for theindividual diaphragm seal system, a calculation is required.

1) For oxygen applications the following values per BAM test (Federal Institute for Materials Research and Testing) apply:
Maximum temperature

Maximum oxygen pressure> 60 °C to 100 °C

30 barto 60 °C

> 100 °C to 175 °C

50 bar
25 bar

WIKA data sheet IN 00.06 ∙ 05/2022

Page 7 of 8

Influencing factors on the measurement
Response time
A combination of the individual components generally causes
a delay in the output of the measured value. This delay isreferred to as the response time and varies depending on theassembly.
Factors such as the control volume of the measuring instrument as well as the capillary length and its associatedcross-section are included in the calculation. It can therefore
be concluded that the response time increases with a large
control volume or a long capillary. This effect can be counteracted by selecting a measuring instrument with a smaller
control volume, a shorter capillary or a capillary with a larger
cross-section.
In addition to the geometric variables of the diaphragm sealsystem, the viscosity of the fill fluid, among other things, mustalso be taken into account. The higher the viscosity value, themore viscous the medium. An optimisation of the responsetime can thus be realised through the selection of a fill fluidwith a lower viscosity.
Furthermore, the temperatures applied influence the physicalproperties of the system fill fluid. If the temperature increases,
then the medium becomes less viscous and the responsetime shortens. Conversely, the response time of the measuring instrument increases as the temperature drops due to theincreasing viscosity.

The opposite effect is seen when the temperature drops. The
decrease in volume now causes the diaphragm to deflect inthe direction of the diaphragm bed. The temperature reduction causes a negative zero point offset due to the restoring
force of the diaphragm.
Height difference
Any height difference between the pressure measuring
instrument and the diaphragm seal (this applies especiallywhen using capillaries) affects the measurement. This is dueto the hydrostatic pressure of the liquid column in the capillary. The display is reduced when the pressure measuring
instrument is positioned higher than the diaphragm seal.
It is increased when the pressure measuring instrument ispositioned lower. This height difference must be known when
designing the entire system at the factory so that it can betaken into account accordingly.
Note: After mounting, a zero point test and, if necessary, azero point correction is recommended.

Temperature effectDiaphragm seal systems are generally filled at room
temperature. If there are now temperature changes in theenvironment or in the process, these have a negative effecton the output values of the measuring instrument. The reason
is due to the change in the physical properties of the system
fill fluid. If the measuring system experiences an increase intemperature, there is an increase in volume which leads toa deflection of the diaphragm in the direction of the process.
The restoring force of the diaphragm simultaneously ensuresa positive zero point offset on the measuring instrument.
To counteract this error, large diaphragm diameters should
be chosen due to their low rigidity. Other factors that counteract the zero point offset are a lower dead volume of the entire
system and a lower thermal expansion coefficient of the fillfluid.

05/2022 EN© 09/2010 WIKA Alexander Wiegand SE & Co. KG, all rights reserved.
The specifications given in this document represent the state of engineering at the time of publishing.
We reserve the right to make modifications to the specifications and materials.

WIKA data sheet IN 00.06 ∙ 05/2022

Page 8 of 8WIKA Alexander Wiegand SE & Co. KG
Alexander-Wiegand-Straße 30
63911 Klingenberg/GermanyTel.
+49 9372 132-0
Fax+49 9372 132-406
info@wika.de
www.wika.de