Operating Principle
A diaphragm seal is attached to the process connection of the instrument. The hermetically sealed system is vacuum filled with liquid, or fill fluid. The pressure applied to the diaphragm is hydraulically transmitted to the pressure instrument.
Diaphragm Seal and Instrument Assembly
To ensure that the pressure is correctly transmitted from the diaphragm to the instrument the following must be
considered:
1. Fluid Displacement
a. Volumetric Displacement: The volume of fluid a diaphragm can displace
b. Control Volume or Instrument Displacement: The volume of fluid needed to obtain full deflection of the instrument, or 0-100% range.
To obtain a successful transfer of pressure from the diaphragm to the pressure instrument the volumetric displacement of the diaphragm must be greater than the control volume of the instrument
2. Instrument Pressure Range
Instruments offering low pressure ranges , typically have a high control volume. Consequentially, these instruments require a high volumetric displacement diaphragm seal.
3. Fill Fluid
When selecting the appropriate fill fluid for the application please consider the following:
a. Safety:In the unlikely event of a rupture of the diaphragm, the filling fluid will come into contact with the process medium. Applications where oxidizing agents are present (eg, chlorine, bromine, oxygen, etc.) require a halocarbon filler, as they can cause fires or violent reactions if glycerin, silicone or oil are used.
b. Pressure Ranges: Applications in vacuum, compound or low pressure require low viscosity fluid must be filled with low viscosity fluids as silicone. Glycerin is high viscosity (1,300 cSt) and not recommended for such applications.
c. Temperature: The filling fluid must be capable of withstanding the min./max. of the process temperature. The following table provides a list of common fluids and their temperature classifications
4. Mounting
a. Pressure Instruments: Gauges, transducers and pressure switches can be direct or remote mounted.
b. The fill fluid must be coded on both seals and instruments.
c. The seal instrument connection must match the instrument process connection
d. Differential Pressure Instruments: Differential gauges, switches and transmitters must be mounted to seals using a capillary. A low viscosity fluid is required for remote mount systems. Each instrument requires two seals and two capillaries
5. Response Time
This is the period of time required for the pressure instrument to indicate 90% of the value of a sudden pressure variation. Unfortunately, response time cannot be quantified due to the many variables involved yet it is understood to be influenced by:
a. Configuration of the System:
i. Direct or Remote Mounted: Direct mounted assemblies provide faster response times.
ii. Pressure or Differential Pressure.
iii. Length of Capillary: Limiting the length of capillary will improve response time.
iv. Multiple Instrument Assemblies.
b. Fill Fluid: Low viscosity fill fluids will have reduced response time.
c. Diaphragm Size: Larger diaphragms, will respond faster to changes in pressure
Diaphragm Seal assembly in Differential Pressure transmitter .
1.Balanced DP system
2.Unbalanced DP system
Where seal, mount, capillary, or fill is not identical. A complete assembly includes one diaphragm seal on the HP side AND one diaphragm seal on the LP side.
Diaphragm Seal assembly in Pressure transmitter .
Also read
Mounting Techniques of Diaphragm Seals
