The measured pressure is transferred from the diaphragm to the filling liquid and passes through the capillary to the measuring chamber of the pressure transmitter. The interior of the diaphragm seal and of the capillary, as well as the measuring chamber of the transmitter, are filled gas-free by the filling liquid.

Transmission response

The transmission response of a remote seal is characterized by the following variables:

• Temperature error
• Response time

Temperature error

Temperature errors are caused by the change of volume of the filling liquid due to temperature variations. To select the right remote seal you must calculate the temperature error.

Below you will find an overview of the factors which influence the size of the temperature error, as well as information on how to calculate the temperature error.

The temperature error is dependent on the following variables:

• Rigidity of the diaphragm used
• Filling liquid used
• Influence of the filling liquid underneath the process flanges or in the connection shank of the pressure transmitter
• Internal diameter of the capillary: The bigger the internal diameter, the bigger the temperature error
• Length of the capillary: The longer the capillary, the bigger the temperature error

Diaphragm rigidity

The rigidity of the diaphragm is of decisive importance. The bigger the diameter of the diaphragm, the softer the diaphragm and the more sensitively it reacts to temperature-induced changes in volume of the filling liquid.

The result is that small measuring ranges are only possible with large diaphragm diameters.

Other factors apart from diaphragm rigidity which also play a role:

• Diaphragm thickness
• Diaphragm material
• Coatings if present

Filling liquid

Every filling liquid reacts to temperature variations with a change of volume. Temperature errors can be minimized by selecting a suitable filling liquid, but the filling liquid must also be appropriate for the temperature limits and operating pressure. Furthermore, the filling liquid must also be physiologically harmless.

Since the filling liquid is present under the diaphragm, in the capillary and under the process flange of the pressure transmitter (or in the connection shank), the temperature error must be calculated separately for each combination.

Note:

When operating in the low-pressure range, also during commissioning, it is recommended to use a vacuum-proof remote seal (see Selection and Ordering data).

An example of a temperature error calculation can be found in the section "Technical Specifications".

Response time

The response time is dependent on the following factors:

• Internal diameter of the capillary: The bigger the internal diameter, the shorter the response time
• Viscosity of the filling liquid The greater the viscosity, the longer the response time
• Length of the capillary: The longer the capillary, the longer the response time
• Pressure in the pressure measuring system: The higher the pressure, the shorter the response time

Recommendations

The following should be observed to obtain an optimum combination of transmitter and remote seal:

• Choose the biggest possible diameter for the remote seal. The effective diameter of the seal diaphragm is then bigger and the temperature error smaller.
• Choose the shortest possible capillary. The response time is then shorter and the temperature error smaller
• Choose the filling liquid with the least viscosity and the smallest coefficient of expansion. Make sure, however, that the filling liquid meets the process requirements with regard to pressure, vacuum and temperature. And ensure that the filling liquid and the medium are compatible with one another.
• Note the following points for use in the low-pressure range:
  • The pressure transmitter must always be positioned below the lowest spigot.
  • The operating range of some filling liquids is very limited with regard to the permissible temperature of the medium.
  • A vacuum-proof remote seal is necessary for continuous operation in the low-pressure range.
• Recommendations for the minimum span can be found in the section "Technical data".

Note

The remote seals listed here are a selection of the most common designs. On account of the large variety of process connections, certain remote seals which are not listed here may be available nevertheless.

Other versions can be:

• Other process connections, standards
• Aseptic or sterile connections
• Other dimensions
• Other nominal pressures
• Special diaphragm materials, including coatings
• Other sealing faces
• Other filling liquids
• Other capillary lengths
• Sheathing of capillaries with protective hose
• Calibration at higher/lower temperatures etc.

Please contact your Siemens Regional Office for more information.