The measured pressure is transferred to the fill fluid by the diaphragm and enters the measuring chamber of the pressure transmitter through the capillary. The fill fluid completely fills the inside of the diaphragm seal, the capillary and the measuring chamber of the pressure transmitter so that it is free of gas.

Transmission response

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

• Temperature error
• Adjustment time

Temperature error

Temperature errors are caused by changes in the volume of the fill fluid as a result of temperature fluctuations. To select the right remote seal, you must calculate the temperature error.

Below is an overview of the factors which affect the extent of the temperature error, and information on how to calculate the temperature error.

The temperature error depends on the following variables:

• Rigidity of the diaphragm used
• Fill fluid used
• Effect of the fill fluid underneath the process flanges or in the connection shank of the pressure transmitter
• Inside diameter of the capillary: The greater the inside diameter, the greater the temperature error
• Capillary length: The longer the capillary, the greater the temperature error

Diaphragm rigidity

The rigidity of the diaphragm is extremely important. The greater the diameter of the diaphragm, the softer the diaphragm and the more sensitive it is to temperature-induced changes in the volume of the fill fluid.

Large-diameter diaphragms are therefore always required for small measuring ranges.

Apart from diaphragm rigidity, the following factors are also important:

• Diaphragm thickness
• Diaphragm material
• Any coatings

Fill fluid

Temperature fluctuations cause volume changes in all fill fluids. Choosing the right fill fluid can minimize the temperature error; however, the fill fluid must be appropriate for the temperature limits and operating pressure. The fill fluid must also be harmless to health.

There is fill fluid underneath the diaphragm, in the capillary and under the process flange of the pressure transmitter (or in the connection shank). The temperature error must therefore be calculated separately for each combination.

Note:

A vacuum-resistant remote seal is recommended for continuous low-pressure operation at 500 mbar a or below, including during commissioning (see ordering data).

You can find an example of how to calculate temperature error in "Technical specifications".

Adjustment time

The adjustment time depends on the following factors:

• Inside diameter of the capillary: The greater the inside diameter, the shorter the adjustment time.
• Fill fluid viscosity: The greater the viscosity, the longer the adjustment time.
• Capillary length: The longer the capillary, the longer the adjustment time.
• Pressure in the pressure measurement system: The higher the pressure, the shorter the adjustment time.

Recommendations

For the best possible pressure transmitter and remote seal combination, please note the following:

• Use a diameter as large as possible for the remote seal. This makes the effective diameter of the remote seal diaphragms larger and reduces the temperature error.
• Use a capillary as short as possible. This reduces the adjustment time and the temperature error.
• Use the fill fluid with the lowest viscosity and smallest coefficients of expansion. Make sure, however, that the fill fluid meets the high-pressure and low-pressure and temperature process requirements. The fill fluid and the medium must also be compatible.
• Please note the following points for operation in the negative pressure range:
  
  • The pressure transmitter must always be positioned below the lowest shank.
  • The operating range of some fill fluids may be extremely limited in terms of the admissible temperature of the medium.
  • A vacuum-resistant remote seal is required for continuous operation in the low-pressure range.
• You can find recommendations for the minimum span in "Technical specifications".

Note

The remote seals listed here are merely a selection of the most common remote seals. As there are a wide range of process connections, this list may not include all remote seals available.

Other versions may include:

• Different process connections and standards
• Aseptic and sterile screwed glands
• Different dimensions
• Different nominal pressures
• Special diaphragm materials and coatings
• Different sealing surfaces
• Different fill fluids
• Different capillary lengths
• Capillary sheathed in protective tubing
• Calibration at higher/lower temperatures, etc.

Please contact your local Siemens office for further information.

Negative pressure service

Liquids, such as silicone oils, inert or those suitable for food, are used in remote seal systems for transmission of the process pressure to the pressure transmitter.

In each liquid, particles have the tendency to leave the liquid compound with increasing temperature (transition from liquid to gaseous aggregate state). This means the vapor pressure increases with increasing temperature and is dependent on the substance or mixture being present.

The higher the temperature and the lower the associated process pressure in the liquid, the more difficult it gets to guarantee the desired transmission properties of the fill fluid and therefore the measuring arrangement.

Plus the sealing elements at the transmitter must be designed so that a diffusion of molecules from the atmosphere into the remote seal system is prevented due to the constantly occurring negative pressure.

In addition to the influencing variables process pressure and process temperature, the vapor pressure curve of the fill fluid at the remote seal end and the stiffness of the remote seal membrane impact the functionality of the remote seal in the negative pressure range.

This means you have to pay special attention to the physical properties of fill fluids with applications in the negative pressure range.·

There are three stages for the negative pressure resistance:

• Standard design of the remote seal without additional protective measures, suitable for the overpressure range and low negative pressure range. This design is identified with (1) in the diagrams below in section 3.
• Negative pressure service with suitable seals and treated fill fluid, identified with (2) in the diagrams below in section 3. Here you select the order codes V01, V03 or V04, depending on the mounting type.·
• Extended negative pressure service with more extended treatment of the fill fluid and the remote seals, identified in the diagrams below with (3). Here you select the order codes V51, V53 or V54, depending on the mounting type.

There are two more areas in the diagrams. The area (4) identifies an area that has to be clarified with Technical Support prior to placing the order. The area (5) describes the area in which the remote seal fill fluid is permanently destroyed and the entire remote seal is therefore without function.

Technical specifications of the remote seal fill fluids

The suitable negative pressure service is specified with the pressure/temperature curves of the respective liquids described below.

Note: For reasons of operational safety, the transmitter must not exceed the height of the remote seal - with differential pressure applications, the height of the bottom remote seal - for measurements in the negative pressure range. The associated installation types B, C1, C2 or H are described at the end of this section under the topic "Measuring arrangements".

Selection of the required negative pressure service

The procedure for determining the required negative pressure service is described below using the silicone oil M5 as fill fluid. The minimum existing process pressure of a fictitious process is 200 mbar_abs (2.9 psi) (at a maximum process temperature of 150 °C (302 °F)). This intersection is identified by an "X" in the diagram below. This means the negative pressure service V01, V03 or V04 (depending on the application) is sufficient in this example.

The suitable negative pressure resistance is determined this way for all other fill fluids

Note:
Note the response times according to the table "Response times" (see Technical Specifications).

Negative pressure applications with silicone oil M5

Negative pressure applications with silicone oil M50

Negative pressure applications with high-temperature oil

Negative pressure applications with halocarbon oil (inert fill fluid)

A BAM approval for process temperatures up to 60 °C (140 °F) and system pressures up to 50 bar (725 psi) is available for the oxygen application.

Negative pressure applications with food oil (FDA-listed)