Siemens SITRANS P - Manual (Page 151)

SITRANS P measuring instruments for pressure
Remote seals for transmitters and pressure gages
Technical description
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Miniature diaphragm seal with diaphragm flush with front
• Miniature diaphragm seals
The quick-release remote seals are used above all in the food in-
dustry. Their design means that the measured medium cannot
accumulate in dead volumes. The quick-release clamp present
on the remote seal means that quick dismounting is possible for
cleaning.
Clamp-on seal
Clamp-on seal with quick-release design (left) and for flange mounting
With clamp-on seals, the pressure is first measured using a cy-
lindrical diaphragm positioned in a pipe, and then transmitted to
the pressure transmitter by means of the filling liquid.
The clamp-on seal is a special design for flowing media. It con-
sists of a cylindrical pipe in which a cylindrical diaphragm is em-
bedded. Since it is completely integrated in the process pipe, no
turbulences, dead volumes or other obstructions to the flow oc-
cur. Furthermore, the clamp-on seal can be cleaned by a pig.
The following types of clamp-on seals exist:
• Quick-release clamp-on seals, e.g. to DIN 11851, SMS stan-
dard, IDF standard, APV/RJT standard, clamp connection etc.
The quick-release facility attached to the remote seal enables
the seal to be removed quickly for cleaning purposes.
• Clamp-on seals for flanging to EN or ASME.
• Clamp-on seals with customer-specific process connections.
Note:
The pressure data on the transmitter and the remote seal must
be observed with regard to pressure/temperature behavior.
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Function
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 dia-
phragm seal and of the capillary, as well as the measuring cham-
ber 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
• Adjustment time
Temperature error
Temperature errors are caused by the change of volume of the
filling liquid due to temperature variations. To select the right re-
mote 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 diam-
eter, 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 big-
ger 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 appropri-
ate for the temperature limits and operating pressure. Further-
more, 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 transmit-
ter (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 commis-
sioning, 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".
© Siemens AG 2008