Siemens P technical specifications 151, Transmission response, Temperature error

© Siemens AG 2008

SITRANS P measuring instruments for pressure

Remote seals for transmitters and pressure gages

Technical description

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.

■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-	2
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 FI 01 · 2009

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