Application of remote flange transmitter in special environment

1 Introduction
Under high vacuum high temperature or low temperature process measurement conditions, a considerable part of the remote flange pressure and differential pressure transmitters are not fully applied, not the short service life of the transmitter, or the poor measurement accuracy. The actual measurement error of some applications with no problems on the surface is quite large. Therefore, how to correctly select and manufacture remote flange pressure and differential pressure transmitter under high vacuum, high temperature or low temperature process conditions is a real problem.

2, Problem analysis
Under high vacuum or low temperature process conditions, the following factors will affect the normal operation of the remote flange transmitter:
(1) The tightness of the threaded connection.
Most remote flange transmitters connect the transmitter to the process connection by using threads on both ends of a capillary. In high temperature applications, these two potential leak points may cause the filling oil to be thermally expanded and squeezed out from the threaded joint, resulting in measurement errors. When the vacuum is applied, the sensing diaphragm is subjected to an outwardly attracting force. The internal filling oil of the transmitter is under negative pressure
In the vacuum state, the outside air is likely to penetrate from the threaded connection point into the capillary tube, seriously affecting the accuracy of the measurement.
(2) The boiling point of the extender oil decreases as the pressure decreases.
In high temperature applications, when the process temperature exceeds the boiling point of the fill oil at this pressure, a small amount of fill oil is vaporized to create another pressure applied to the sensor of the transmitter, causing errors in the measurement. If the vaporization pressure is too large, the unrecoverable convexity of the measuring diaphragm will cause the transmitter to be damaged. This is the main reason for the damage of remote flange pressure and differential pressure transmitters in vacuum applications.
(3) Although the oil is an oily substance, it will contain more or less dissolved gas. These dissolved gases will release and expand under high vacuum and high temperature application conditions, and generate a pressure to be transmitted to the sensor of the transmitter. Cause measurement errors.
(4) Under low temperature environment (generally below -10 °C), the shrinkage of the filling oil volume in the capillary at low temperature will destroy the normal pressure transfer process, resulting in measurement error.

3, The solution
In order to solve the above problems, we have developed a new production process for remote flange pressure and differential pressure transmitters used under high vacuum, high temperature or low temperature process conditions.
(1) All-welded structure of the remote flange pressure differential pressure transmitter.
The connections between the capillary and the transmitter, the process connection are all sealed by soldering to eliminate possible leaks.
(2) Accurately control the filling amount of the filling oil according to the application conditions.
According to the application conditions of the user and the characteristics of various filling oils, the filling amount of the filling oil is appropriately reduced by precise calculation to compensate for the influence of the expansion of the filling oil. In the application of low temperature environment, the filling amount of filling oil will be appropriately increased according to the specific application conditions to compensate for the influence of volumetric shrinkage of the filling oil.
(3) Special filling oil degassing treatment.
The filling oil is specially treated to minimize the amount of dissolved gas therein.
(4) Special calculation for remote flange transmitter.
Transmitter manufacturers always promote their own transmitters with high precision and high stability, but for the remote flange pressure with capillary, differential pressure transmitter measurement accuracy and dynamic response in various applications Time, but no supplier can provide their exact application indicators. Because these indicators are ever-changing with the process conditions and remote flange pressure, the type of differential pressure transmitter, previously used by experience, there is no specific theoretical basis.
In order to quantitatively analyze the working conditions of remote flange pressure and differential pressure transmitter under various measurement conditions, understand the actual measurement accuracy and dynamic response performance under specific process conditions, and provide a reasonable and scientific selection of the transmitter. Guide to the type and installation position, combined with theory and a large number of specific application experience, developed and designed a set of remote flange pressure, differential pressure transmitter selection, calculation method, for remote flange pressure, differential pressure transmitter Selection provides a very good theoretical basis


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