Impact of diaphragm seal systems on transmitter measurement

Diaphragm transmitter is composed of pressure / differential pressure transmitter, diaphragm seal, filling liquid and

connection structure of direct installation or capillary. Diaphragm seals, filling solutions and connection structures in

 the form of direct mounting or capillary are called diaphragm seal systems. The diaphragm pressure transmitter 

is mainly used in the following occasions:

1) The medium to be measured is a liquid that is corrosive, crystalline, viscous, easily vaporizable, and contains

 suspended matter.

2) The process medium may freeze or solidify in the transmitter or impulse line.

3) The process temperature is outside the normal operating range of the transmitter, and the temperature cannot 

be adjusted below the temperature limit value with the impulse pipeline.

4) The process liquid is very dangerous and cannot enter the control area where the transmitter is located.

5) The application requires a sanitary connection structure.

The diaphragm of the transmitter
detection component is designed to avoid direct contact between the capacitive

 sensing element and the measured fluid, so that the capacitive element has good and stable working conditions, 

and at the same time, the transmitter's adaptability to various measured differential pressure fluids can be improved.

However, adding a diaphragm seal system will affect the overall system performance of the transmitter, mainly 

reflected in the impact on system accuracy and response time.

1 .Influence on system accuracy

The influence of diaphragm type transmitter on measurement accuracy mainly includes two aspects: pressure loss 

from diaphragm to measuring diaphragm;Zero drift caused by changes in ambient temperature.The pressure loss

 from the diaphragm to the measured diaphragm is mainly related to the zero drift caused by the change of ambient temperature, the volume of the filling liquid in the capillary tube, the thermal expansion coefficient of the filling liquid 

and the properties of the diaphragm

2.Pressure loss from diaphragm to measuring diaphragm During the operation of the diaphragm transmitter, when the 

process pressure is applied, the diaphragm is displaced. The measured pressure is indirectly transmitted to the transmitter through the liquid filling system and the capillary tube. During this period, the diaphragm to the measuring diaphragm 

will be caused. Pressure loss [3]. The loss refers to the differential pressure detected by the measuring diaphragm 

when the measured fluid acts on the high-pressure side and low-pressure side diaphragm of the detection part at high pressure pH and low pressure PL, respectively.

transmitter  (4)

transmitter  (6)