yokogawa EJX438B wireless gauge pressure transmitter with remote diaphragm seals

Figure 1: A pressure transmitter with remote diaphragm seals.



The transmitter with a diaphragm seal system is composed of a diaphragm seal system, the capillary and filling liquid. It is suitable for any applications in which the transmitter need to be isolated with the processing. In engineering design, this kind of transmitter is often used. And it is widely used in the petrochemical industry to measure the liquid level, interface or flow rate of corrosive, viscous, and crystallization media.

But the measurement accuracy and response time of the diaphragm seal system could be affected by many factors. For one thing, there are internal factors including the diaphragm structure, capillary diameter and length, and characteristics of the filling liquid. For another, the mounting environment, mainly the ambient temperature, also affects them.

Since diaphragm seal systems and capillaries transfer the process pressure to the transmitter, it is critical to reduce the influence of temperature and improve the response time. Therefore, this article analyzes the three factors and summarizes the notes in the design and selection to help engineering designers to choose and install the diaphragm seal system correctly.

1. Diaphragm Hardness & Diameter

Diaphragm hardness is an important parameter affecting its performance in the temperature change. When the filling liquid expands or contracts with the temperature, the diaphragm with lower hardness (good elasticity) bears a smaller opposite reaction force. With the temperature changing, the generated reverse pressure acts on the inductive diaphragm of the transmitter, thus causing the measurement error.



diaphragm seal with threaded connection

Figure 2: Diaphragm seal.



The smaller hardness makes the elasticity better. In the elastic range, the diaphragm with good elasticity can absorb the changed volume of the filling liquid, and reduces the pressure influence. Therefore, it could overcome the error to a large extent.

When the filling liquid volume changes due to the temperature, the pressure measurement error of the large diameter diaphragm is small, while that of the small diameter diaphragm is large.

For instance, a transmitter, with a single-sided seal system, of which the filling liquid is DC200, the seal diaphragm is 316LSS. And the pressure measurement error caused by the temperature change of every 20 K is listed in Table 1. For the transmitters with the two-sided seal system, the error for every 20 K temperature change is 25 % of the value in Table 1.



       (kPa)

DN

Diaphragm error

Capillary error

System error(sensitive elements)

40

0.87

0.30

0.90

50

0.29

0.03

0.20

80

0.09

0.09

0.03


Table 1: Pressure measurement error caused by the temperature change of every 20 K.

2. Filling Liquid

The filling liquid transfers the pressure, which expands or contracts with the change of temperature. Its fluidity and expansion characteristics will affect the performance of the remote diaphragm seal system. The filling fluid characteristics are shown in Table 2.



Filling liquid

Temperature range

Relative density

Coefficient of thermal expansion/(ml° ℃-1)

Viscosity/mPa °s

DC200 silicone oil

-40~205℃

0.945

0.00108

9.5

DC704 silicone oil

0~315℃

1.07

0.00053

44

Inert fill fluid(halohydrocarbon)

-45~175℃

1.85

0.000864

6.5

SyltherM XLT silicone oil

-75~150℃

0.85

0.000666

1.6

Glycerinum, water

-15~95℃

1.13

0.00019

12.5


Table 2: Characteristics of filling fluid (25℃)

* Coefficient of Thermal Expansion

Filling liquid with a smaller coefficient of thermal expansion could reduce the error caused by temperature change. The expansion varies with the volume of the filling liquid. Therefore, the length of the capillary should be as short as possible and the diameter should be as small as possible to reduce the volume of the filling liquid and the error.

* Viscosity

The filling liquid viscosity is a measure of its flow rate. The viscosity, the length, and the inner diameter of the capillary all affect the friction resistance. The greater the friction resistance, the longer the response time. Selecting filling liquids with low viscosity will speed up the response.



response time of four types of filling liquid, including high temperature silicone oil, silicone oil, vegetable oil, and inert oil

Figure 3: Filing fluid response time (T90%/s) of DN50 diaphragm with a 1-mm capillary at 20℃.



The smaller the capillary diameter is, the bigger the resistance to pressure transmission and the smaller the velocity. A large capillary inner diameter can accelerate the transmission and shorten the response time. The longer the capillary, the longer the distance the pressure signal travels, increasing the response time.

Whether used for measurements of differential pressure, flow rate, or liquid level, the diaphragm size, capillary length at both ends, and filling liquid of the seal system in positive and negative pressure chamber should be the same. The expansion of capillary at both ends is the same to minimize the measurement error caused by the change of ambient temperature.

3. System Precision

Zero error is due to the variation of the temperature. And the characteristics of filling liquid, temperature and length of the capillary determine the zero offset.

For DN80, 316 L, silicone oil diaphragm seal system, the capillary is 5 meters, the ambient temperature is 45 ℃. The "TK" static pressure curve of the capillary sealing system is shown in Fig. 4.



static pressure curve

Figure 4: A static pressure curve is according to the capillary length.



From the figure, we could conclude that if the capillary is 5 meters, the static pressure would be 140 Pa/10K.
Temperature variation = ambient temperature - temperature calibration = 45 ℃-25 ℃ = 20 ℃
p = T ×140 Pa/10 K =280 Pa
So, the zero offset is 280 Pa.

When measuring the liquid level of a double flange type transmitter, as shown in Fig. 5, the length of the capillaries at both ends would be identical, and it could overcome the influence of the expansion with the ambient temperature change, but not the influence of the static pressure.
It is difficult to offset the static pressure produced by the change in filling fluid density. Therefore, the filling liquid whose density is less affected by temperature change is a good choice, that is, you should use the filling liquid with a small coefficient of thermal expansion. The densities in Table 2 are measured at 25 ℃, which would change with the ambient temperature.



how to install a double flange type transmitter with a diaphragm seal system

Figure 5: Installation instruction of a double flange type transmitter.



At 25 ℃, if -Δp1 represents the zero point, Δp1 =ρ2 gH.
When the variation of filling liquid density is Δp , the actual zero point: -Δp2 =(p2 +Δp)gH.
Zero offset: Δp =ΔpgH

When it is the long capillary, the filling liquid with a high coefficient of thermal expansion, and the varying ambient temperature and the large measuring range, the zero offset would be bigger. The solution is to make temperature compensation for the change of filling liquid density (when the accurate measurement is required) or reduce the expansion static pressure to extremely low by changing the mounting position and stabilizing the ambient temperature.

4. Summary

In short, when choosing a differential pressure transmitter with the flange seal system, there should be considering the measurement error due to temperature change, as well as characteristics, temperature, and response time of the media during the measurement.

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