The pressure parameter is an important parameter in the industrial production process, and directly participates in the control of the production process and the chain protection operation of the equipment. On the basis of the rapid progress in silicon, micromachining technology, ultra-large integrated circuit technology, and research on material preparation and characteristics, pressure sensors have become a type of sensor with very mature technology, very stable performance and very high cost performance among all kinds of sensors. , the achievement of the importance of pressure transmitters in the field of industrial control. Since the pressure transmitter is a field instrument that is in direct contact with the measured medium, and often works in harsh environments such as high temperature, low temperature, corrosion, vibration, shock, etc., how to make the stability and accuracy of the pressure transmitter meet the requirements of different industries Condition requirements are the first problems to be solved in engineering applications. This paper starts with the application of pressure transmitter and discusses its problems in engineering practice.
【1 Zero adjustment】
Fuji pressure transmitter needs to be adjusted after installation to eliminate the influence of installation location, pipeline pressure and other factors. The field generally uses the field operator and the instrument hardware adjustment button to realize the zero point adjustment of the pressure transmitter. There are certain differences between the two adjustment methods. The zero-point adjustment of the transmitter output by the handheld terminal can be consistent with the original set value, but its adjustment range is small. Using instrument hardware to perform zero point adjustment is actually to reset the range and force the current measured value to be the output value corresponding to the transmitter's 4 mA. The upper range limit is automatically adjusted, that is, the range span and characteristics of the transmitter remain unchanged. in actual operation
During the process, it is more suitable to adjust the zero point through the handheld communicator.
【2 Instrument grounding】
Fuji transmitter grounding is mainly used to shield interference. The transmitter generally has two grounds, which are internal grounding and external grounding. The internal grounding of the transmitter is located on the terminal side of the transmitter, and is marked with a grounding symbol, and the ground of its internal circuit is insulated from the housing. The external ground terminal is usually not a standard configuration of the transmitter, and should be specified when ordering. The internal grounding of the transmitter and the grounding of the casing must be connected correctly. When the internal grounding of the electrical interface is used as the casing grounding of the transmitter, it may not be fully grounded; when the transmitter casing is not properly grounded, the transient voltage protection cannot be achieved. In the project, it is strictly forbidden to connect the transient voltage protection
Run the ground wire together with the signal wire because the ground wire will carry excessive current in the event of a lightning strike.
【3 Application of Remote Diaphragm】
Taking Fuji pressure transmitter as an example, the pressure transmitter with remote diaphragm is mainly composed of pressure transmitter, remote diaphragm, process connection and capillary. In general, such pressure transmitters with remote diaphragms should be used as little as possible. The high precision and high stability of various transmitters are only for single transmitters, but there are no exact application indicators for remote transmitters with capillaries. In industrial measurement, the use of pressure transmitters with remote diaphragms is common in the following conditions: In high temperature applications, without using ordinary pressure piping to cool down, the high temperature medium is isolated from the transmitter; the medium to be measured Corrosive to the wetted part of the transmitter, especially the diaphragm of the transmitter sensor; the measured medium is highly viscous or easy to solidify or crystallize due to changes in the environment, process, and temperature; measurement of container liquid level; liquid Density or liquid/liquid interface measurement; the process requires the container or pipeline to be measured to reduce dead angles as much as possible; it is not desirable to have a pressure-inducing pipeline entering the medium.
【3. 1 Small range pressure transmitter]
The remote diaphragm is not suitable for small range pressure measurement. For remote diaphragms with different pressure-sensitive areas, there will be corresponding very small recommended ranges. If the measurement range is smaller than the recommended very small application range in the actual application process, the overall performance of the instrument will be lost, and sometimes the measurement requirements cannot be met at all. When measuring liquid level, pressure transmitters with small ranges generally need to do zero-point migration. The zero-point migration determines the selection of the instrument range. Under the condition that the accuracy of the instrument can be guaranteed, the instrument with a larger range can be used.
【3. 2 Capillary selection and oil filling]
After adding a set of remote capillary and remote diaphragm to the Fuji pressure transmitter, the influence of various factors on the measurement of the transmitter needs to be considered. The measurement accuracy and dynamic response time of the transmitter will vary greatly, and the error caused is likely to be very large. The length, thickness and material of the capillary are the key factors. The principle of length is to meet the working conditions and to be as short as possible; the selection of roughness needs to consider the physical properties of oil filling and the requirements of dynamic response time. The material is determined according to the usage environment. When determining whether the pressure transmitter can meet the requirements, the thickness and length of the capillary, the characteristics of various filling oils, the size and material of the remote diaphragm, the range of the transmitter, and the installation method can be integrated under the specified process conditions. Various influencing factors, according to the relevant software calculation, the conclusion of whether the transmitter is suitable for the process conditions is obtained. Various oil filling options for pressure transmitters further broaden the range of applications. However, in low temperature, high temperature, sanitary and other occasions, the requirements for oil filling are strict. General principles for selecting filler oil: The working point of process pressure and temperature must be at the stage where the temperature of the selected filler oil changes little with pressure; DC200 silicone oil has good comprehensive performance and is a very commonly used filler oil; Silicone oil for high temperature measurement usually has If the viscosity is relatively large, a thicker capillary must be selected to cooperate with it, so as to ensure reasonable dynamic response performance; whether the filling oil and the measuring medium will react in contact is also a content that must be considered in the selection of oil filling.
【4 Pressure transmitters in harsh environments】
With its advanced technology, convenient installation, high precision, and low maintenance, pressure transmitters have been widely used in harsh environments and have been affirmed. However, there are still a large number of surface problems, in fact, the measurement error is a larger example [3]. The reason is mainly based on unreasonable application.
【4. 1 Negative pressure environment]
In a negative pressure environment, the requirements for the transmitter are mainly reflected in the airtightness of the transmitter and the choice of oil filling. At the same time, it is necessary to avoid the long-term negative pressure state of the wetted diaphragm. Diaphragms that are under negative pressure for a long time are easily deformed, which seriously affects the service life and measurement accuracy. In order to ensure the overall airtightness of the pressure transmitter, all welding must be adopted. In the negative pressure environment, the working range of various filling fluids will be greatly affected, and it is prone to decomposition, gasification and other phenomena, which will lose the pressure guiding function and affect the measurement. In addition to the influence of the above-mentioned pressure transmitter itself, the Fuji pressure transmitter can also be better applied in the negative pressure environment through a reasonable installation position.
【4. 2 Corrosive wear environment]
In the environment of corrosion and wear, there are mainly two ways to protect the liquid-contacting diaphragm, that is, adding a protective cover to the diaphragm and selecting a special metal with high hardness and corrosion resistance. Different methods can be adopted according to the actual working conditions. In the environment of corrosion and abrasion in the measurement environment, the method of adding a protective cover to the diaphragm is an effective solution for pressure measurement. However, the application of this cover in the measurement will definitely have a certain impact on the measurement performance, and in the actual application process, there is no dynamic analysis of the impact of performance changes. In a strong corrosive environment, the isolation diaphragm of special material solves the problem of damage to the diaphragm by the measuring medium, but users usually ignore the corrosion problem at the welding point of the diaphragm. In order to solve such problems, special materials should be used as a whole.
【4. 3 High temperature environment]
The use of Fuji pressure transmitters in high temperature environments mainly focuses on the selection and insulation of oil. Such as the cracking of the pressure transmitter in the asphalt industry, this phenomenon is mainly caused by the gasification and volume expansion of the transmitter oil filling during the high temperature process. In addition, due to improper cooling measures at the installation nozzle, the process temperature is too high, which exceeds the very high value of the diaphragm, which is one of the main reasons for the damage to the diaphragm. The solution is to use oil-filled oil with a wider temperature range and take cooling measures at the same time.
