的基本原理是、将一个空间用敏感元件（多用膜盒）分割成两个腔室，分别向两个腔室引入压力时，传感器在两方压力共同作用下产生位移（或位移的趋势），这个位移量和两个腔室压力差（差压）成正比，将这种位移转换成可以反映差压大小的标准信号输出。 The basic principle of a differential pressure transmitter is to divide a space-sensitive element (multi-purpose capsule) into two chambers, and when the pressure is introduced into the two chambers, the sensor generates displacement (or (Displacement trend), this displacement is proportional to the pressure difference (differential pressure) of the two chambers, and this displacement is converted into a standard signal output that reflects the magnitude of the differential pressure.
In actual construction, there are many types of structure of the sensitive element, the form of the cavity, the method of displacement conversion, and the format of the standard signal.
Differential pressure transmitter principle
The process pressure passes through the isolation diaphragm on both sides or one side, and the filling liquid acts on the tensioned measurement diaphragm in the delta element (that is, the sensitive element). The measurement diaphragm and the capacitor plates on the insulators on both sides form a capacitor. When no pressure is applied or the pressure on both sides is equal, the measuring diaphragm is in the middle position, and the capacitance of the two capacitors is equal. When the pressure on the two sides is not the same, the measuring diaphragm is displaced, and the displacement is proportional to the pressure difference. This kind of displacement is transformed into a differential capacitor formed on the capacitor plate. The differential capacitor is converted into a two-wire current signal of 4-20mADC by electronic circuits. The working principle and differential pressure transmission of pressure transmitters and absolute pressure transmitters The device is the same, except that the pressure in the low-pressure chamber is atmospheric or vacuum.
Principle and application of differential pressure transmitter
There are many types of mechanical transmitters, such as resistance strain gauge differential pressure transmitters, semiconductor strain gauge differential pressure transmitters, piezoresistive differential pressure transmitters, inductive differential pressure transmitters, and capacitive differential pressure transmitters. Devices, resonant differential pressure transmitters and capacitive acceleration sensors. But the most widely used is the piezoresistive differential pressure transmitter, which has a very low price and high accuracy and good linearity. Below we mainly introduce such sensors.
In understanding the piezoresistive force sensor, we first understand the element such as a resistance strain gauge. The differential pressure transmitter resistance strain gauge is a sensitive device that converts the strain change on the DUT into an electrical signal. It is one of the main components of a piezoresistive strain transmitter. The most commonly used resistance strain gages are metal resistance strain gages and semiconductor strain gages. There are two types of metal resistance strain gauges: wire strain gauges and metal foil strain gauges. Usually, the strain gauge is tightly adhered to the substrate that generates mechanical strain through a special adhesive. When the substrate undergoes a stress change, the resistance strain gauge also deforms together, so that the resistance value of the strain gauge changes, so that The voltage applied to the resistor changes. Such strain gauges usually have small changes in resistance when subjected to force. Generally, these strain gauges form a strain bridge, which is amplified by subsequent instrumentation amplifiers and then transmitted to the processing circuit (usually A / D conversion). And CPU) display or actuator.
Internal structure of metal resistance strain gauge
It consists of a base material, a metal strain wire or strain foil, an insulation protection sheet, and a lead-out wire. According to different purposes, the resistance value of the differential pressure transmitter resistance strain gauge can be designed by the designer, but the value range of the resistance should be noted: the resistance value is too small, the required driving current is too large, and the heat of the strain gauge causes The temperature itself is too high, and the use in different environments makes the resistance value of the strain gauge change too much, the output zero point drift is obvious, and the zeroing circuit is too complicated. The resistance is too high, the impedance is too high, and the ability to resist external electromagnetic interference is poor. Generally it is about several tens to several tens of thousands of ohms.
Working principle of resistance strain gauge:
金属电阻应变片的工作原理是吸附在基体材料上应变电阻随机械形变而产生阻值变化的现象，俗称为电阻应变效应。 The working principle of the differential pressure transmitter metal resistance strain gauge is that the resistance of the strain resistance absorbed on the base material changes with the mechanical deformation, which is commonly known as the resistance strain effect. The resistance value of the metal conductor can be expressed by the following formula:
R = ρ * L / S
Where: ρ——resistivity of metal conductor (Ω · cm2 / m)
S——Sectional area of conductor (cm2)
L——the length of the conductor (m)
We take the metal wire strain resistance as an example. When a metal wire is subjected to an external force, its length and cross-sectional area will change. It can be easily seen from the above formula that the resistance value of the differential pressure transmitter will change. When a wire is stretched by an external force, its length increases, and its cross-sectional area decreases, and the resistance value increases. When the metal wire is compressed by an external force, the length decreases and the cross section increases, and the resistance value decreases. As long as the change in resistance is measured (usually the voltage across the resistance), the strain of the strained wire can be obtained.
是丈量变送器两端压力之差的变送器,输出规范信号(如4~20mA,1~5V)。 The differential pressure transmitter is a transmitter that measures the pressure difference between the two ends of the transmitter and outputs a standard signal (such as 4 ~ 20mA, 1 ~ 5V). Different from ordinary pressure transmitters, differential pressure transmitters have two pressure interfaces. Differential pressure transmitters are generally divided into positive pressure terminals and negative pressure terminals. Under normal conditions, differential pressure transmitters have positive pressure. The pressure at the end should be greater than the pressure in the negative pressure section to measure. The practical application of the differential pressure transmitter in the oil depot project, the design conditions: 2000m3 oil tank, diameter d = 14.5m, height can get the actual oil inventory G, from the formula can also know its density ρh = 14m. One-time meter: Z351 / 1151DP flange type flameproof differential pressure temperature transmitter of Shandong Feichuang Instrument Co., Ltd. is selected. The flange type is used to avoid sedimentation of the bottom of the tank and infarct the pressure tube. . Secondary meter: Use the company's FWP series intelligent light bar to display the current alarm device, universal signal input, can change the range arbitrarily, use the light bar to display the liquid level, and digitally display the tonnage of oil products. Take 6 # tank as an example, S = π × r2 = 3.14 × 7.252 = 165m2, and the height is 14m. On the top of the oil tank, a differential pressure transmitter is designed to install a liquid level alarm to prevent oil overflow, as a double insurance. In the application, the measured value is directly ton, so no matter what kind of oil is stored in the oil tank, the value shown in the secondary table is the number of tons of oil in the oil tank, which prevents the trouble of measuring the density and stopping conversion. Differential pressure transmitters are usually put in and out of the warehouse using a pump to pass through an ellipse gear meter. Because the accuracy of the flow meter is limited, the maximum is only 0.2.