In the previous article "How to choose a MEMS pressure sensor?Smart Sensor tells you (1)", we focused on three factors that need to be considered in the selection of pressure sensors, that is, the purpose of the pressure sensor, the pressure range and the accuracy requirements. Today, we continue to bring you other aspects that need to be considered when choosing the right pressure sensor, including electrical requirements, operating methods, temperature ranges, and pressure sealing requirements.

Consideration of electrical requirements

Generally, the output of piezoresistive pressure sensors is an analog signal, and the full-scale output voltage can reach 100-150mV. The voltage of the long-distance output signal will be attenuated, and the current signal output should be used. The current can be amplified by a pressure transmitter and a current signal of 20 mA or less can be output. In practice, conditioning circuits are used to convert the analog signal output from the MEMS sensor into a suitable standard signal for the user to select.

The analog signal output by the MEMS sensor is converted by A/D and V/F to obtain a digital signal or frequency signal. After the signal is conditioned, the digital signal can output I2C/SPI, the analog signal can output 0~5V and 0~10V (absolute voltage output and proportional voltage output), and the current signal can output 4~20mA.

Pressure sensors typically employ two methods of excitation sources: constant current sources and constant voltage sources. There is a difference between these two methods and their effects are different:

1) The constant current source excitation is conducive to the compensation of thermal sensitivity drift, and the constant current source excitation is used in the general accuracy measurement;

2) Constant voltage excitation cannot directly provide sensitivity temperature compensation effect, but when excitation with constant voltage source, a thermistor or diode can be connected in series outside the bridge to compensate for the thermal sensitivity drift, and the measurement accuracy depends on the accuracy of the constant voltage source regulator device;

In addition, the excitation power supply of the pressure sensor can also be divided into proportional excitation and fixed excitation:

1) Proportional excitation is to connect the pressure transmitter bridge directly to the power supply, when the power supply changes, the sensitivity and zero point of the pressure sensor will change accordingly;

2) Fixed excitation means that there is a reference voltage inside the sensor conditioning circuit, and the pressure sensor bridge is powered by the reference voltage for excitation. The reference voltage is constant and independent of the supply voltage. The reference voltage remains unchanged as long as the supply voltage varies within a specified voltage range. As a result, the output of the sensor remains unchanged and is not affected by the supply voltage.

Consideration of the way it works

The operation mode of the pressure sensor is also an important consideration in the selection. For example, the measurement of gas pressure with a sensor is different from the measurement of liquid pressure. Gas is a compressible fluid, which stores a certain amount of compression energy when pressurized, and releases it with kinetic energy when depressurized, exerting a shock wave on the sensor's elastic membrane. The liquid is an incompressible fluid, and when the pressure sensor is installed, there is no room for tightening the screw, which may increase the pressure of the liquid, exceed the pressure limit of the elastic membrane, and cause the elastic membrane to rupture.

When the operating environment of the pressure sensor is harsh, such as large vibration, shock, or large electromagnetic interference, the sensor puts forward more stringent requirements. It is not only required that the overvoltage capacity of the pressure sensor be enhanced, but also that the mechanical seal is reliable and anti-loosening, and the leads, pins and outer wires of the sensor itself should be electromagnetically shielded and the shield should be well grounded.

In addition, the compatibility of the pressure sensor with the fluid medium being measured should be considered. For example, the elastic membrane structure of the sensor should be separated from the corrosive medium, and when the sensor detects the pressure of flammable and explosive media, a small excitation current is used to prevent sparks and sparks when the elastic membrane ruptures, and to increase the pressure resistance of the pressure sensor jacket.

Consideration of the operating temperature range

Pressure sensors, like all physical device systems, can cause errors or even fail to use in environments with extreme temperatures. Generally, the temperature range of pressure sensors is divided into five categories: ordinary commercial grade, the range is -10~60°C, industrial grade, the range is -25~80°C, the automotive grade, the range is -40~125°C, the military grade, the range is -55~125°C, and the special grade, the range is -60~350°C.

The pressure sensor is available in commercial grade for indoor applications and industrial grade for outdoor applications. Measures can also be taken to thermally isolate the sensor from the environment or to heat or cool it, with general commercial grade applications below -10°C or above 60°C. The choice of temperature range should also take into account the electronic temperature characteristics and mechanical temperature characteristics of the sensor.

Changes in temperature affect the sensor in two ways, either by causing zero drift and by affecting the output of the entire range. In order to eliminate the effects of temperature, various temperature compensation techniques need to be applied. The wider the operating temperature range, the more difficult the compensation technology will be, and the greater the calibration effort.

Consideration of pressure sealing requirements

The pressure seals commonly used for pressure sensors are rubber pads (or O-rings), epoxy resin, PTFE pads, taper hole fits, pipe thread fits and welding. The sealing material used determines the operating temperature range of the pressure sensor.

The MEMS pressure sensor of Beijing Smart Sensor Technology Co., Ltd. has a variety of electrical output methods, including digital signal output I2C/SPI, analog signal output and current signal output, and can be customized according to actual application scenarios; In terms of working temperature range, the pressure sensor of Smart Sensor also performs outstandingly, which can still achieve high precision and high stability in a wide temperature range, for example, the working temperature range of Smart Sensor can reach -40~125°C, and the accuracy of the whole temperature range is maintained at ±1%FS.

In short, choosing the right pressure sensor requires a number of factors to be compared and considered in order to finally choose the right product. With various comprehensive and leading technical indicators, Smart Sensor pressure sensors have won the favor of more and more suppliers!