Differential pressure gauge: the pressure sentinel of industry
1. Working principle: complete link from sensing response to signal presentation
The core of the differential pressure gauge is to convert the pressure difference into an observable signal. Different types of instruments achieve this conversion through differentiated sensing structures, which can be divided into two technical paths: mechanical type and electronic type.
(2) Electronic differential pressure meter: electrical conversion of mechanical signals
The electronic differential pressure gauge triggers changes in electrical characteristics through the deformation of the sensing element, and outputs a digital signal after circuit processing, which has higher precision and intelligent capabilities:
Piezoresistive: The core is a semiconductor pressure-sensitive chip. The silicon diaphragm undergoes elastic deformation under the action of pressure difference, which causes the piezoresistive coefficient of the silicon lattice to change, thereby unbalancing the resistance value of the Wheatstone bridge. After compensating for temperature drift through the laser trimming process, it can output an electrical signal that is linearly related to the voltage difference, with an accuracy of ±0.1%~±0.5%, a response time of less than 5ms, and supports 4-20mA and HART protocol transmission.
Capacitive type: With parallel plates as the sensing core, the pressure difference causes the distance between the moving plate and the fixed plate to change, resulting in a change in capacitance. This type has extremely high sensitivity and can measure tiny pressure differences at the μPa level. It has good stability and small temperature drift, making it suitable for special scenarios with stringent accuracy requirements.
(1) Mechanical differential pressure gauge: amplification and conduction of mechanical deformation
Mechanical differential pressure gauges do not require a power supply and rely on the mechanical deformation of the sensing element and the signal amplification of the transmission mechanism to achieve measurement. Common types and principle details are as follows:
Liquid column type: Using the physical principle of "ΔP = ρgh" (ρ is the density of the liquid, g is the acceleration of gravity, h is the height difference of the liquid column), the pressure difference is converted into the height difference of the liquid in the glass tube. Water, alcohol or mercury is commonly used as the medium. The structure is simple and the accuracy can reach ±0.5%, but the response is slow and is easily affected by the installation angle and temperature. For example, when calibrating low-pressure gas pressure in the laboratory, the pressure difference can be directly read by observing the liquid level difference on both sides of the U-shaped tube.
Diaphragm type: An elastic diaphragm made of metal or rubber is used as the core sensing element. The pressure difference will push the diaphragm to deform. After the deformation is amplified by a lever or gear transmission mechanism, it drives the pointer to deflect on the dial. Its measuring range covers several Pa to tens of kPa. It has good shock resistance and an accuracy of about ±1%~±2.5%. However, it has mechanical wear problems and is suitable for flow monitoring and blockage alarm in industrial sites.
Spring tube type: Two symmetrical spring tubes are connected to the high and low pressure ends respectively. The pressure difference causes the spring tubes to deform in opposite directions. After the deformation is transmitted through the "I"-shaped bracket, it is converted into pointer readings through the gear mechanism. This structure has strong high-pressure resistance, can measure MPa-level pressure differences, and has excellent impact resistance. It is widely used in petrochemical high-pressure pipeline monitoring.