Is the air compressor running smoothly? Check if these four tests have been conducted properly.

The current mature level detection solutions mainly include the following types:

The float-type level sensor (reed switch type) is the most common old method. The structure is very simple: a float is attached to a tube, and a string of reed switches and resistance chains are installed in the tube. The float moves up and down with the liquid level, and the magnets inside attract the reed switches, connecting different positions of the resistors. The transmitter converts this signal into a 4-20mA analog output. The biggest advantage of this solution is that it is inexpensive and does not depend on the medium. It works well with water, oil, and refrigerants. However, the mechanical components such as the float and connecting rod are prone to clogging and jamming, and they wear out over time.

The piezoelectric transducer-type liquid level switch has been used more and more recently. Its principle is somewhat like hitting a metal fork - the probe uses high-frequency vibration, and when immersed in the liquid, the vibration frequency changes. The circuit detects this change and gives a switch signal. This type of device is generally used for high and low level alarms and does not measure continuous liquid levels, but it has a fast response and is resistant to dirt, and is not afraid of bubbles and slight scaling. In our previous site, the mechanical float often got stuck, but after replacing it with a piezoelectric transducer, there have been no more problems.

Ultrasonic liquid level measurement is a non-contact solution. The sensor is attached to the outer side of the tank wall, and the longitudinal wave propagates along the tank wall. The level of the liquid affects the propagation loss of the sound wave, and measuring the signal strength can calculate the liquid level. The advantages are that it does not require drilling holes or contact, and is very friendly to the sealing system. However, this system is relatively expensive and is usually used for key monitoring of core units.

Currently, the mainstream high-temperature detection sensors in the compressor industry fall into three categories:

Thermocouples (Thermocouple) are veterans in high-temperature applications. Their principle is that two different metals are welded together, and when the temperature changes, a tiny thermoelectric potential is generated at the junction, and measuring this voltage can determine the temperature. The most commonly used type is K-type (nickel-chromium - nickel-silicon), which can handle temperatures up to several thousand degrees. The exhaust ports of industrial compressors are suitable for using thermal heads. The disadvantage is that it has an additional "cold end compensation" hassle and the signal is relatively weak (at the millivolt level), which is prone to interference.

Pt100 / Pt1000 platinum resistors are typical resistance-based temperature measurement methods. The resistance of platinum has a fixed linear relationship with temperature (0°C is exactly 100Ω), and measuring the resistance can inversely calculate the temperature. The accuracy of Pt100 resistors is much higher than that of thermocouples (up to ±0.1°C), and they perform well in the temperature range of -200°C to 600°C. Nowadays, the water and oil temperatures of many compressors are directly measured using Pt100, and the three-wire or four-wire connection methods can significantly reduce wiring errors and make on-site adjustments easier.

NTC thermistors are a low-cost, fast-response solution. They are made of semiconductor materials and their resistance decreases by a certain amount for every slight increase in temperature. Their advantages are small size and fast response (at the millisecond level), but the temperature range is narrow, generally not exceeding 150°C, and they are not suitable for high-temperature conditions. However, they are widely used in some small, mobile compressors, which are cheap and easy to use.

The thermal mass flowmeter is specifically designed for gases such as compressed air. Its working principle can be understood as follows: The sensor probe is equipped with two temperature probes. One is continuously heated (maintaining a fixed temperature difference higher than that of the gas), while the other is a reference probe for measuring the temperature of the gas itself. When the gas flows, it will absorb some heat from the heating probe, and the faster the heat dissipation, the faster the gas flows and the larger the volume. Based on this heat dissipation amount, the mass flow can be directly calculated.

This solution has several particularly practical benefits: Firstly, it directly outputs mass flow (Nm³/h), eliminating the need for additional temperature and pressure compensation, saving a lot of trouble. Secondly, the measurement range is very wide (1:100 or even 1:200), capable of measuring both full production high flow rates and small leakage signals. Thirdly, it has no moving parts, is not prone to wear, has extremely low pressure loss, and does not waste energy.

The vortex flowmeter is another commonly used solution. It utilizes the Karman vortex street principle - when a fluid flows past a triangular column, alternating vortices are generated on both sides, and the frequency of the vortices is strictly proportional to the flow velocity. Its advantage is high temperature and pressure resistance, and a sturdy structure, making it very reliable in main pipelines and large-diameter applications. However, it outputs volumetric flow rate and needs to consider temperature and pressure corrections. If the gas contains moisture or oil, the accuracy will be affected.

In the current compressor industry, ceramic pressure sensors are one of the most widely used solutions. It utilizes the "piezoresistive effect" principle - pressure directly acts on the ceramic diaphragm, causing a slight deformation of the diaphragm, and the resistance value on the backside of the thick film resistor (forming a Wheatstone bridge) changes accordingly. The bridge can then output a voltage signal proportional to the pressure. Why are ceramics so useful? Because ceramics themselves have high elasticity, corrosion resistance, wear resistance, impact resistance and vibration resistance, and they have particularly good stability, with an annual drift of better than 0.2% FSO. They can operate stably in a wide temperature range of -40°C to 135°C, even in the harsh working conditions of compressors.

Diffused silicon pressure sensors are also common. The principle is similar to that of ceramic types, except that the sensitive element uses silicon material, and the accuracy is often higher, but it is also more sensitive to temperature and electromagnetic interference. It is well-suited for some high-precision testing scenarios.

Stainless steel strain gauge sensors retain the sturdy and reliable characteristics of metals, can withstand strong pressure pulsations and impacts, and are suitable for large-load scenarios such as hydraulic systems. However, their overall stability and long-term drift control are not as good as the ceramic solution.

In the application of air compressors, pressure sensors are mainly responsible for maintaining stable air pressure output, improving the working efficiency and compression air quality of the air compressor. Some new intelligent compressor platforms even adopt an isolation manifold module design, making it convenient to remove and repair the pressure sensor without emptying the internal medium of the system, thus improving maintenance convenience.