Sensor Applications in HVAC

The concept of HVAC includes heating, ventilation, and air conditioning, so it has a broader concept than central air conditioning. As can be seen from Figure 1, HVAC is an important environmental and security technology that has been condensed from the contradictory and unified relationship between people and the environment over a long period of development. After years of development, the application of HVAC has penetrated into various sectors of the national economy, playing an important role in promoting economic development and improving people's living standards, and sometimes even a critical role.

The heating system consists of heat sources, heat dissipation equipment, heat transmission pipelines, and control components. Its technical function is to input heat energy into the space, compensate for its heat loss, and reach the indoor temperature requirements. The ventilation system consists of fans , exhaust or return ports, purification devices, air ducts, and control components. Its technical functions are ventilation, heat prevention and cooling, improving indoor environment, and preventing internal and external environmental pollution. As for the air conditioning system, it is composed of cold and heat sources, air outlet equipment and terminal devices, fans, water pumps , pipes, vents, control components, etc. It relies on the fully processed and suitable parameters and good quality air conditioning medium to transfer and exchange energy and mass with the controlled environment space to achieve on-demand control of indoor air temperature, humidity, cleanliness and other parameters. In HVAC, more and more emerging sensors have been developed and applied. This article will mainly introduce the application of humidity sensors and CO2 sensors in HVAC. The application of humidity sensors is in the ascendant . There are 20-30 types of humidity measurement in principle, but humidity measurement has always been one of the difficult problems in the world's metrology field. A seemingly simple value, in-depth study, involves quite complex physical-chemical theoretical analysis and calculation. Beginners may ignore many factors that must be paid attention to in humidity measurement, thus affecting the reasonable use of sensors. Common humidity measurement methods are: dynamic method (dual pressure method, dual temperature method, shunt method), static method (saturated salt method, sulfuric acid method), dew point method, dry-wet bulb method and electronic sensor method. However, for the requirements of precise sensing and real-time control in HVAC control, the electronic humidity sensor method is a development trend. Electronic humidity sensor products and humidity measurement belong to the industry that emerged in the 1990s. In recent years, great progress has been made in the field of humidity sensor research and development at home and abroad. Humidity sensors are rapidly developing from simple humidity sensors to integrated, intelligent, and multi-parameter detection, creating favorable conditions for the development of a new generation of humidity measurement and control systems, and also raising humidity measurement technology to a new level. The first lithium chloride electric humidity sensor based on the principle of resistance-humidity characteristics was developed by FW Dunmore of the American Bureau of Standards. This component has a high degree of accuracy, simple structure, low price, and is suitable for measurement and control of normal temperature and humidity. The measurement range of lithium chloride elements is related to the lithium chloride concentration and other components of the humidity sensitive layer. The effective humidity sensing range of a single element is generally within 20% RH. For example, the humidity sensing range corresponding to a concentration of 0.05% is about (80~100)% RH, and the corresponding range of a concentration of 0.2% is (60~80)% RH, etc. It can be seen that when measuring a wider humidity range, components of different concentrations must be combined and used together. The number of components that can be used for full-scale measurement is generally 5. The measurable range of lithium chloride hygrometers using the component combination method is usually (15-100)%RH. Some foreign products claim that their measurement range can reach (2-100)%RH. The HS1101 humidity sensor adopts a patented solid polymer structure with fast response time, high reliability and long-term stability. It is fully interchangeable without calibration. The HS1101 humidity sensor is equivalent to a capacitor Cx in the circuit. Its capacitance increases with the humidity of the measured air. In the relative humidity range of 0%-100%RH, the capacitance changes from 160pF ​​to 200pF, and its error is no more than ±2%RH, the response time is less than 5s, and the temperature coefficient is 0.04pF/℃.

Figure 1. Definition of HVAC

CO2 sensors are becoming more and more important in HVAC control.

The higher the content of carbon dioxide (CO2) in the air, the greater the impact on the human body. When the carbon dioxide content is higher than 0.7%, the human body will feel uncomfortable. When it exceeds 10%, the human body will become comatose and die. When it reaches 20%, people will die within a few seconds. Therefore, in places where people are densely populated, the carbon dioxide content is a very important parameter, which is directly related to human comfort and safety. For the control of carbon dioxide, it is essential to use carbon dioxide sensors for detection and calculation.

For the HVAC system of a building, the advantages of using carbon dioxide sensors are mainly reflected in the following three points:

● Improve the living environment and make people feel more comfortable.

● Reduce energy consumption. Just like controlling humidity at each point and lighting according to actual needs, the exchange of fresh air is controlled by carbon dioxide content to keep the environment at the best comfort level, while the operating costs are minimized.

● Lower maintenance costs. In HVAC management, by monitoring the entire system components, building managers can design and perform maintenance more effectively. Here, the TGS4160 CO2 sensor is an electrochemical gas sensitive element. When the element is exposed to the CO2 gas environment, an electrochemical reaction will occur.

In order to keep the sensor at the most sensitive temperature, it is generally necessary to provide a heating voltage to the heater for heating, but the change of the heating voltage will directly affect the stability of the sensor, so the heating voltage must be stable and its range should be within 5.0±0.2 VDC. In order to ensure the accurate measurement of CO2, in addition to ensuring the stability of the heating voltage and temperature compensation for changes in ambient temperature, it is more important to measure the potential value ΔEMF between the two electrodes, rather than the absolute potential value EMF, because there is a good linear relationship between ΔEMF and CO2 concentration changes. Although the absolute value of EMF increases with the increase of ambient temperature, ΔEMF remains constant, and it is basically unaffected by temperature in the temperature range of -10℃ to +50℃.

The ΔEMF value can be obtained by the following formula:

ΔEMF = EMF1 - EMF2

, where EMF1 is the EMF value of 350 ppm CO2; EMF2 is the EMF value of the measured CO2.

Under the conditions of temperature of 20℃±2℃, humidity of 65±5%RH, heating voltage of 5.0±0.05 VDC, and preheating time of 7 days or more, the EMF value of the sensor at a concentration of 350 ppm is 220~490 mV, and the ΔEMF value at a CO2 concentration of 350~3500 ppm is 44~72 mV. Therefore, in the actual measurement application circuit, in addition to using an operational amplifier with high input impedance (≥100GΩ) and low bias current (≤1 pA), the measured signal must be processed according to the characteristics of the sensor. A single-chip microcomputer is usually used to process the signal and the signal is processed by programming by oneself. (