Did you know your body's ability to cool itself from the environment depends on temperature and humidity? Whether it's humans, plants, or food, using an environmental temperature and humidity sensor is essential to maintaining comfort. Temperature and humidity control indoor conditions and are critical to detecting weather conditions. Similarly, the agriculture industry uses them to detect plant fungal or disease growth.The relationship between temperature and humidity helps us understand the inner workings of the sensor. Depending on the type of sensor we choose, there are several ways to sense the environmental condition. In this blog, we will discover how these sensors work and their applications in various domestic or commercial applications. Moreover, we will explain the output of these sensors and their calibration process.Temperature and Humidity RelationshipTemperature and humidity are directly related to the habitability of humans and plants. However, they are critical in controlling process parameters in industrial applications. Let's have a look at the science behind temperature and humidity, as it's essential to build an understanding of how sensors work:◀ Temperature: Hot or Cold?Temperature is a quantitative assessment of the environment's degree of hotness and coldness. At the atomic level, it represents the average kinetic energy of the atoms vibrating and colliding with each other. Temperature sensors are used daily for cooking, refrigeration, air conditioning, and weather detection.◀ Temperature and HeatHeat transfer always occurs from a hot object to a cold object. The fast-moving atoms in a hot object collide with the slow-moving ones in a cold object, causing heat transfer. Temperature is critical in understanding heat transfer modes such as conduction, convection, and radiation.◀ Temperature ScalesQuantitatively measuring the temperature depends on the scale that we choose. There are mainly three scales to measure temperature:● Celsius (°C)● Fahrenheit (°F)● Kelvin (K) ◀ Humidity: Water in the AirHave you ever taken a steam bath? Or did you feel like you were taking one in dense fog conditions? In either case, the humidity in the environment reaches 95% to 100%. By definition, humidity is the amount of water vapor in the air. The increased air humidity or dense fog formation can result from sudden environmental temperature changes. ◀ Types of HumidityMeasuring humidity depends on the type. There are two main ways to measure humidity, and each type has its applications. ● Absolute Humidity: The mass of water in a specific volume of air. Its measurement unit is g/m3.● Relative Humidity: The maximum amount of water vapor the air can hold at a specific temperature is relative humidity. It is the standard unit for humidity measurement, represented in percentage (RH%). ◀ Perception of Temperature Based on HumidityHumans, animals, and plants perceive humidity and temperature in different ways. Humans rely on the perspiration process to keep their bodies cool. Animals use other methods to cool down, such as panning and the presence of fur. Plants use transpiration to keep them cool. However, the process of perspiration and transpiration is directly affected by the air's humidity level. If the humidity is 100%, the heat exchange process stops.To incorporate this effect, we have devised a scale called humidity index. It is a threshold determined by scientists beyond which heat stress starts to occur. It is the primary reason why temperature and humidity sensors are always present together in air conditioning systems.Role of Temperature and Humidity SensorsTemperature and humidity sensors, in combination, detect, measure, and report the dampness and degree of hotness or coldness. Multiple types of temperature and humidity sensors are available, and their usage depends mainly on the application.▍Different Types Of Temperature Sensors● NTC ThermistorsNTC stands for Negative Temperature Coefficient. As the temperature increases, the resistance decreases across the thermistor. Due to their non-proportional behavior, NTC accuracy and precision were challenging to manage. However, modern digital circuits overcame the problem, and now NTC provides a better temperature sensitivity coefficient, around 10 times that of the popular RTDs. The measuring range of NTC thermistors is between -55 and +200C. ● Resistance Temperature Detectors (RTDs)RTD is a temperature sensor that uses resistance to measure temperature. The resistance in RTDs is directly proportional to the environment's temperature. It consists of a thin wire wrapped around a glass or ceramic core. Depending on the accuracy of the detectors, the RTD can have three or four wires of platinum wrapped around a core. We continuously measure the resistance across this wire, which increases with temperature. RTDs can respond to the changing temperature within 0.5 to 5 seconds, which makes them ideal for wide-scale applications. The measuring range of RTDs is between -200 to 600°C.● ThermocouplesThermocouple sensors use the Seebeck effect to measure temperature. Two different metals join at one end, which is heated. The other end of the metals connects to a cold junction maintained at a reference temperature. A voltmeter measures the change in voltage between the two wires, representing the temperature. There are many types of thermocouples, such as K, J, N, R, S, B, T, and E-type. Each of them has a specific tolerance range and color coding. Some types can measure 0 to 1600°C.● Semiconductor-Based SensorsTemperature sensors that use semiconductors rely on the P-N junction. When a circuit applies forward bias to the PN junction, the semiconductor sensor works at its heart. It causes a current to flow directly proportional to the temperature, making these temperature sensors ideal for electronics. Their range is typically between –55ºC to +150ºC.▍Different Types Of Humidity Sensors ● Capacitive Humidity SensorsCapacitance is the property of a material to store electric current. However, the ability to store current may change depending on the moisture content in the air. Materials that change capacitance with humidity are called hygroscopic dielectrics. A capacitive humidity sensor uses a sandwich biscuit-like configuration. The dielectric sits between two electrodes. One electrode is porous to allow moisture to pass through. The voltage difference across the element and the charge stored within the dielectric represent its capacitance. It can provide RH in percentage. ● Resistive Humidity SensorsA hygroscopic material, such as salt, is placed between two electrodes to form a resistive humidity sensor. As the humidity increases, the circuit's resistance changes, directly providing the relative humidity in the air. ● Thermal Humidity SensorsAbsolute humidity, the total presence of water in the air irrespective of temperature, is calculated using thermal humidity sensors. Two temperature sensors collectively form a thermal humidity sensor. One temperature sensor is directly exposed to moisture, while the other is kept dry. The comparison in thermal conductivity of dry and wet sensors provides absolute humidity.How Temperature and Humidity Sensors Work?The output of all sensors is either voltage, current, or capacitance. These values are just numbers until processed, filtered, and amplified until the signal quality is good for the data acquisition. To understand, we can divide the whole workings of temperature and humidity sensors into 5 steps:1. CalibrationCalibration and standardization go hand in hand. Every sensor can give a unique output depending on the manufacturing process and material characteristics. It becomes vital to ensure that the sensor is reading accurately. A highly accurate instrument or device is a benchmark for all temperature and humidity sensors under production. The device is traceable to national or international specifications to ensure accuracy and consistency.2. Signal ConditioningHere, filtration, amplification, or all other signal processing techniques are applied to ensure the accuracy of the output result.3. Analog to Digital ConversionThe output of a humidity or temperature sensor is either current, voltage, or capacitance. It is essential to convert this into a digital signal, which can then go into the processing system. ADCs are sometimes embedded in the sensor or inside the data processing device.4. Data ProcessingA microprocessor or computational device converts the data into perceivable temperature and humidity values.5. Unit ConversionFinally, the system converts the values into the user's desired units, such as Fahrenheit, Celsius, Kelvin, Relative Humidity (%), or absolute humidity (g/m3).Industries that Utilize Temperature and Humidity Sensors♦ Applications of Temperature and Humidity Sensors● Agriculture (crop monitoring, irrigation)● Healthcare (patient comfort, drug storage)● HVAC systems (energy efficiency, comfort)● Meteorology (weather forecasting)● Food Industry (quality control, storage)♦ Types of Sensors Based on Application➔ Leaf Wetness SensorCapacitive humidity sensors are essential to the agriculture industry, providing critical data for optimizing crop management and preventing disease outbreaks. The sensor is in the shape of a leaf. The dielectric constant changes as moisture or water collects on the leaf's surface. The change in the dielectric capacity directly converts into humidity values that can help in disease prevention, irrigation management, frost prediction, and spray timing selection. ➔ Atmospheric Temperature and Humidity SensorThese sensors mainly target the meteorology industry but can sometimes be used in other manufacturing industries. They primarily consist of a protective material with an ingress protection (IP) rating. The protective shell can house multiple sensors, such as temperature, humidity, and pressure. The detection range for temperature is between -40 to 60ºC. Humidity is between RH0-100%, and pressure is between 10-110kPa. They are handy for weather detection and prediction. ➔ HVAC Temperature & Humidity SensorIn air conditioning systems, temperature, and humidity sensors play a vital role in maintaining habitability for the residents. Humans feel comfortable at 22 °C to 27 °C and a relative humidity of 40% to 60%. HVAC temperature and humidity sensors ensure that the ventilation system adjusts to the demand for cooling and heating or increases moisture content based on their detection. ConclusionThe invention of computers and microprocessors has led to a significant global shift. Modern temperature and humidity sensors use digital technology to enhance their detection capability and provide accurate results. From agriculture and healthcare to industrial processes and environmental monitoring, these sensors play a pivotal role in ensuring optimal conditions and safeguarding human health. As technology advances, we expect changes in these technologies that will only improve further.
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