1、 Core principle: The essence of humidity control

Firstly, it should be understood that controlling humidity in the test chamber is essentially controllingwater vapor pressureWe usually change the partial pressure of water vapor inside the box in two basic ways:

1. humidifyInject water vapor (usually atomized water or steam generated by heating) into the box.

2. dehumidifyCondense the water vapor inside the box into water and discharge it.

The 'humidity curve' is the dynamic and precise balance between humidification and dehumidification processes at different temperature points to achieve the target humidity value.

Key concepts: Dew point temperature and relative humidity

• dew point temperatureThe temperature at which air cools to saturation without changing its water vapor content and pressure. It directly reflects the content of water vapor in the air.

• relative humidityThe percentage of the actual water vapor partial pressure in air at a certain temperature to the saturated water vapor partial pressure at the same temperature.This is a variable strongly correlated with temperature.

This means that in order to achieve a humidity curve, the control system mustSimultaneously and collaboratively control temperature and humidity.

2、 Key hardware for achieving humidity curve

1. sensor：

• Temperature and humidity sensorUsually, it is a capacitive or resistive sensor used for real-time monitoring of temperature and relative humidity inside the box. Its accuracy, stability, and response speed directly affect the control effect. Advanced devices will use multiple sensors to ensure measurement accuracy.

2. actuator：

• Dehumidification of refrigeration systemThis is the most essential dehumidification method. By starting the compressor for cooling, the surface temperature of the evaporator (located in the air circulation duct inside the box) is reduced to below that of the air inside the boxdew point temperatureWhen humid air flows through a cold evaporator, water vapor condenses into water that adheres to the fins and flows to the drainage box, thereby reducing humidity.

• Auxiliary dehumidificationFor the requirement of extremely low humidity, some equipment may be equipped withdryerornitrogen purgingSystem.

• Boiler humidificationHeating water with electric heating tubes to produce pure steam, with fast humidification speed and high control accuracy, is the mainstream and high-precision equipment choice.

• Ultrasonic humidificationBy high-frequency vibration, water is atomized into micrometer sized particles, which has a fast humidification response but requires high water quality and may produce impurities.

• Electrode humidificationCurrent flows directly through water, utilizing the resistance of water to generate heat and steam, which is now less commonly used.

• Humidification system：

• dehumidification system：

3. brain：Multi channel PID controller

• This is the core of achieving 'running the curve'. It is not a simple switch, but an intelligent microprocessor.

• multi-channelCan simultaneously receive signals from temperature and humidity sensors, and independently output commands to multiple actuators such as heaters, refrigeration compressors, humidifiers, etc.

• PID control: ThroughProportions, integrals, and differentialsAlgorithms calculate precise control variables to enable the system to quickly and smoothly reach the set value without overshoot or oscillation.

3、 Implement the control logic of "humidity running curve"

Assuming we have set up a（25°C, 60%RH）to（60°C, 95%RH）The curve of the heating and humidification section.

The internal processing flow of the controller is as follows:

1. Target value analysisThe controller calculates the current time based on the preset curvetarget temperatureandTarget humidityFor example, the goal for the 10th minute is (35 ° C, 75% RH).

2. Collaborative control decision-making：

• Calculate target humidityThe controller, based on the current target temperature (35 ° C) and target relative humidity (75% RH), uses the built-inHumidity conversion formula or table lookup methodCalculate the corresponding value at this timeTarget humidity (or target dew point temperature)Assuming the calculated target dew point temperature is 30 ° C.

• decision：

• Scenario A: Humidification is requiredIf the current dew point temperature measured by the sensor (e.g. 28 ° C) is lower than the target dew point temperature (30 ° C), it indicates that there is insufficient moisture in the air. At this point, the controller will startHumidification systemInject steam into the box (such as a boiler).

• Scenario B: Dehumidification is requiredIf the sealing of the box is good or the sample releases moisture during the heating process, resulting in the current dew point temperature (such as 31 ° C) being higher than the target dew point temperature (30 ° C), the controller will startdehumidifyAt this point, it will startcompression refrigerationLet the evaporator cool down (e.g. to 25 ° C) and condense excess moisture.

• Temperature control circuitThe controller detects that the current temperature (e.g. 30 ° C) is lower than the target temperature (35 ° C), so it startsheaterMeanwhile, in order to ensure the heating rate, it maySuppress refrigeration compressorThe job.

• Humidity control circuitThis is the most complex and core part.

3. Dynamic balance and decoupling：

• Throughout the entire process,Heating, cooling, humidificationThese three actions will affect each other. For example, while cooling and dehumidifying, it will also lower the temperature inside the box, which conflicts with the demand for heating; If cold steam is injected during humidification, it will also slightly affect the temperature.

• The controller is equipped withdecoupling controlAlgorithms can predict and compensate for these mutual interferences. For example, when strong cooling and dehumidification are required while maintaining high temperatures, it will simultaneously increase the power of cooling and heating, forming a dynamic balance to ensure that both temperature and humidity can accurately follow the curve.

4、 Practical operation steps (user perspective)

1. programmingWrite test programs on the touch screen or upper computer software of the test chamber.

• target temperature

• Target humidity

• runtime

• Slope (e.g. from 25 ° C to 60 ° C for 30 minutes, the slope is approximately 1.17 ° C/minute)

• segment settingsDivide the entire test curve into multiple segments.

• Set parameters for each segmentIncluding:

• Loop settingIf a temperature and humidity change process needs to be repeated, the number of cycles can be set.

2. Startup and Monitoring：

• Set value curve

• Actual value curve

• The working status of each actuator (percentage of power output for heating, cooling, and humidification)

• Start the program and the device will start running automatically.

• The real-time monitoring interface will display:

• A well controlled system should have the actual value curve closely aligned around the set value curve.