An HVAC humidity sensor or transmitter monitor is a device that plays a vital role in maintaining optimal humidity levels within a building or controlled environment. It continuously measures the amount of moisture (water vapor) present in the air, which is expressed as relative humidity (RH).

There are two main types of HVAC humidity devices:

• Sensors: These devices simply measure and convert the relative humidity level into an electrical signal. This signal may be voltage, current, or resistance-based depending on the sensor type. Sensors typically require additional equipment to interpret and utilize the data.
• Transmitters: These devices combine a sensor with internal electronics that convert the sensor’s electrical signal into a standardized output format, such as a 4-20mA current loop or a digital Modbus communication protocol. Transmitters can directly connect to building automation systems (BAS) or controllers for real-time monitoring and control purposes.

Maintaining proper humidity levels is crucial for several reasons:

• Comfort: Excessive humidity can lead to a stuffy and uncomfortable feeling, while very dry air can cause irritation to the skin and respiratory system. A balanced humidity level between 30% and 50% RH is generally considered ideal for occupant comfort.
• Health: High humidity levels can promote the growth of mold and mildew, which can trigger allergies and respiratory problems. Conversely, excessively dry air can worsen respiratory issues and cause dry skin.
• Building Protection: Improper humidity control can contribute to structural damage. High humidity can lead to warping of wood, peeling of paint, and growth of mold on building materials. On the other hand, excessively dry air can cause wood to crack and furniture to become brittle.
• Efficiency: Optimal humidity levels can improve the efficiency of HVAC systems. In cooling mode, proper dehumidification reduces the cooling load on the air conditioner. In heating mode, maintaining some moisture in the air prevents dry air from absorbing heat from occupants, reducing the heating load.

The placement of humidity sensors and transmitters depends on the specific application and the desired control strategy. Here are some common locations:

• Return air duct: This is a common location to monitor the overall humidity level within a building.
• Mixed air plenum: Here, the sensor can monitor the conditioned air being supplied to the building after mixing of return and fresh air.
• Outdoor air intake: Monitoring outdoor humidity can be helpful for controlling dehumidification strategies during cooling seasons.
• Humidified spaces: In spaces requiring specific humidity control, such as greenhouses, museums, or server rooms, sensors can be placed within the conditioned space itself.

Several factors come into play when selecting an HVAC humidity sensor or transmitter:

• Accuracy and Stability: Look for sensors with high accuracy specifications and good long-term stability to ensure reliable readings.
• 応答時間： Consider the response time of the sensor, especially if it is critical to react quickly to humidity changes.
• Operating Range: Choose a sensor with an operating humidity range that covers the expected conditions within your building.
• Output Signal: Select a sensor or transmitter with an output signal that is compatible with your existing building automation system or controller.
• Durability: The sensor should be suitable for the environment it will be installed in, considering factors like temperature extremes, dust, and potential presence of corrosive elements.

Most humidity sensors require minimal maintenance. However, some best practices include:

• Refer to manufacturer’s recommendations: The specific maintenance procedures may vary depending on the sensor model.
• Periodic cleaning: Following the manufacturer’s instructions, gently clean the sensor probe to remove any dust or debris buildup that could affect readings.
• キャリブレーション： Some sensors may require periodic calibration to ensure accuracy. Consult the manufacturer’s recommendations for calibration intervals and procedures.

By implementing an HVAC humidity monitoring system, you can ensure optimal comfort, protect your building structure, and potentially improve the efficiency of your HVAC system.