Humidity Sensors

Optimal humidity level in indoor air strongly contributes to human comfort. A key to successful humidification control starts with reliable humidity measurement. The recommendations for indoor air relative humidity vary from country to country. ASHRAE Standard 62.1-2016 recommends that relative humidity in occupied spaces be controlled to less than 65% to reduce the likelihood of conditions that can lead to microbial growth. Health and Safety Executive in the United Kingdom recommends relative humidity in the range of 40 to 70% in the workplace environment. At higher temperatures, the relative humidity should be at the lower end of this range. Similarly OSHA, Occupational Safety & Health Administration in the United States, recommends controlling indoor air humidity in the range of 20-60%. Humidity measurements in building automation are useful not only for maintaining a pleasant indoor air quality, but also as a control parameter for economizers, ventilation and cooling towers. Usually the control target is to find a fine balance between the human comfort and energy efficiency. Several humidity measurements are needed in order to get sufficient amount of information for accurate control.

Vaisala has a wide range of products for indoor humidity measurements. Here are just a few examples. The cost-effective GMW80 Series wall mount humidity transmitters combine excellent stability with easy installation and reliable operation. The HMW90 Series and the GMW90 Series is a natural choice in locations that require more functionalities, such as digital output and high accuracy. Vaisala’s wide offering is also available for cooling towers, economizers, heating and demand controlled ventilation. Depending on the need, there are several options available for ducts, from cost-effective HMD80 Series to full metal body duct probe HMD60 Series.

What Makes Vaisala Humidity Sensors Unique?

Vaisala is the original innovator of the thin-film capacitive humidity measurement technology, which has now become the industry standard in humidity measurement. All Vaisala’s humidity sensors incorporate Vaisala HUMICAP® technology. A capacitive thin-film polymer sensor consisting of a substrate on which a thin film of polymer is deposited between two conductive electrodes. The sensing surface is coated with a porous metal electrode to protect it from contamination and exposure to condensation. The substrate is typically glass or ceramic. The thin-film polymer either absorbs or releases water vapor as the relative humidity of the ambient air rises or falls. The dielectric properties of the polymer film depend on the amount of absorbed water. As the relative humidity around the sensor changes, the dielectric properties of the polymer film change, and so does the capacitance of the sensor. The instrument’s electronics measure the capacitance of the sensor and convert it into a humidity reading. Vaisala HUMICAP® sensors guarantee quality and reliability, with their reputation for accuracy, excellent long-term stability, and negligible hysteresis.

Excellent Long-Term Stability

Insensitive To Dust & Most Chemicals

Chemical Purge Option For Stable Measurements 

Sensor Heating Even In Condensing Environments

Full Recovery From Condensation


The sensor location should be carefully chosen to represent the conditions in the controlled space. Locations close to the discharge of the supply air duct, near exterior doors and windows, and inside walls which are exposed to solar radiation should be avoided. Locate sensors away from heat and moisture sources and make sure that a gentle air flow is allowed around the humidity sensor. Avoid locations where air circulation is prevented e.g. by interior textiles or furniture. If humidity is centrally controlled at the air handler level, it is recommended to add additional space sensors to critical spots or problematic areas e.g. shower rooms. In duct installation, locate the sensor in a place where it can be easily maintained and calibrated. Make sure that the duct seams are tightly sealed to avoid air leakage. For supply air duct installations, select a robust humidity transmitter which withstands humidity fluctuations, condensation, and outside air contaminants. Avoid locations near the cooling and heating coils.

CO2 Sensors

The environment in which you’re measuring CO2 plays an important role in determining the best choice of sensor for your application. For a laboratory environment, you will need an accurate and traceable measurement device with good chemical durability for both the sensor and the enclosure. Investing in a device with exchangeable measurement probes or modules is recommended. Other options could include field adjustment against a calibrated handheld device or with calibration gases. Vaisala Indigo series transmitters have an exchangeable GMP252 CO2 measurement probe and come with a calibration certificate and IP65-rated housing. The unit can tolerate most common disinfectants as well as hydrogen peroxide, which is commonly used for decontamination procedures. The exchangeable probe helps to maintain traceability by minimizing calibration downtime.

Sensors for public spaces require operation in spaces occupied 24/7 and minimal maintenance. The products should also be unobtrusive and have tamper-proof enclosures. Vaisala GMW80-series transmitters meet all these requirements. For green buildings and other demanding environments where high measurement accuracy is required we recommend the Vaisala GMW90 series. The sensor chosen for measuring CO2 concentration in outdoor environments is one of the most important for the entire building because the measurements it takes are used to control the building’s fresh air intake. For this type of application, Vaisala offers the GMP252 transmitter, which is designed for demanding high-humidity environments.

What Makes Vaisala’s CO2 Sensors Unique?

Vaisala uses a single-beam and dual-wavelength NDIR (Non-dispersive infrared) technology for measuring CO2. The patented CARBOCAP® technology is used in all its CO2 sensors. This technology incorporates a unique tunable band pass filter that was developed by Vaisala and manufactured in our own state-of-the-art cleanroom. This sensor allows for measurement of both the CO2 absorption signal and a reference signal using a single set of components. Because of this, Vaisala is getting a real reference measurement in every measurement cycle. This eliminates the most common drift factors normally associated with all other NDIR sensors. The results of this breakthrough are excellent long-term stability, a long service life and the ability to function in the most demanding environments, including warm, humid, and dusty environments. The benefits of Vaisala’s CARBOCAP® sensors are:


The unique design of CARBOCAP® results in recommended calibration intervals of up to 5 years.


Suitable for use in spaces occupied 24/7 as well as dusty and dirty industrial environments.

Long lifetime

Its microglow light source is rated for 15+ years of stable light output providing many years of trouble-free use.

Out Of The Box Performance

Shows correct values immediately after power-up because the CARBOCAP® does not need to “learn” its environment like other technologies do.


We recommend installing wall-mounted sensors at head height in a representative location and not behind a cupboard, in the ceiling, or close to an air inlet duct. Make sure that there is no underpressure in the room. If there are any leaks you may need to seal the cable feed-throughs. Also, make sure that you have entered the correct elevation to ensure accurate measurement. It is not recommended to install two measurement devices on top of each other. This is not a problem for CO2 sensors, but if you install a humidity or temperature sensor directly above of a CO2 sensor, the heat from the CO2 sensor’s light source will distort the measurement results.

Outdoor Humidity Sensors

Facility managers must rely on their outdoor sensors to provide accurate information. Sensor drift and instability can be detrimental, especially to high-energy consumption facilities such as data centers. A high quality outdoor humidity transmitter with tight tolerance is required for calculating other parameters such as dew point, enthalpy, and wet bulb. A reliable outdoor sensor will recover from saturated conditions, and some sensors even have built-in heating cycles to prevent saturation.

Vaisala’s outdoor humidity transmitters incorporate the HUMICAP® sensing technology, resulting in accuracy and stability even under extremely harsh outdoor conditions. The HUMICAP® sensors fully recover from saturation. Optional heating to prevent condensation is also available for high humidity conditions. Outdoor transmitters come with flexible installation options and are available with solar radiation and precipitation shields, which provide excellent ventilation while blocking both direct and reflected solar radiation. For example, the Humidity and Temperature Transmitter Series HMS80. Watch Webinar On Outdoor Humidity Measurement here.


Vaisala’s outdoor transmitters come with flexible installation options and are available with solar radiation and precipitation shields, which provide excellent ventilation while blocking both direct and reflected solar radiation.

  • Locate sensors away from heat and moisture sources.
  • Avoid areas near exhaust fans or shaded areas that may affect measurements.
  • Avoid locations where air circulation is obstructed by structures or equipment.
  • Use a mounting kit for installation in inlet ducts or a pole-mast installation kit for cooling towers or building rooftops.
  • Use a radiation shield to protect the sensor from precipitation and solar radiation.

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Dpstar Group
No 35, Jalan OP ½, Pusat Perdagangan One Puchong,
Off Jalan Puchong, 47160 Puchong,
Selangor Darul Ehsan, Malaysia.
Email: [email protected]

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