Chapter 2: Input Devices and Sensors
Part 2: Analog Devices

Humidity Measurements

Humidity is the presence of water vapor in air. The amount of water vapor present in air can affect human comfort and numerous material properties. It is a parameter that HVAC designs often must take into account and therefore can be a required measurement in HVAC control schemes. The amount of water vapor in air can be defined by one of several ratios, which include relative humidity, humidity ratio, specific humidity, and absolute humidity. By far the most common measurement of humidity in the HVAC industry is relative humidity (RH).

Relative humidity is the ratio of partial water vapor pressure in an air-water mixture, to the saturation vapor pressure of water at the same temperature. This is analogous to the ratio of the number of water molecules per unit volume of the mixture to the number of water molecules that would exist in a saturated mixture at the same temperature.

Types of Relative Humidity Sensors
Relative Humidity sensors are used in DDC control systems for HVAC to measure relative humidity in conditioned spaces and ducts. Commonly applied sensor types include thin-film capacitance, bulk polymer resistance, and integrated circuit type. The integrated circuit type combines a sensor (commonly of the capacitance type) and some of the signal conditioning circuitry to form a solid-state device. Relative humidity can also be measured along with dew point and other humidity measurements by chilled mirror hygrometers. See the Chilled Mirror Hygrometers section in the section on Dew Point Measurement.

Thin Film Capacitance
Thin film capacitance sensors operate on the principle that changes in relative humidity cause the capacitance of a sensor (made by laminating a substrate, electrodes, and a thin film of hygroscopic polymer material) to change in a detectable and repeatable fashion. Because of the nature of the measurement, capacitance humidity sensors are combined with a transmitter to produce a higher-level voltage or current signal. Key considerations in selection of transmitter sensor combinations include range, temperature limits, end-to-end accuracy, resolution, long-term stability, and interchangeability.

Capacitance type relative humidity sensor/transmitters are capable of measurement from 0-100 % relative humidity with application temperatures from -40 to 200 °F. These systems are manufactured to various tolerances, with the most common being accurate to ±1%, ±2%, and ±3%. Capacitance sensors are affected by temperature such that accuracy decreases as temperature deviates from the calibration temperature. Sensors are available that are inter-changeable within plus or minus 3% without calibration. Sensors with long term stability of <±1% per year are available.

Bulk Polymer Resistance
Bulk Polymer Resistance sensors use the principle that resistance change across a polymer element varies with relative humidity and is measurable and repeatable. As with capacitance humidity sensors, polymer resistance sensors are combined with transmitters to produce a higher-level voltage or current signal.

Bulk polymer resistance humidity sensor/transmitters are commonly capable of measurement from 0-100 % relative humidity with application temperatures from -20 to 140 °F. These systems are manufactured to various tolerances, with the most common being accurate to ±2%, ±3%, and ±5%. Some manufacturers rate their published accuracy to the 20 - 95 % RH ranges. Resistance sensors are affected by temperature such that accuracy decreases as temperature deviates from the calibration temperature. Bulk polymer resistance humidity sensors are not commonly interchangeable. Sensors with long term stability of <±1% drift per year are available.

Installation
Sensors are commonly enclosed in at least a louvered plastic or metal enclosure. Sensors for rugged use are usually enclosed by a filtering element such as a plastic or stainless steel screen, or a sintered metal cup or tube. Mounting methods are similar for all the technologies in common use.

Continue with Analog Devices: Dew Point Mesurements