DEFINITION OF TERMS RELATED TO HUMIDITY:

There are many terms associated with humidity which can lead to confusion.

Relative humidity

Relative humidity is defined as the ratio of the water vapor pressure to the saturation vapor pressure (over water) at the temperature of the gas.

RH = Pw/Pws * 100

Water vapor

Water vapor can be treated as a gas. At a particular temperature air for example can only hold so much water vapor. The higher the temperature the more water vapor it can hold. When saturated the relative humidity would be 100 % RH, so relative humidity describes how close to saturation the air is. It is important to remember that in a process with a high relative humidity a small drop in temperature will cause the humidity to rise and the environment to saturate. Rapid temperature changes in a an environmental chamber for example could also cause condensation.

Dewpoint

Is the temperature at which air becomes saturated with water and begins to condense - forming a dew. Therefore at 100 % RH the ambient or process temperature equals the dewpoint temperature. The more negative the dewpoint temperature is from the ambient temperature the less the risk of condensation and the drier the gas or air stream.

Wet bulb

Traditionally this was the temperature indicated by a thermometer whose bulb is wrapped in a wet sheath. The wet bulb temperature and the dry bulb temperature (i.e air temperature) would then be used to calculate relative humidity or dewpoint. Alternatively charts or tables can be used.

Mixing ratio

Is often used in drying applications and is the ratio of the mass of water vapor to the mass of dry air with which it is associated. Vaisala products give this output as grams of water per kilogram of dry air (g/kg).

Absolute humidity

This is often confused with mixing ratio. It is the ratio of the mass of water vapor to the unit volume of moist air. Vaisala products give this output as grams of water per cubic metre of air (g/m3). 

ELECTRONIC RH SENSORS

DEFINITION OF TERMS RELATED TO ELECTRONIC RH SENSORS:

When looking at the specification of an electronic RH sensor the following should be taken into account

Accuracy

Accuracy is a much abused term in humidity measurement because it is difficult to define and many unrealistic claims are made. However it is still regarded as a major factor when choosing an instrument. Of equal importance are other factors such as stability, hysteresis, temperature dependence and chemical tolerance. Also see the section on calibration. The majority of Vaisala products are compared and adjusted against factory standards during manufacture. These factory standards are traceable to various national standards. Some Vaisala products are automatically supplied with Vaisala calibration certificates.

Linearity

This is a measure of the offset from a specified straight line joining the end points of the working range. In the case of Vaisala sensors the accuracy specification includes the linearity and hysteresis errors.

Hysteresis

This is the difference in output in %RH when the humidity is increased from a set point by a specified amount then decreased by the same amount (e.g from 10 %RH to 75 %RH then back to 10 %RH, over a one hour period). Vaisala includes this error in the accuracy figure.

Repeatability

Describes the ability of a sensor to repeat its performance in a number of tests.

Stability

Gives an indication of drift over a period of time. Typically the HUMICAP sensor has stability of ±0.5%RH/year in normal air conditions, which makes it the most stable humidity sensor available today.

Temperature dependence

Shows the effect of temperature change on the RH output value. Over a large temperature range this change can be substantial. In Vaisala products the temperature compensation built in.

Response time

Is the speed of response of the sensor when subjected to a step change in humidity. This varies depending on the temperature, air flow and type of filter fitted. A typical response time with Vaisala products is 15sec.

Vaisala RH meters employ the following types of sensors:

Relative Humidity (RH)

Vaisala produced the World’s first thin film capacitive humidity sensor in 1973. Described as the HUMICAP the technology of this sensor has been continuously improved over the years. The most important features of the sensor are the operating range (relative humidity and temperature), stability, accuracy, repeatability, hysteresis, chemical tolerance and speed of response.

The principle of operation of the sensor is based upon a change in capacitance as a polymer film absorbs and desorbs water vapor. The input stage of the transmitter electronics must also be capable of measuring this small non linear capacitance change with good accuracy and repeatability.

 

    Temperature (T)

Many Vaisala products also have a temperature sensor built in because it is often important to know the temperature of the environment at the point where the humidity is being measured. Vaisala instruments use platinum resistance Pt100 or Pt1000 sensors to measure temperature. This type of sensor is a precision resistor which increases in resistance with temperature. The platinum as a material does not change over time, so the change in resistance is very stable and repeatable.

 

VARIABLES IN ELECTRONIC RH MEASUREMENT:

Temperature differences

It is important that the humidity sensor is at the same temperature as the process or environment being measured. At 20C and 50 %RH a +/-1C difference between the sensor and zone of measurement will cause an error of +/-3 %RH. At 90 %RH the error is +/-6 %RH. At high humidities a temperature difference of a few degrees can cause water to condense on the sensor. In an unventilated space it may take hours to dry out. Vaisala humidity sensors will start to function normally as soon as the water has evaporated but if the water is contaminated the life of the sensor may be shortened and calibration may change.

Other influences

Care must be taken to avoid hot or cold spots that are not representative of the environment being measured. A sensor located near a door, room humidifier, heater or air conditioning inlet duct for example would be subject to rapid humidity or temperature changes and may appear to be unstable. Avoid also sample flows where the gas temperature can drop below the dewpoint temperature.

Chemical tolerance

Certain chemicals in the atmosphere surrounding the probe head could also attack or contaminate the sensor. Vaisala sensors have good chemical tolerance and we have a comprehensive list of the effect of various concentrations on sensor performance.If you have any queries about a particular chemical please contact us. It is important to know if the sensor head could be subjected to any periodic sterilization routines.

 

ELECTRONIC RH CALIBRATION

In case there is no high accuracy reference meter available, the Vaisala products with a ± 1%RH accuracy is recommended to send back to the Vaisala Lab for recalibration. With ± 2%RH and ± 3%RH accurate products also the salt solution calibration(see below) can be used.

Laboratory calibration

Relative humidities can be generated in specially adapted chambers using various saturation techniques and compared against transfer standard humidity or dewpoint probes. However the cost is significant and can only be justified where there is a need for regular humidity calibrations. Saturated salt calibration at two points is recommended for the majority of Vaisala probes. Using this method standard value relative humidity environments are generated in small containers containing selected aqueous saturated salt solutions. The Vaisala HMK11 uses this principle. A saturated solution of Lithium Chloride (LiCl) for example will generate a constant humidity of 11.3 %RH and Sodium Chloride (NaCl) will generate a humidity of 75.5 %RH (both at 20°C). See table below.

Under ideal conditions the accuracy is better than ± 1%RH. However due to possible temperature variations and other error sources the accuracy is reduced to ± 2.5 %RH. Other salt values are also available. For further information on saturated salts please refer to the American ASTM standard E104-85, the German standard DIN50008 and the work by L. Greenspan, published in the Journal of Research by the National Bureau of Standards (now NIST) Vol 81A.

Temperature stability and equilibrium

When making comparisons between a calibrated meter and a fixed probe for example it is important that the two probe heads are as close together as possible and they are at the same temperature. For example at 50 %RH and 20°C a 1°C difference between the 2 heads would mean a difference in RH of 3 %RH. At 90 %RH the corresponding error is 6 %RH. Rapid changes in temperature will also make a calibration check difficult. This consideration is particularly important when a meter is being used as a transfer standard to check a number of fixed installation probes in different ambient conditions. The temperature of the meter sensor head, hence the temperature readout, must be allowed to stabilise before a comparison reading is taken.

In applications where a probe is to be operated in extremes of temperature or over a wide temperature range the temperature dependence of the sensor and electronics must also be taken into account. For example a temperature dependence of ± 0.04 %RH/°C means that over an operating temperature range of 50°C the change in RH could be up to 2 %RH.

Comparisons between instruments and other humidity measurement techniques:

RH (relative humidity) measurement can be accomplished by several means. A sling psychrometer can be used to measure RH. This is done using a wet and a dry thermometer; the RH is proportional to the temperature of the wet bulb when compared to the temperature of the dry thermometer. This system is tried and true but not as accurate as digital and synthetic hair hygrometers. 

Synthetic Hair Hygrometers:

Synthetic hair hygrometers are exactly that—a moisture sensitive "hair" that contracts or expands depending on the amount of water in the air. This contraction and expansion can easily be measured on a dial marked with the % RH. Synthetic hair hygrometers are accurate, but only in a limited range. If your application involves small fluctuations in RH%, then these are economical and effective devices. We do not recommend these as portable units, because the reaction time is slow, and moving rapidly through changing environments will affect the calibration and accuracy of the units. 

Problems often occur when using different techniques to compare and calibrate humidity probes. For example whirling hygrometers often give a higher reading when compared to a fixed probe. Only the thermometers in a whirling hygrometer can be calibrated and errors are often caused in such instruments through poor maintenance, incorrect usage and temperature differences. Errors can easily be introduced when using conversion tables as the relationships between humidity variables are complex. In cases of dispute the calibration of the instruments involved and their traceability should be carefully checked. For example if two meters have a specified RH accuracy of ±2 %RH and ±3 %RH respectively the difference in reading could be up to 5 %RH and both meters would theoretically be in calibration.

Frequency of calibration

This depends on the relative humidity, temperature and other environmental factors. As a general guide a calibration period of 1 year is recommended for temperatures up to 35°C and humidities up to 70 %RH. Above these values the humidity should initially be checked at six monthly intervals.

Humidity calibration certificates

An individual certificate showing two or more comparison points will give an indication of the accuracy and linearity of a humidity product at the time of calibration. A NIST calibration certificate can be provided with an additional cost.