Humidity, dew point and associated variables
Variables used to describe humidity include
Partial vapor pressure
Wet bulb temperature
Parts per million
Equilibrium relative humidity
The expression moisture content mostly applies to solids and liquids.
It is not a quantitative term when applied to a gas.
Water vapor pressure (Pw)
The pressure exerted by the water vapor present in air or a gas.
Saturation vapor pressure of water (Pws)
The maximum pressure the water vapor can exert at a particular temperature. The higher the temperature, the more water vapour the air can hold.
HUMICAP (sensor) for relative humidity measurement
Relative humidity (RH)
This is the ratio of the partial water vapor pressure (Pw) to the water
vapor saturation pressure (Pws) at a particular temperature.
It is measured as a percentage: %RH = lOO%*(Pw/Pws)
Relative humidity is strongly temperature dependent. Pressure will also change the RH. For example, if a process is kept at a constant temperature, the RH will increase by a factor of two if the pressure is doubled.
Dew point temperature (Td)
Is the temperature at which air becomes saturated with water and begins
to condense, forming dew. At 100 %RH the ambient temperature equals
the dewpoint temperature. The more negative the dewpoint is from the
ambient temperature, the less the risk of condensation and the drier the
air. Dewpoint is not temperature dependent, hut is affected by pressure.
Frost point temperature (Tf)
If the dewpoint temperature is below freezing the term frostpoint temperature
is sometimes used. Tf is always slightly higher than Td below 0 °C as the
water vapor saturation pressure of ice is different to water.
Absolute humidity (a)
Refers to the mass of water in a unit volume of moist air at a given
temperature and pressure. Usually expressed as grams per cubic meter
of air. It can be confused with mixing ratio.
Mixing ratio (x)
Is the ratio of the mass of water vapor to the mass of dry gas. It is
dimensionless but often expressed in grams per kilogram of dry air.
It is mainly used in drying processes or ventilation systems for calculating
water content when the mass flow of air is known.
Wet bulb temperature )Tw)
Traditionally, this was the temperature indicated by a thermometer wrapped
in a wet sheath. The wet bulb temperature and the ambient temperature
can then he used to calculate relative humidity or dewpoint. Tables and
charts can also be used.
Parts per million (ppm)
Is the ratio of water vapor to the dry gas or total (moist) gas. It is sometimes
used for low levels of moisture and expressed by mass/volume or mass/weight.
Equilibrium relative humidity (ERH)
This is the value of the relative humidity of the air surrounding a substance
when there is no net exchange of moisture.
Water activity (aw)
Is similar to equilibrium relative humidity and uses a scale of 0 to 1, instead
of 0% to 100%.
Is the amount of energy required to bring a gas to its current state from a
dry gas at 0 °C. Mostly used in air conditioning calculations.
Vaisala has a choice of products for measuring relative humidity, temperature
and dewpoint. Some products also have built-in calculation options to give
outputs in terms of other humidity variables.
The terms probe, transmitter and sensor are all used to describe a product
that measures humidity. The probe is the part of the product that contains
the humidity sensor and may he rigidly fixed to the transmitter housing or
connected by a flexible cable. The transmitter provides the output signal.
The critical part is the sensor. Vaisala designs and iiianufactures various
families of sensors.
HUMICAP - for relative humidity
DRYCAP - for low dewpoint
DEWCAP - for high accuracy dewpoint
DEWCAP (sensor) for high accuracy measurement.
What to look for in a sensor
The majority of Vaisala RH sensors work over the full range from 0 to
100 %RH. The choice of product depends on the application and operating
temperature especially at the extremes of humidity (>90 %RH and less than
In general, this term is much abused and many unrealistic claims are
made on data sheets. Vaisala products are compared and adjusted during
manufacture against factory standards that are traceable to national
standards. This calibration chain is detailed in the certificates now supplied
with the majority of our products when shipped from the factory. Other
important considerations associated with accuracy include:
* Linearity over the working range
* Hysteresis and repeatability
* Stability over a period of time
* Temperature dependence of the sensor
Our aim is to provide "accuracy that lasts". This reflects our commitment
to provide accurate and reliable sensors with excellent long-term stability.
Is the speed of response of the sensor when subjected to a step change
in humidity. Other factors include temperature, airflow, and type of filter
fitted. A blocked filter, for example, will give a slower response.
Certain aggressive chemicals in the air can attack or contaminate the probe
head or sensor. Vaisala sensors have good chemical tolerance and we have
extensive application knowledge of the effect of various chemical concentrations.
Examples of concentrations that may affect the sensor are:
* Organic solvents - 1,000 to 10,000 ppm
* Corrosive agents (strong acids) - 1 to 10 ppm
* Weak acids - 100 to 1,000 ppm
* Bases - 10,000 to 100,000 ppm
Some products have a chemical purge (re-gain) feature. This heats the
ensor and purges it of chemicals that may have gathered in the sensor
over a period of time.
Choice of product
No single product can cover the full range of applications. Therefore the
following must he taken into account:-
Choose a measuring point that gives a true picture of the environment.
Above 60 °C the transmitter electronics should be mounted outside the
process and a suitable high temperature probe head inserted into
Beware of temperature differences
When mounting the probe head into a process, avoid temperature drops
along the body of the probe. If there is a large temperature difference
between the probe head and external environment, the whole probe
should be mounted in the process and the cable entry point insulated.
If there is a risk of condensation, the probe should be mounted
horizontally to avoid water dripping down the probe/cable and saturating
the filter. See figure 1
Measurement error at 100%RH when the difference between
the ambient and sensor temperature is 1 Deg.C.
Air should he allowed to circulate around the probe head to ensure the sensor and process is at the same temperature. At 20 °C and 50 %RH a 1 °C difference between the sensor and zone of measurement will cause an error of 3 %RH. At 90 %RH the error is 6 %RH. See figure 2.
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