What is Humidity?
Humidity in simple terms is the amount of water vapour in the air.
Different measures of humidity:
Absolute humidity is the quantity of water vapour in a given volume of air, expressed by weight.
Specific humidity is a ratio of weight quantities of water vapour to dry air, such as 1:200, for example.
Relative humidity is how much water vapour can possibly be held by the air, in its current condition of temperature and pressure.
Relative Humidity in detail:
Relative humidity is the most commonly measured value used term for measurement and control
Relative humidity is the ratio of the current absolute humidity to the highest possible absolute humidity (which depends on the current air temperature). A reading of 100 percent relative humidity means that the air is totally saturated with water vapour and cannot hold any more.
Measuring Relative humidity
Hygrometers are instruments used for measuring relative humidity. A simple form of a hygrometer is specifically known as a psychrometer and consists of two thermometers, one of which includes a dry bulb and one of which includes a bulb that is kept wet to measure wet-bulb temperature. Modern electronic devices use temperature of condensation, changes in electrical resistance, and changes in electrical capacitance to measure humidity changes.
Dry Wet Sensor Measurement
Our range of controllers and indicators which are based on the classical dry/wet method of measuring humidity give very accurate readings.
Sensors being enclosed in stainless steel tubes ensures a long life and the unit can be used even in corrosive environments at very high levels of humidity.
Two sensors can be used to measure air temperature and relative humidity using the wet and dry bulb method. Improved design provides for faster and more accurate temperature sensing.
Temperature stability of the new design allows realistic temperature measurement accuracy’s of ± 0.2°C over the range.
The sensing element consists of a semiconductor integrated circuit which provides an output voltage proportional to the ambient temperature. The relationship between temperature and output voltage is extremely linear due to the high internal gain of the integrated circuit.
For air temperature and/or humidity readings using the wet/dry bulb method, we recommend that the wet and dry bulb sensors are located such that a slight breeze is available near the sensors.
Our sensor enclosure is supplied with a water tank assembly suitable for wet/dry humidity measurement.
Capacitive Sensor Measurement
In such systems, the sensor element is built out of a film capacitor on different
substrates (glass, ceramic, etc.). The dielectric is a polymer which absorbs or releases water proportional to the relative environmental humidity, and thus changes the capacitance of the capacitor, which is measured by an onboard electronic circuitry.
Such systems can be made very compact and do not require any system for wetting any part of the sensor. They are suitable for clean environments where humidity and temperature are expected to be moderate.
Calibrating Humidity Sensors
Salts as listed below have relatively stable Humidity Meter levels and are well documented for the temperatures as given in the table.
|
Salt/T |
5.0 |
10.0 |
15.0 |
20.0 |
25.0 |
|
Lithium chloride |
11.3 |
11.3 |
11.3 |
11.3 |
11.3 |
|
Magnesium chloride |
33.6 |
33.5 |
33.3 |
33.1 |
32.8 |
|
Potassium carbonate |
43.1 |
43.1 |
43.1 |
43.2 |
43.2 |
|
Sodium bromide |
63.5 |
62.2 |
60.7 |
59.1 |
57.6 |
|
Sodium chloride |
75.7 |
75.7 |
75.6 |
75.7 |
75.3 |
|
Potassium chloride |
87.7 |
86.8 |
85.9 |
85.1 |
84.3 |
|
Potassium sulphate |
98.5 |
98.2 |
97.9 |
97.6 |
97.3 |
Using saturated salts for calibrating instruments
The dry salt is spread about 3 mm deep in a shallow tray that occupies most of the bottom of an airtight box. Water is added to moisten the salt. Do not add more water than is needed to make the salt look damp. The instrument is then laid on a grid supported above the tray. Electronic sensors can be inserted through a hole in the box which is made reasonably airtight with a split rubber bung. Allow one hour for equilibration when the instrument is bulky. 20 minutes for small sensors.
Note that saturated salts that have equilibrium RH below ambient will continue to absorb water indefinitely and overflow. The 98% RH calibration can only be done in a room with constant temperature.
