Dew Point Meter

Basic Principle of Dew Point Meter

By Cooling at constant pressure if the temperature of air is reduced, the water-vapour in the air will start to condense at a particular temperature. This temperature is called dew point temperature.

Description of Dew Point Meter

The main Parts of arrangement are

• A shiny surface (mirror) is fixed with a thermocouple.
• A nozzle is providing a jet of air on the mirror.
• A light source focused constantly on the mirror.
• A photo cell to detect the amount of light reflected from the mirror.

Operation of Dew Point Meter

• The mirror is constantly cooled by a cooling medium. The cooling medium is maintained at a constant temperature.
• To this mirror is attached a thermocouple whose leads are connected to a millivoltmeter.
• Constantly a light is made to fall in an angle on the mirror and the amount of reflected light is sensed by a photo cell.
• Now an air jet is made to fall in an angle on the mirror and the water-vapor (moisture) contained in the air starts condensing on the mirror and they appear as small drops (dew) on the mirror.
• This moisture (dews) formed on the mirror reduces the amount of light reflected from the mirror and it is detected by photocell. When for the first time, there is a change in amount of transmitted light; it becomes an indication of dew formation.
• At this instance (that is, when the due formation is detected first), the temperature indicated by the thermocouple attached to the mirror becomes the dew point temperature.
• Thus this arrangement is used to determine the time at which the dew appears for the first time and dew point temperature.

Application of dew point meter

• This instrument is used on ships to protect cargoes from condensation damage by maintaining the dew point of air in holds lower than the cargo temperature.
• Used in industries for determining dew point.

Limitations

There are limitations in cooling fluids and light measurement.

Electrical Humidity Sensing Absorption Hydrometer

Basic Principle:

A change in resistance with change in humidity is taken as a measure of humidity.

This is type of Electrical Humidity sensing Absorption Hydrometer, know more about mechanical humidity sensing methods using Hair Haydrometer

Description:

The main Parts of this arrangements are, two metal electrodes which are coated and separated by a humidity sensing hygroscopic salt (lithium chloride). The leads of the electrodes are connected to a null – balance wheat stone bridge.

Operation:

When the humidity of the atmosphere is to be measured, the electrodes coated with lithium chloride are exposed to atmosphere. Humidity variation causes the resistance of the chemical (lithium chloride) to change. That is, the chemical absorbs moisture or loss moisture and causes a change in resistance.

Higher the humidity (RH) in the atmosphere more will be the humidity absorbed by lithium chloride and lower will be the resistance.

Lesser the humidity (RH) in the atmosphere less will be the humidity absorbed by lithium chloride and higher will be the resistance.

The change in resistance using a Wheatstone bridge and this change in resistance becomes a measure of humidity (RH) present in the atmosphere.

Applications

These hydrometers are used under constant temperature conditions to measure humidity of atmosphere.
The accuracy of this instrument is in +/- 25%.
The response is of the order of few seconds.

Limitations

These instruments should not be exposed to 100% humidity as the resulting absorption by the chemical (lithium chloride) might damage the instrument.
If these devices are not used under constant temperature conditions, temperature correction must be made.

Hair Hydrometer

Basic Principle

Due to humidity, several materials experience a change in physical, chemical and electrical properties. This property is used in transducer that are designed and calibrated to read relative humidity directly.

Hair hydrometer is a type of absorption hydrometer and uses the mechanical humidity sensing technique.

Certain hygroscopic materials such as human hair, animal membranes, wood, paper, etc., undergo changes in linear dimensions when they absorb moisture from their surrounding air. This change in linear dimension is used as the measurement of humidity present in air.

Description of Hair Hydrometer

The main Parts of hair hydrometer are,

Human hair is used as the humidity sensor. The hair is arranged in parallel beam and they are separated from one another to expose them to the surrounding air/atmosphere. Number of hairs are placed in parallel to increase mechanical strength.

This hair arrangement is placed under small tension by the use of a tension spring to ensure proper functioning.

The hair arrangement is connected to an arm and a link arrangement and the link is attached to a pointer pivoted at one end. The pointer sweeps over a humidity calibrated scale.

Operation of Hair Hydrometer

When the humidity of air is to be measured, this air is made to surround the hair arrangement and the hair arrangement absorbs the humidity from the surrounding air and expands or contracts in the linear direction.

This expansion or contraction of the hair arrangement moves the arm & link and thus the pointer to a suitable position on the calibrated scale and thus indicating the humidity present in the air/atmosphere.

Note: These Hair hydrometers are called membrane hydrometers when the sensing element is a membrane.

Applications of Hair Hydrometer

These hydrometers are used in the temperature range of 0’C to 75’C.
These hydrometers are used in the RH (Relative Humidity) range of 30 to 95%.

Limitations of Hair Hydrometer

These Hydrometers are slow in Response
If the Hair hydrometer is used constantly, its calibration tends to change.

Sling Psychrometer

Sling Psychrometer is used to measure both the dry bulb and wet bulb temperatures at time. These temperatures are a measure of humidity content in air.

Description of Sling Psychrometer

The main parts of the instrument are

The instrument frame which holds the thermometers.

One mercury in glass thermometer whose sensing bulb is bare to directly contact the air and to measure the temperature which is called as the dry-bulb temperature.

One mercury in glass thermometer whose sensing bulb is covered with a cotton or muslin wick made wet with pure water. This sensing bulb covered with the cotton wick moistened is made to contact the air and the temperature indicated by this thermometer is called as the wet bulb-thermometer.

The instrument frame carrying the thermometer is covered by a glass casing.

A swivel handle is attached to frame-glass casing – thermometer arrangement to ensure that the air at the wet bulb always in immediate contact with the wet wick.

When a thermometer bulb is directly exposed to an air-water vapour mixture, the temperature indicated by the thermometer is the dry-bulb temperature.

When a thermometer bulb is covered by a constantly wet wick and if the bulb covered by the wet wick is exposed to air water vapour mixture, the temperature indicated by the thermometer is the wet bulb temperature.

Operation of Sling Psychrometer.

In order to measure the dry bulb and wet bulb temperature, the Psychrometer frame – glass covering – thermometer arrangement is rotated at 5 m/s to 10 m/s to get the necessary air motion.

Note: An important condition is that correct/accurate measurement of wet bulb temperature is obtained only if air moves with velocity around the wet wick. In order to get this air velocity, the Psychrometer is being rotated.

The thermometer whose bulb is bare contacts the air indicates the dry bulb temperature.

At the same time, the thermometer whose bulb is covered with the wet wick comes in contact with the air and when this pass on the wet wick present on the bulb of the thermometer, the moisture present in the wick starts evaporating and a cooling effect is produced at bulb. Now the temperature indicated by the thermometer is the wet bulb thermometer which will naturally be lesser than the dry bulb temperature.

Note: If the Psychrometer is rotated for a short period, then the wet bulb temperature recorded will not be proper.
Note: If the Psychrometer is rotated for a longer period, the wick will get dried soon and the wet bulb temperature will not be at its minimum value.

Application of Sling Psychrometer

1. It is used for checking humidity level in air-conditioned rooms and installations.
2. It is used to set and check hair hygrometer.
3. It is used in the measurement range of 0 to 100% RH.
4. It is used for measuring wet bulb temperature between 0’C to 180’C.

Limitation of Sling Psychrometer

1. The measured medium is disturbed due to the act of measurement. The evaporation process at the wet bulb will add moisture to the air.
2. It cannot be used in automation requirement situations.
3. It cannot be used for continuous recording purpose.
4. If the wick is covered with dirt, the wick will become stiff and its water absorbing capacity will reduce, however, a stiff/dirty wick will resume normalcy when boiled in hot water.

Humidity / Dampness Measurement

Introduction:

The amount of water vapour contained in air or gas is called as humidity. This humidity affects human comforts and many industrial process as in case of chemical industries, garments industries, paper industries, food industries, leather industries, pharmaceutical industries, precision equipment manufacturing, etc. hence, study of humidity is important. Let us define some common terms related to humidity measurement.

Humidity

The amount of water vapour contained in air or gas is called as humidity. It is usually measured as absolute humidity, relative humidity or due point temperature.

Dry air

When there is no water vapour contained in the atmosphere, it is called dry air.

Moist air

When there is water vapour present in the atmosphere, it is called moist air.

Saturated air

Saturated air is the moist air where the partial pressure of water-vapour equals the saturation pressure of steam corresponding to the temperature of air.

Absolute humidity

It is the mass of water vapour present per unit volume. In other words, it is the quantity of the water vapour present in air and its unit is grams per cubic meter of air.

Relative humidity RH

Relative humidity = (water vapour pressure actually present)/(water vapour pressure required of saturation) at a given.

Here a comparison is made between the humidity of air and humidity of saturated air at the same temperature and pressure.
It should be noted that if relative humidity is 100%, it is saturated air. That is, the air contains all the moisture it can hold.
It should also be noted that the degree of saturation (percentage of relative humidity) of air keeps on changing with temperature.

Humidity ratio or specific humidity

For a given volume of air water vapour mixture,

Humidity ratio = (mass of water – vapour)/mass of dry air

Dew point temperature

By continuous cooling at constant pressure if the temperature of air is reduced, the water-vapour in the air will start to condense at a particular temperature. The temperature at which the water vapour stats condensing is called as dew point temperature.

Dry-bulb temperature

When a thermometer bulb is directly exposed to an air-water vapour mixture, the temperature indicated by the thermometer is the dry-bulb temperature.
This dry-bulb temperature is not affected by the moisture present in the air, that is, the temperature of air is measured in a normal way by the thermometer.
The dry bulb is used to distinguish the normal temperature measured from the temperature measured by the wet-bulb.

Wet-bulb temperature

When a thermometer bulb is covered by a constantly wet wick, and if the bulb covered by the wet wick is exposed to air-water vapour mixture, the temperature indicated by the thermometer is wet-bulb temperature.

When air is passed on the wet wick present on the bulb of the thermometer, the moisture present in the wick strats evaporating and this creates a cooling effect at the bulb. The bulb now measures the thermo dynamic equilibrium temperature reached between the cooling effected by the evaporation of water and heating by convection.

Wet-bulb depression

Wet-bulb depression = (dry bulb temperature) – (wet bulb temperature)

Always dry-bulb temperature is higher than the wet bulb temperature.

Percentage humidity

Percentage humidity = weight of water vapour in a unit weight of air/ weight of water vapour in same weight of air if the air were completely saturated at the same temperature.