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Updated : 16/10/2016

GHowSAW Humidity

Introduction

Humidity is the amount of water vapor in the air. Water vapor is the gas phase of water and is invisible. Humidity indicates the likelihood of precipitation, dew, or fog. Higher humidity reduces the effectiveness of sweating in cooling the body by reducing the rate of evaporation of moisture from the skin. This effect is calculated in a heat index table, used during summer weather.

There are three main measurements of humidity: absolute, relative and specific. Absolute humidity is the water content of air. Relative humidity, expressed as a percent, measures the current absolute humidity relative to the maximum for that temperature. Specific humidity is a ratio of the water vapor content of the mixture to the total air content on a mass basis.

Climate

While humidity itself is a climate variable, it also interacts strongly with other climate variables. The humidity is affected by winds and by rainfall. At the same time, humidity affects the energy budget and thereby influences temperatures in two major ways. First, water vapor in the atmosphere contains "latent" energy. During transpiration or evaporation, this latent heat is removed from surface liquid, cooling the earth's surface. This is the biggest non-radiative cooling effect at the surface. It compensates for roughly 70% of the average net radiative warming at the surface. Second, water vapor is the most important of all greenhouse gases. Water vapor, like a green lens that allows green light to pass through it but absorbs red light, is a "selective absorber". Along with other greenhouse gases, water vapor is transparent to most solar energy, as you can literally see. But it absorbs the infrared energy emitted (radiated) upward by the earth's surface, which is the reason that humid areas experience very little nocturnal cooling but dry desert regions cool considerably at night. This selective absorption causes the greenhouse effect. It raises the surface temperature substantially above its theoretical radiative equilibrium temperature with the sun, and water vapor is the cause of more of this warming than any other greenhouse gas.

Sensors

There are various devices used to measure and regulate humidity. A device used to measure humidity is called a psychrometer or hygrometer. A humidistat is a humidity-triggered switch, often used to control a dehumidifier.

Humidity is also measured on a global scale using remotely placed satellites. These satellites are able to detect the concentration of water in the troposphere at altitudes between 4 and 12 kilometers. Satellites that can measure water vapor have sensors that are sensitive to infrared radiation. Water vapor specifically absorbs and re-radiates radiation in this spectral band. Satellite water vapor imagery plays an important role in monitoring climate conditions (like the formation of thunderstorms) and in the development of future weather forecasts.

Electronic sensors such as the HIH-3610 Series delivers instrumentation-quality RH (Relative Humidity) sensing performance in a low cost, solderable SIP (Single In-line Package). Available in two lead spacing configurations, the RH sensor is a laser trimmed thermoset polymer capacitive sensing element with on-chip integrated signal conditioning. The sensing element's multilayer construction provides excellent resistance to application hazards such as wetting, dust, dirt, oils, and common environmental chemicals.
This sensor is eminently sutable for how weather sensing and is generally interfaced with a DS2438 slave device as shown below.


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Water vapour pressure

In a closed container partly filled with water there will be some water vapour in the space above the water. The concentration of water vapour depends only on the temperature. It is not dependent on the amount of water and is only very slightly influenced by the presence of air in the container.

he water vapour exerts a pressure on the walls of the container. The empirical equations given below give a good approximation to the saturation water vapour pressure at temperatures within the limits of the earth's climate.

Saturation vapour pressure, ps, in pascals:

ps = 610.78 *exp( t / ( t + 238.3 ) *17.2694 )

where t is the temperature in degrees Celsius

The svp below freezing can be corrected after using the equation above, thus:

ps ice = -4.86 + 0.855*ps + 0.000244*ps2

The next formula gives a direct result for the saturation vapour pressure over ice:

ps ice = exp( -6140.4 / ( 273 + t ) + 28.916 )

Water vapour concentration

The relationship between vapour pressure and concentration is defined for any gas by the equation:

p = nRT/V

p is the pressure in Pa, V is the volume in cubic metres, T is the temperature in degrees Kelvin (degrees Celsius + 273.15), n is the quantity of gas expressed in molar mass ( 0.018 kg in the case of water ), R is the gas constant: 8.31 Joules/mol/m3

To convert the water vapour pressure to concentration in kg/m3: ( Kg / 0.018 ) / V = p / RT

kg/m3 = 0.002166 *p / ( t + 273.16 )   

where p is the actual vapour pressure

Relative Humidity

The Relative Humidity (RH) is the ratio of the actual water vapour pressure to the saturation water vapour pressure at the prevailing temperature.

RH = p/ps

RH is usually expressed as a percentage rather than as a fraction. The RH is a ratio. It does not define the water content of the air unless the temperature is given. The reason RH is so much used in conservation is that most organic materials have an equilibrium water content that is mainly determined by the RH and is only slightly influenced by temperature.

Notice that air is not involved in the definition of RH. Airless space can have a RH. Air is the transporter of water vapour in the atmosphere and in air conditioning systems, so the phrase "RH of the air" is commonly used, and only occasionally misleading.

The Dew Point

The water vapour content of air is often quoted as dew point. This is the temperature to which the air must be cooled before dew condenses from it. At this temperature the actual water vapour content of the air is equal to the saturation water vapour pressure. The dew point is usually calculated from the RH. First one calculates ps, the saturation vapour pressure at the ambient temperature. The actual water vapour pressure, pa, is:

pa= ps * RH% / 100

The next step is to calculate the temperature at which pa would be the saturation vapour pressure. This means running backwards the equation given above for deriving saturation vapour pressure from temperature:

Let w = ln ( pa/ 610.78 )

Dew point = w *238.3 / ( 17.2694 - w )

This calculation is often used to judge the probability of condensation on windows and within walls and roofs of humidified buildings.

The dew point can also be measured directly by cooling a mirror until it fogs. The RH is then given by the ratio

RH = 100 * ps dewpoint/ps ambient

Concentration of water vapour in air

It is sometimes convenient to quote water vapour concentration as kg/kg of dry air. This is used in air conditioning calculations and is quoted on psychrometric charts. The following calculations for water vapour concentration in air apply at ground level.

Dry air has a molar mass of 0.029 kg. It is denser than water vapour, which has a molar mass of 0.018 kg. Therefore, humid air is lighter than dry air. If the total atmospheric pressure is P and the water vapour pressure is p, the partial pressure of the dry air component is P - p . The weight ratio of the two components, water vapour and dry air is:

kg water vapour / kg dry air = 0.018 *p / ( 0.029 *(P - p ) )
  = 0.62 *p / (P - p )

At room temperature P - p is nearly equal to P, which at ground level is close to 100,000 Pa, so, approximately:

kg water vapour / kg dry air = 0.62 *10-5 *p

Thermal properties of damp air

The heat content, usually called the enthalpy, of air rises with increasing water content. This hidden heat, called latent heat by air conditioning engineers, has to be supplied or removed in order to change the relative humidity of air, even at a constant temperature. This is relevant to conservators. The transfer of heat from an air stream to a wet surface, which releases water vapour to the air stream at the same time as it cools it, is the basis for psychrometry and many other microclimatic phenomena. Control of heat transfer can be used to control the drying and wetting of materials during conservation treatment.

The enthalpy of dry air is not known. Air at zero degrees celsius is defined to have zero enthalpy. The enthalpy, in kJ/kg, at any temperature, t, between 0 and 60C is approximately:

h = 1.007t - 0.026   below zero: h = 1.005t

The enthalpy of liquid water is also defined to be zero at zero degrees celsius. To turn liquid water to vapour at the same temperature requires a very considerable amount of heat energy: 2501 kJ/kg at 0C

At temperature t the heat content of water vapour is:

hw = 2501 + 1.84t

Notice that water vapour, once generated, also requires more heat than dry air to raise its temperature further: 1.84 kJ/kg.C against about 1 kJ/kg.C for dry air.

The enthalpy of moist air, in kJ/kg, is therefore:

h = (1.007*t - 0.026) + g*(2501 + 1.84*t)

g is the water content in kg/kg of dry air

The Psychrometer

The final formula in this collection is the psychrometric equation. The psychrometer is the nearest to an absolute method of measuring RH that the conservator ever needs. It is more reliable than electronic devices, because it depends on the calibration of thermometers or temperature sensors, which are much more reliable than electrical RH sensors.

The psychrometer, or wet and dry bulb thermometer, responds to the RH of the air in this way:

Unsaturated air evaporates water from the wet wick. The heat required to evaporate the water into the air stream is taken from the air stream, which cools in contact with the wet surface, thus cooling the thermometer beneath it. An equilibrium wet surface temperature is reached which is very roughly half way between ambient temperature and dew point temperature.

The air's potential to absorb water is proportional to the difference between the mole fraction, ma, of water vapour in the ambient air and the mole fraction, mw, of water vapour in the saturated air at the wet surface. It is this capacity to carry away water vapour which drives the temperature down to tw, the wet thermometer temperature, from the ambient temperature ta :

( mw - ma) = B( ta- tw)

B is a constant, whose numerical value can be derived theoretically by some rather complicated physics (see the reference below).

The water vapour concentration is expressed here as mole fraction in air, rather than as vapour pressure. Air is involved in the psychrometric equation, because it brings the heat required to evaporate water from the wet surface. The constant B is therefore dependent on total air pressure, P. However the mole fraction, m, is simply the ratio of vapour pressure p to total pressure P:  p/P. The air pressure is the same for both ambient air and air in contact with the wet surface, so the constant B can be modified to a new value, A, which incorporates the pressure, allowing the molar fractions to be replaced by the corresponding vapour pressures:

pw - pa= A* ( ta- tw)

The relative humidity (as already defined) is the ratio of pa, the actual water vapour pressure of the air, to ps, the saturation water vapour pressure at ambient temperature.

RH% = 100 *pa/ p= 100 *( pw - ( ta- tw) * 63) / ps

When the wet thermometer is frozen the constant changes to 56

The psychrometric constant is taken from: R.G.Wylie & T.Lalas, "Accurate psychrometer coefficients for wet and ice covered cylinders in laminar transverse air streams", in Moisture and Humidity 1985, published by the Instrument Society of America, pp 37 - 56. These values are slightly lower than those in general use.

To check your calculation, take air at 20C and 15.7C wet bulb temperature. The RH is 65%. The water vapour pressure is 1500 Pa. The water vapour concentration in kg/m3 is 0.011, in kg/kg it is 0.009. The dew point is 13C.

Installation

Picking the right location for your temperature sensor can be challenging because there are so many factors that influence the temperature. The measured value should represent the ambient air temperature in the shade. For example, if you mount it too close to your house, radiated heat can cause it to read too high at night. Mount it too well-shielded on the north side of your house and it may stay too cold.

When choosing a potential location, keep these things in mind:

  • Because of the close interelationship beteeen temperature and humidity is is strngly advised that the humidity sensor be installed as close as possible to the temperature sensor.

  • Don't mount your sensor directly in the sun. Use some sort of a shield to keep the sun from influencing your measured humidity. You can mount your humidity sensor in a tree, use an existing structure for shade, or build a Stevenson Screen (see below).

  • Mount your sensor at least 1 metre above the ground to minimize ground-heating effects. In general, higher is better.

  • Choose a location over a "cool" surface such as grass or bushes. Mounting your temp sensor over a "hot" surface such as an asphalt driveway will cause high readings

  • Avoid mounting near a structure that will radiate heat. A block wall that gets direct sun exposure during the day will radiate heat for many hours after the sun sets.

  • Temperatures above your roof could easily reach 20 degrees above the ambient temperature.

  • The module needs to be protected from direct water contact and contamination caused by insects, dirt, birds, and debris.

  • Consider weatherproofing the temperature module's circuit board.

  • If you're mounting a temperature sensor indoors, keep it away from heater and air conditioning vents as well as heat producing appliances such as televisions.

Once you have identified a potential location, try testing your module in that spot. Build a temporary data cable and install the temperature sensor. Capture data for a day or two, then plot it using your favourite tool such as Excel. Review the data, does it look right and is it reasonably close to what the National Weather or the local airport Service recorded for your area?

Screens

If the location selected for your temperature sensor is in direct sunlight for any part of the day then you will require a Stevenson Screen of one sort or another. Without any screening a temperature sensor subject to direct sunlight may record temperatures up to 10°C above the true ambient temperature. For further information regarding why a radiation screen is required and several sources on how to make one please consult Citizen Weather Observer Program

My choice was to mount the temperature and humidity sensors in a sensor housing 2 metres above ground under the eaves of a south facing wall of my house. The housing is mounted away from the wall to minimise the effect of the thermal mass of the wall.

       

Instructions for the construction of this housing can be found here. The author of the document is Bill Ellis but unfortunately I no longer have the original URL.

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