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Updated : 16/10/2016

GHowSAW Weather Temperature Sensors

Introduction

There are numerous temperature sensors available from IC's to RTD's and thermocouples all of which can be accessed by a one-wire slave. But since we are concerned here only with atmospheric temperatures within the range -40°C to +50°C we can more easily use the DS18S20 one-wire temperature slave or alternatively we can access the temperature function built in to one-wire slaves such as the DS2438 battery monitor. In my experience, it is preferable to use the dedicated DS18S20 temperature sensor over the inbuilt temperature sensor of other devices, because the latter are less accurate and generally report a temperature 2-3°C higher than a DS18S20 mounted close by.

Please see the page on One-Wire Devices for more information on using other temperature sensors such as thermocouples.

Other Temperatures

Whilst ambient temperature is the most commonly presented weather temperature there ar several other forms of temperatures that are commonly quoted to describe the current weather:

Wind chill

Popularly referred to as wind chill factor, Wind Chill is the perceived decrease in air temperature felt by the body on exposed skin due to the flow of cold air and is more commonly used in regions where there is a risk of frost-bite exposure

Wind chill temperatures are always lower than the air temperature for values where the formula is valid.


Where Twc is the wind chill index, based on the Celsius temperature scale, Ta is the air temperature in degrees Celsius (°C), and V is the wind speed at 10 metres standard anemometer height, in kilometres per hour (km/h).

When the apparent temperature is higher than the air temperature, the heat index is used instead.

Heat Index

The heat index (HI) or humiture or humidex is an index that combines air temperature and relative humidity in an attempt to determine the human-perceived equivalent temperature — how hot it feels. The result is also known as the "felt air temperature" or "apparent temperature". For example, when the temperature is 32 °C with very high humidity, the heat index can be about 41 °C.

Where :
C1 = -41.3216667, C2 =3.26324648, C3 =5.63518404, C4 =-0.357943801, C5 =-3.12176486 10-2, C6 =-3.04539833 10-2, C7 =5.60972904 10-3, C8 =1.35819481 10-3, C9 =-9.08520988 10-6

The above formula for approximate HI is only valid for temperatures above 27°C with a relative humidity of 40% or higher.

Physical Effects of Heat Index
27 to 32 °C  caution
Possible fatigue.
Physical activity could lead to heat cramps.
32 to 40 °C extreme caution
Possible heat cramps and exhaustion.
Physical activity could lead to heat stroke.
40 to 54 °C danger
Likely heat cramps and exhaustion.
Probable heat stroke.
above 54 °C extreme danger
Imminent heat stroke.

Dew Point

The dew point is the temperature below which the water vapour in a volume of humid air at a given constant barometric pressure will condense into liquid water at the same rate at which it evaporates. Condensed water is called dew when it forms on a solid surface.

The dew point is a water-to-air saturation temperature and is associated with the relative humidity. A high relative humidity indicates that the dew point is closer to the current air temperature so that at a relative humidity of 100% the dew point is equal to the current temperature and that the air is fully saturated with water. When the dew point remains constant and temperature increases, relative humidity decreases

The, saturated vapour pressure Ps in Pascals can be calculated from the temperature T (°C) using the formula below:

Reference: Tetens, O., 1930: Uber einige meteorologische Begriffe. Zeitschrift fur Geophysik, Vol. 6:297. There are more accurate formulæ given in standard reference works, such as the Smithsonian Tables, but this version is adequate for all but high accuracy laboratory studies.

The next step is to calculate the temperature at which Pa would be the saturation vapour pressure. Using the relationship beteen Pa, Ps and Relative Humidity (RH) we can equate the Pa pressure to the vapour pressure at the dew point (Td)

Rearranging we abtain the following relationship for dew point

An alternative formula for calculating dew point: Set x = (1 - 0.01 x RH) where RH is the relative humidity expressed as a percent (a number between 1 and 100). eg If the relative humidity is 38 percent, x = 0.62.

Then calculate:
DPD = (14.55 + 0.114T)x + ((2.5 + 0.007T)x)^3 + (15.9 + 0.117T)x^14
where T is the temperature in degrees Celsius.

This calculation yields the difference between the temperature and dew point in degrees Celsius.

Finally, compute the dew point:-
TD = T - DPD.
The answer is in degrees Celsius.

The psychrometer, or wet and dry bulb thermometer

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 only limitation to the psychrometer is that it is difficult to use in confined spaces (not because it needs to be whirled around but because it releases water vapour).

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 calculate a wet-bub temperature the best way is to calculate the dew point then use a Skew-T diagram.A blank Skew-T diagram can be found here at this link:

Skew-T.pdf

For finding the wet-bulb temperature, first find the elevation of your location. Next, at the elevation of your location, plot the air temperature (in degrees Celsius) and the dewpoint temperature on the chart. Take the air temperature up the dry adiabat line and the dewpoint temperature up the theta line until they meet. At the point where they meet, come back down the moist (or wet) adiabat to the elevation of your station. This will be the wet-bulb temperature.

For information on how to read and understand a Skew-T diagram, see the following link :
http://www.theweatherprediction.com/thermo/

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:

  • Don't mount your sensor directly in the sun. Use some sort of a shield to keep the sun from influencing your measured temperature. You can mount your temperature 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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