Thermometry

The art or science of temperature observation

History

• No one person credited with the invention of the thermometer; developed over time

• Avicenna used this principal to show that hotness and coldness of air caused water in a closed tube partially filled with air to expand and contract; 11th century– Likely conducted the first air temperature

measurement

History

• Galileo – invented the thermoscope (galileo thermometer); reflected changes in sensible heat

• Galileo�s thermoscope is comprised of glass sphere�s filled with aqueous alcohol of slightly different densities

• Guiseppe Biancani – Developed first clear diagram of a thermoscope - 1617

History

• Robert Fludd (1638) – First thermoscope containing a scale (thermometer) – vertical tube, with a bulb at the top and the end

immersed in water– water level in the tube is controlled by the

expansion and contraction of the air– One major drawback

History

• Ferdinando II de' Medici (Grand Duke of Tuscany) – Invented first modern-style thermometer with a sealed tube partly filled with alcohol, with a bulb and stem

• Each inventor and each thermometer was unique—there was no standard scale

History

• Christiaan Huygens (1665) - suggested using the melting and boiling points of water as standards

• Carlo Renaldini (1694) - proposed using them as fixed points on a universal scale

• Isaac Newton (1701) - proposed a scale of 12 degrees between the melting point of ice and body temperature

History

• Daniel Gabriel Fahrenheit (1724) – Invented Fahrenheit scale based on three fixed points of temperature. Used Hg for first time.– First: frigorific mixture of ice, water, and

ammonium chloride, a salt, and waiting for it to reach equilibrium. Reading was 0F

– Second: reading taken in still water as ice is just forming on the surface. 32F

– Third: reading when placed under the arm or in the mouth. 96F

History

• Anders Celsius (1742) - proposed a scale with zero at the boiling point and 100 degrees at the melting point of water

• Jean Pierre Cristin (1743) – proposed to flip the Celsius scale and named it the Centigrade scale

Thermometry

• Air temperature is one of the most fundamental of all meteorological measurements, also most influenced by exposure

• Accurate measurements can be made, but can be hard due to cost, reliability and power requirements

• Errors in measurement are very common across networks

Temperature

• What is meant by the term “temperature”? What are you actually measuring?

Common Misnomer

• Temperature is NOT hot or cold, it is high or low

• Hot and cold applicable to measurements against some set-point

Effects of Errors

• Errors in excess of 2 to 3 degrees Celsius not uncommon

• Numerical models greatly impacted by a difference of 1 degrees Celsius

• Climate models greatly impacted by a difference of 0.2 degrees Celsius

Measurement Quality

• Accuracy not limited by technology• Limited by ability to use it while avoiding

exposure error

Temperature Scales

• Preferred scales are Kelvin and Celsius– Can be used almost interchangeably– Fahrenheit mainly used just in the U.S.–What are the 4th and 5th scales?

Defunct Scale

• Joseph-Nicolas Delisle (1732) – used Mercury and developed the Delisle scale (Degrees De or D)– Boiling point was 0, freezing point was 150– Ranged up to 2400 degrees– Popular in Russia in the 19th century

Triple point

• Temperature and pressure where all three phases can coexist–Water (6.106 hPa)

• 273.16 Kelvin• 0.01 Celsius• 32.018 Fahrenheit

• Why is this important in temperaturemeasurement?

Requirements for Proper Measurements

• What precautions must be taken when measuring temperature during the day?

• What precautions must be taken when measuring temperature during the night?

• We know air is a very effective insulator. What does this mean for measuring temperature?

Requirements for Proper Measurements

• What effect can precipitation have on temperature measurement?

• What type of shielding is ideal for temperature measurement?

• What level of sensitivity should be expected from a thermometer used to measure air temperature?

Requirements for Proper Measurements

• Sensor should be robust, stable in calibration, easy to use. How long should it’s operational life be (on average?)

• What effects can air pollution have on the sensor? What types of error(s) can this lead to?

• What is the standard height above ground for temperature measurements?

Minimizing Exposure Error

• Sensor needs to be completely shielded by shield

• Radiation errors can reach extremes when there is maximum solar radiation, light winds and a highly reflective ground surface (ie snow)

Site and Exposure Requirements

• WMO Guidance: “The best site for [temperature] measurements is over level ground, freely exposed to sunshine and wind and not shielded by, or close to, trees buildings and other obstructions.”

• Why avoid trees? How far away from obstructions should the thermometer be?

Bad Siting Areas

• Very sheltered positions. Why?

• Locations which may result in significant additional reflected radiation.

• Rooftop, chimneys, eaves. Why?

• Sites with significant topographic shelter. Why?

Bad Siting Areas

• Masts or towers where the screen is significantly sheltered by the mast structure or where height is above 2 meters

• Areas of tarmac or concrete

Minimizing Exposure Error, Thermometer Shielding

• Aspirated Radiation Shield– Protects sensor from the elements– Fan forces air over the sensor

• Passive shield– No fan– Impedes natural airflow as little as possible– Only blocks 80% of incoming solar radiation

Thermometer Shielding

Stevenson Screen

• First described by Thomas Stevenson in 1866• Louvered Screen, passive• Still widely used today• Typically house a max/minThermometer• What is the typical cost?

AWS Screen

• What does AWS stand for?

• Come in various sizes and shapes• Does size/shape of this type of

screen matter? Why or why not?

• How do you pick the best screen?

Aspirated Shield• Provide constant flow of air across

the sensor• Why is this advantageous to non-

aspirated shields?

• Why not switch all observations over to aspirated shields?

• What are the disadvantages to using this shield?

Temperature Sensors

• Categorized according to the physical principal they use– Thermal expansion*– Thermoelectric*– Electrical resistance*– Electrical capacitance

Thermal Expansion

• Temperature is determined through the use of bimetallic strips and liquid-in-glass thermometers

Linear Expansion

• One-dimensional length change with temperature

• ΔL =

Volume Expansion

• Change in volume with temperature• ΔV =

Bimetallic Strips

• Pair of metals with different thermal expansion coefficients

• Strip maintains it�s shape at temperature bonding took place

• Temperature change causes bending of the metal

• Typically found in thermostats

Bimetallic Strip Deflection

• y =

Liquid-in-Glass Thermometer

• Glass tube with a bulb at one end filled with liquid and attached to, or etched with, a scale

• Liquid is typically either mercury or alcohol• Sheathed vs unsheathed– Sheathed

• Encased in outer glass sheath, with scale engraved– Unsheathed

• Graduations marked on thermometer stem or attached scale

– Advantages/disadvantages to these?

Liquid-in-Glass Thermometer

• Three classifications for these thermometers determined by immersion requirements– Partial– Total– Complete

Partial Immersion

• Should be placed in a bath liquid (water or oil) until the bulb and a small portion of stem (typically indicated by an immersion line) are immersed in the liquid to be measured

• Can be used for calibrating other sensors

Total Immersion

• Bulb and portion of the stem containing the thermometric fluid are immersed

• Can also be used for calibrating other sensors

Complete Immersion

• Bulb and stem are completely immersed in the liquid

• Used for air measurement

Special Types of Liquid-in-Glass Thermometers

• Minimum thermometer– Uses alcohol with a dumbbell in the stem– Thermometer is mounted horizontally– Alcohol flows around the dumbbell as

temperature increases– As temperature decreases, the alcohol miniscus

drags the dumbbell down to the minimum measurement

Special Types of Liquid-in-Glass Thermometers

• Maximum thermometer– Uses mercury and has a constriction in the stem– Bulb is mounted slightly higher than the rest of

the column– Temperature increase causes volume increase and

mercury is forced through constriction– Temperature decrease causes break at

constriction point

Expansion

• As temperature increases, the glass and thermometric fluid expand according to:ΔVd =

Static Sensitivity of Liquid-in-Glass Thermometer

• S = _________ assuming r is constant• What are the units of static sensitivity?• What does it mean?

Thermocouples

• Thermocouple is an electrical apparatus that can generate a current proportional to the amount of heat it is exposed to

• Thomas Johann Seebeck ( 1821) - discovered that when any conductor (such as a metal) is subjected to a thermal gradient, it will generate a voltage (Seebeck effect)

Thermocouples

• Two dissimilar metals are joined at each end (at points known as junctions)

• If each end experiences a different temperature (i.e. temperature gradient), a voltage will develop

Thermocouples

• the magnitude of the effect depends on the metal in use

• Using a dissimilar metal to complete the circuit creates a circuit in which the two legs generate different voltages, leaving a small difference in voltage available for measurement

Thermocouples

• Thermocouples measure the temperature difference between two points, not absolute temperature

• In traditional applications, one of the junctions (cold junction) is maintained at a known (reference) temperature

Thermocouples

• Having available known cold junctions is impractical outside laboratory environments

• Artificial cold junctions (thermistor or diode) have been have been implemented as an artificial cold junction

• Leads must be composed of same wires• How can you increase their sensitivity?

Common Thermocouple Metals

• Type T – Copper Constantan• Type J – Iron and Constantan• Type E – Nickel (10% Chromium) and

Constantan• Type K – Nickel (10% Chromium) and Nickel

(5% aluminum and silicon)

Transfer Equation

• Mathematical equation in terms of spatial or temporal frequency, of the relation between the input and output of a (linear time-invariant) system

Thermocouple Transfer Equation

• ΔV =

Electrical Resistance Sensors

• Two types– Resistance Temperature Detectors (RTDs)– Thermistors

RTDs

• Many materials exhibit variations in their properties with changing temperature

• Metals or semiconductors that show variation in electrical resistance can be used as temperature sensors

RTDs

• Platinum is the most commonly used metal– Stable– Resists Corrosion– Easily Workable– High Melting Point– Can be Highly Purified– Simple and Stable Resistance-Temperature

Relationship– Drawback – Sensitive to strain; bending can

change resistance

Platinum Sensor Resistance

• RT =

Thermistors

• An electrical resistor whose resistance is greatly reduced by heating

• Typically composed of metallic oxides• Popular because their high resistance makes

them less sensitive to lead wire resistance and are available in many configurations

Temperature Sensor Exposure

• If the temperature sensor is exposed to the air, the instrument will indicate a temperature

• Temperature sensor may differ from the actual air temperature

• Heat flows toward or away from a temperature sensor by conduction, convection, and radiation

Radiative properties

• Sensor exists in radiative environment• Sensor radiates heat to cooler surfaces or has

heat radiated to the sensor from hotter surfaces

Accurate Temperature Measurement

• Sensor must be in good thermal contact with the air (air is a poor conductor)

• Usually requires air circulation to promote heat transfer by convection

• Sensor must be protected from heat flow along the sensor support and radiative heat transfer

• Central problem for immersion sensing instruments in meteorology is coupling

Heat Transfer Equations

• Conduction (HC) =

• Radiation (HR) =

• Convection (HV) =

• What are the units of all three?

Heat Transfer Variables

• kM =• AM =• TS, TM, TA =• ΔX =• D, L =

Heat Transfer Variables

• αS =• R =• c =• VA =