Met 163: Lecture 4 Chapter 4 Thermometry This week TUE: thermometry lecture THU: how to obtain data from RAWS, RAWS data analysis using MATLAB software,

Met 163: Lecture 4Chapter 4

Thermometry

This weekTUE: thermometry lecture

THU: how to obtain data from RAWS, RAWS data analysis using MATLAB software, RAWS break-down.

Outline

• Introduction

• Thermal expansion - bimetallic strip, Liquid-in-glass thermometer

• Thermoelectric Sensors - thermocouple

• Electrical Resistance Sensors - thermistors, Resistance Temperature Detectors (RTD)

• Exposure of Temperature Sensors

Air Temperature

The measurement of air temperature dates back to the time of Galileo(1564-1642).

It is perhaps the premier measurement of meteorological parameters.

Measurements of temperature are not free of errors or problems due to response time, accuracy of instrument, and operational need.

Errors in the measurement of air temperature in excess of 2-3 °C are not uncommon.

Galileo thermometer

- made of a sealed glass cylinder containing a clear liquid and several glass vessels of varying densities.

- As the temperature of most liquids increases, their density decreases.

Air Temperature

Errors in the measurement of air temperature in excess of 2-3 °C are not uncommon.

These errors are acceptable for the general public.

However, numerical models of all scales of motion are greatly affected by errors as large as 1 °C.

Errors of just 1°C in mesoscale models have been shown to be the deciding factor between no storms initiated and intense storms.

Additionally, errors as small as 0.2 °C can change the prediction of a global climate model.

http://usatoday30.usatoday.com/weather/story/2012/09/14/death-valley-now-the-hottest-spot-in-the-world/57775492/1

September 2012: The Libyan temperature record of 136 degree F from 1922 was determined to be invalid because of the combination of a poor weather instrument, a location in a bad spot for accurate readings and an inexperienced record-keeper.

http://www.wunderground.com/deadheat

Errors in temperature measurements…



Air Temperature

Temperature sensors can be categorized according to the physical principle that they use:

- Thermal expansion- Thermoelectric (thermocouples)- Electrical resistance- Electrical capacitance and some other effects…

Direct indicating instruments can use displacement directly: (liquid-in-glass thermometers, bimetallic strips)

Sensors designed to work with data loggers usually convert the output to a voltage signal.

Air Temperature

Raw output of a temperature sensor may be displacement, voltage, resistance, capacitance, etc.

Temperature SensorT Y1 = X, V, R, C

Air Temperature: Thermal Expansion

One way thermal expansion is exploited to sense temperature is through the use of bimetallic strips and liquid-in-glass thermometers.

In both cases the difference between expansion coefficients of two materials is used.

Linear expansion is given by:

ΔL=αL0ΔΔV=βV0ΔT

Where α is the coefficient of linear expansion, L0 is the length of the

material when ΔT = 0, ΔL is the change in length and ΔT is the temperature change from some arbitrary temperature where L0 was measured.



Bimetallic Strip: A bimetallic strip is a pair of metals with different thermal expansion coefficients that have been bonded together.

At the reference temperature, the temperature at which bonding took place, the strip maintains its original shape.

When the temperature changes, the strip bends in a circular arc, for small deflections, due to the differential expansions of the two components of the strip, where one end is held in a fixed position.

http://video.mit.edu/watch/mit-physics-demo-bimetallic-strip-12046/




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TLKy

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- By coiling a very long strip it becomes much more sensitive to small temperature changes.

Bimetallic strip applicationsFire alarm: the heat of the fire causes the brass and iron strips to expand, resulting in the bending of the bimetallic strip, which on bending touches the screw adjacent to it, thus completing the circuit. Once the circuit is complete, the bell begins to ring.

Thermostats: these sense the air temperature using the differential expansion of two metals to actuate an on/off switch; the system will be switched on when the temperature drops below the set point on the thermostat, and switched off when it rises above,

Air Temperature: Liquid-in-glass Thermometer

A Liquid-in-glass thermometer is a glass tube with a bulb at one end filled with the liquid and a scale fastened to or etched on the glass tube.

The liquid is usually mercury or alcohol. The freezing and boiling points of mercury are -39 °C and 357 °C, respectively. Alcohol can be used down to -62°C.

Q: Why isn't water used as the liquid in the common thermometer?

A: If water was used in the common liquid thermometer we wouldn't be able to measure anything over 100 °C or under 0°C



One way to classifying these thermometers is by the immersion required. Immersion types are partial, total and complete.

A partial immersion thermometer should be placed in the bath liquid until the bulb and a small portion of stem (indicated by an immersion line) are immersed in the liquid to be measured.

For a total immersion thermometer, the bulb and the portion of the stem containing the thermometric fluid are immersed.

Partial and total types are used for calibrating other sensors.


A complete immersion thermometer: the bulb and the entire stem are immersed. This type is used for air temperature measurement.


Two special liquid-in-glass thermometers are used to measure the minimum and maximum temperature: a minimum and maximum thermometer.


The minimum thermometer uses alcohol with a dumbbell in the stem.

The minimum thermometer must be mounted horizontally to work properly.

The alcohol flows around the dumbbell as the temperature increases and leaves the dumbbell in a fixed position. When the temperature decreases, the meniscus of the alcohol does not let the dumbbell pass but drags it down to indicate the minimum temperature.

To reset it, the thermometer is simply tipped upside down.


The maximum thermometer uses mercury and has a constriction in the stem.

The bulb is mounted slightly higher than the rest of the column.

As the temperature increases, the mercury in the thermometer expands and moves freely up the tube past the constriction. When the air temperature begins to drop, the constriction prevents the mercury from flowing back down the tube.

Can be reset by shaking it.


Met 163: Lecture 4Chapter 4

Thermometry

Thermoelectric SensorsThe junction of two dissimilar metals forms a thermocouple.

When the two junctions are at different temperatures, a voltage is developed across the junction.

By measuring the voltage difference between the two junctions, the difference in temperature between the two can be calculated.

Or, If the temperature of one junction is known and the voltage difference is measured, then the temperature of the second junction can be calculated.

Thermocouple

The temperature of the measuring junction (Tair) can be calculated if the temperature of the reference junction (Tref) is known and the voltage difference is measured.

Tair

Tref

Thermocouples

Thermocouples provide:-A wide useful temperature range,-Are inherently differential, -Are rugged-Reliable and inexpensive-And usually have a fast response.

The main disadvantage of thermocouples:-Very low output, on the order of 40 μV/ °C.-Slight nonlinearity-And need for calibration.

Thermocouples

There are some observed laws of thermocouple behavior used as a rule-of-thumb guide to thermocouple circuit design and construction.

Thermocouples

The thermoelectric effects: when one junction has a different temperature than the other, an electromotive force is produced in the circuit and current flows.

The magnitude of the force or potential depends on the temperature difference between the two junctions.

There are three components of the thermoelectric effects: The Seebeck effect, Peltier effect, and Thomson effect.

Thermocouples: Seebeck effect

The Seebeck effect: the conversion of thermal energy to electrical energy.

This effect measures the ease at which excess electrons will circulate in an electrical circuit under the influence of thermal difference.

The change in the voltage is proportional to the temperature difference between the junctions when the ends are connected to form a loop.

http://www.youtube.com/watch?v=jlMERuu4IiU

Seebeck effect: demo

http://www.youtube.com/watch?v=jlMERuu4IiU

Thermocouple

The temperature of the measuring junction (Tair) can be calculated if the temperature of the reference junction (Tref) is known and the voltage difference is measured.

Tair

Tref

Seebeck Effect

Thermocouples: Peltier effect

The Peltier effect: closely related to the Seebeck effect. It represents the thermal effect due to a reversible current through dissimilar materials or through similar metals due to an external source of current.

A current flow in one direction might warm the junction of the two dissimilar materials (and release heat to the surroundings of that junction), whereas if the current was reversed, the junction would cool (and absorb heat from its surroundings).

Thermocouples: Peltier effect

Thermocouples: Thomson effect

The Thomson effect: the absorption or liberation of heat by a homogeneous conductor due to a current flowing through it.

It is primarily evident in currents introduced from external sources and those generated by the thermocouple itself. The ability of a given material to generate heat with respect to both a unit temperature gradient and a unit current, is gauged by the Thomson coefficient.

The importance of the Peltier and Thomson effects is essentially infinitesimal because the heat evolved is negligible compared to the amount of thermal energy available from the environment to the junctions of T1 and T2.

The (Thermocouple) Thermoelectric Laws

The three fundamental empirical laws behind the accurate measurement of temperature by thermoelectric means are the:

1. Law of homogeneous materials2. Law of intermediate materials 3. Law of intermediate temperatures.

Law #1: the voltage across a thermocouple is unaffected by temperatures elsewhere in the circuit, provided the two metals used are each homogeneous.

Thus one can use lead wires made of thermocouple metals.


2. Law of intermediate materials

Law #2: If a third metal is inserted in either A or B and if the two new junctions are at the same temperature, no effective voltage is generated by the third metal. This means that a real voltmeter (or amplifier) can be used.

The terminals of a voltmeter are usually made of a third metal and can be close together. It is important to make sure the terminals of the voltmeter are at the same temperature.


3. Law of intermediate materials

Law #3: If a metal C is inserted in one of the AB junctions, then no net voltage is generated so long as junction AC and BC are at the same temperature.

This means that the two wires or a junction can be soldered together and the presence of the third metal, solder, will not affect the voltage if there is no temperature gradient across the solder junction.


T1

T2Ref.

B A

A+

-

V1

V2

V3G

Fig. 4-5 (a)

Thermocouples

Common thermocouple types

Type Metal

T Copper and constantan J Iron and constantan E Nickel (10% chromium and

constantan) K Nickel and Nickel (5%

aluminum/silicon)

Thermocouples

A thermocouple is inherently a differential temperature sensor; it measures the temperature difference between two junctions.

Absolute temperature measurements can be made only if one of the junctions is held at a known temperature or if an electronic reference junction is used.

A block of metal (aluminum, copper, or any highly conductive metal) can be used for the reference temperature.

This is done by inserting the reference junction of the thermocouple in the block and simultaneously measuring the temperature of the block.

Thermocouples

The Campbell Scientific data loggers have this metal block underneath the wiring panel.

The CSI data loggers have special instructions in their programming language that allows for thermocouple measurements.

See the CR1000 manual.

Thermocouple output voltages

Thermocouples

The most common type of thermocouple used for meteorology is the copper-constantan (Type-T).

Its range of use varies from -200°C - 350°C, but is mostly used in the -60°C to 100°C with an accuracy of ±0.5°C.

Another common type used in meteorology is the Type-E.

Electrical Temperature Sensors

One whose resistance varies as a function of temperature. - Thermistors - Resistance Temperature Sensors (RTDs)

Thermistors

Thermistor or thermal resistor is a hard, ceramic-like electronic semi-conductor, commonly made from a mixture of metallic oxide materials.

Have a very large negative resistance coefficient (i.e., an increase in T by 1°C yields a decrease of 5% in resistance).

Thermistors

• Platinum is most commonly used for precision resistance thermometers because it is stable, resists corrosion, is easily workable, has a high temp melting point, and can be obtained to a high degree of purity.

• Simple and stable resistance-temperature relationship.

• Platinum is sensitive to strain; bending the sensor can change the resistance.

RTD: Resistance Temperature Detectors

Resistance of a platinum sensor is given by


20 (1 )TR R aT bT

with sufficient accuracy for the meteorological temperature range -50 to 50°C. R0 is resistance at 0°CRT= resistance of sensor at temperature T°C

Coefficients depend on purity of platinum;a = 0.00385 or 0.00392°C-1

Because the RTD resistance is fairly low and the change with temperature is small a bridge circuit is often used.


Because the RTD resistance is fairly low and the change with temperature is small a bridge circuit is often used.

The bridge circuit converts resistance to voltage and can be amplified to a reasonable level using an instrument amplifier (CR1000 data loggers have this circuit built in).





Campbell Scientific

Exposure of Temperature Sensors

Unaspirated Radiation Shield

Error in Unaspirated Radiation Shield

Documents

Met 163: Lecture 4 Chapter 4 Thermometry This week TUE: thermometry lecture THU: how to obtain data from RAWS, RAWS data analysis using MATLAB software,