Thermometers

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Thermometers measure temperature.

Measure of hotness or coldness of a body.

Two thermometers do not necessarily give the same reading at the same temperature.

The need for standard thermometers

– use any commercial laboratory thermometer as school standard.

The International Temperature Scale

In practice, accurate measurement of temperature using a real gas thermometer is inconvenient, and requires painstaking laboratory work. So, to reduce the experimental difficulties involved and to standardise temperatures, the International Temperature Scale was adopted.

It was first set up by the International Committee of Weights and Measures in 1927 and was revised or modified in1948, 1960, 1968, 1975 and 1976. The most recent scale was decided on in 1990. This scale of temperature is based on the measured temperature values of a number of reproducible equilibrium states.

In the beginning, six primary points were chosen, then later some secondary points were added, bringing the number to eleven, and since ITS-90 there are now sixteen fixed points. These points are determined using substances of the highest purity, generally at their boiling points, freezing points or triple points.

Except for the triple point of water, values are not defined but are based on experimental results, which change over the years as thermometry techniques improve. In most cases, pressures are taken to be one atmosphere, except for hydrogen which is measured at lower pressures.

The instruments used for interpolating between selected points and beyond the highest point (1357.77 K) are usually specified.

Between 13.8033 K and 1234.93 K a platinum resistance thermometer is used.

Optical pyrometers giving values based on Planck’s law of radiation are used for temperatures above 1234.93 K, taking as reference the radiation emitted from a black body cavity.

At very low temperatures (< 5 K) ITS-90 is defined in terms of the vapour pressure of 32He and 42He. Absolute zero cannot be attained in practice (third law of thermodynamics). Between 3.0 K and 24.6 K a constant volume gas thermometer is used.

The ITS recognises the thermodynamic temperature scale as the basic one, and is constructed so as to approximate thermodynamic temperatures. The unit used is the Kelvin, which is defined to be 1/273.16 of the triple point of water.

Experiment

Graduate two thermometers at ice and steam points.

What two reference points are the celcius and kelvin scale both based on ?

the freezing point of and boiling point of water., these points were selected because they are easily reproduced in a laborotory.

If we have a liquid in glass thermometer and a thermistor and place these thermometers in situations where the they will give measurable value for their thermometric property at each temperature.

The liquid in glass thermometer, mark the height of the liquid at zero degrees Celsius and again at 100 degrees celcius, divide this distance by 100 and mark off each degree.

The thermistor we must use a multimeter, note the resistance at zero degrees Celsius and again at 100 degrees celcius, this time the best thing to do is to

Compare values obtained for an unknown temperature, using a

straight-line graph between reference points.

What two reference points are Farenheit scale based on ?

Practical thermometers, e.g.

• clinical thermometer,

Used by doctors and nurses

Range ??? in and around 37oC

The old style is on the left, but more commonly found are the small electrical thermometers based on a thermistor. Like all thermometers they require a little amount of time to settle, usually giving a beep when the readin can be taken.

Another form of thermometer is the InfraRed radiation thermometer. As you should have been told in Junior Cert Science, all objects above 0K radiate heat in the form of Infrared radiation, these devices absorb the radiation examine the frequency and thus give a reading of the temperature of the object.

• oven thermometers,

Dictates the on/off (ness) of the oven

• boiler thermometers,

Bi-metalic strip

• temperature gauge in a car.

stays in the same place if things are going well (90oC)

Infrared thermometers

Fridge thermometers

allows us to keep food cold, thus preventing the growth of bacteria.

Variation of the resistance of a metallic conductor with temperature

We just wound insulated copper wire around a cylinder (plastic pen-outer) about 100 times.Because it is insulated it wont short, and the plastic keeps it "tidy".

Remove 2cm of insulation from each end and attach croc clips.We leave them in the glycerol all year around, and cover them with clingfilm to keep them clean!

It works well. You will get a very small change in R (about 0.5ohms over 100degrees)

But I tell them that we want it like that otherwise the resistance of our house wiring would change from summer to winter, with all kinds of unwanted effects.

It costs next to nothing and every student can have/make their own one.

A whole range of thermometers for every purpose here.

http://thermometer.co.uk/