The following are some essential conditions governing choice of properties and materials for temperature measurement by the thermometer.
Kinds of Thermometer:
Thermometers may be categorized according to thermometric properties utilized in constructing such thermometers. For instance, we have:
Liquid-in-Glass Thermometers: Such thermometers utilize expansion of liquid in glass tube. As the liquid volume increases with temperature rise, length of liquid along tube differs with temperature.
Constant Volume-Gas Thermometer: That makes use of the given mass of gas whose pressure differs with temperature at constant volume.
Resistance Thermometer: Makes use of variation of resistance of given conductor like platinum with temperature.
Thermo-electric Thermometer: This makes use of variation of electromotive force (emf) developed between pair of junctions of two dissimilar metals with temperature.
This thermometer makes use of two liquids like alcohol or mercury. Mercury-in glass thermometers are commonly utilized in laboratory or in hospitals as clinical thermometers. While, alcohol-in-glass thermometers are utilized in temperate countries as alcohol has the much lower melting point than mercury.
i) Clinical Thermometers:
Clinical thermometer is generally utilized in hospital clinics. It is specially adapted for estimating temperature of human body. Stem of clinical thermometer is graduated from 35oC to 45oC.
It has the constriction in stem near bulb that prevents mercury from entering bulb. Therefore measurement can be read at leisure. Jerking instrument to drive the mercury back then returns broken thread of the mercury. Laboratory thermometers are longer than clinical thermometer s and they have no limitations in them. Besides, they are generally longer with longer range of temperatures from -10oC to about 150oC.
ii) Maximum and Minimum Six's Thermometers:
Apart from laboratory and clinical thermometers, there is yet another mercury-in-glass thermometer known as Maximum and Minimum (six's) thermometer.
Six's thermometer is utilized to evaluate maximum and minimum temperatures on the particular day. Thermometer is composed of long bulb A which has alcohol or any other appropriate liquid. BC is the thread of mercury. Above C, there is more alcohol that partially fills the bulb D. Above the surface of mercury thread at B and C are light steel indices with the spring joined to each steel to prevent it from slipping.
When alcohol in A expands, mercury thread in BC is pushed round transferring index at C upwards and leaving g index B in position. When the alcohol in A contracts, mercury thread BC is drawn back leaving the C index to record the maximum temperature reached. Index B likewise records minimum temperature reached. The magnet is utilized to reset every index by drawing them down to surface of the mercury thread. Minimum and Maximum thermometer is thus utilized to record minimum and maximum temperatures of day. Knowledge of such temperatures over the period of time may be helpful in forecasting weather.
Constant Volume Gas Thermometers:
The two properties which are kept constant in construction of this thermometer are volume (V) and the amount of gas in moles (n). Therefore varying properties are pressure (P) on gas and temperature (t). Let a fixed mass of gas maintained at constant volume in the vessel able to measure pressure P. If P0 and P100 are pressures at ice and steam points respectively and Pt is pressure at some unknown temperature toC. Assuming that equal changes in pressure denote equal changes in temperature, then
Jolly's Constant volume air thermometer will be explained to show how pressures are estimated at constant volume but at varying temperatures.
It comprises of the glass bulb of approx 100cm3 in volume containing dry air. This bulb is joined to fine glass capillary tube and rubber pressure tube that in turn is joined to moveable reservoir of mercury.
There is mark X on capillary tube. This is constant volume mark. Total pressure on air is (H + h) where H is atmosphere pressure and h is height of mercury above mark X.
If P0 = H + h0 P100 = H + h100 and Pt = H + ht
Then t = [(Pt -P0)(P100 - P0) X 1000c
t = [(ht -h0)(h100 - h0) X 1000c
Jolly's constant-volume gas thermometer is not very suitable to use, when compared with mercury-in-glass, resistance and thermo-electric thermometers.
Standard constant-volume hydrogen thermometer as developed by Chappius. It comprises of Platinum Iridium cylinder T that is connected to the manometer. Constant volume is attained through devise P1. A barometer is also included with manometer utilizing a device P2. Vertical distance between P1 and P2 provides total pressure.
The Chappius version is the improvement of Jolly's simple gas thermometer. It is to overcome inaccuracies in Jolly's instrument because of fluctuations in barometric heights in course of experiment. It is also to decrease errors because of capillary depression on mercury that come to play when utilizing tubes of narrow bore in Jolly's instrument.
Platinum Resistance Thermometers:
As name of thermometer suggests, it is the thermometer which depends on resistance of the wire whose values change with temperature. Callendar found out that resistance of pure metals increases as temperature increases. He illustrated that resistance of pure platinum differs with gas thermometer temperature t according to equation.
Rt = R0 (1 + At + Bt2)
Where, Rt and R0 are resistances at toC and 0oC respectively.
A and B are some constants for the given specimen. On platinum resistance scale of temperature, equal changes in resistance indicate equal changes in temperature.
Typical platinum resistance utilized in connection with Wheatstone (Meter) Bridge devised by Callendar and Griffiths. Bridge is utilized to get balance point that allows experiment to find out different values of Rt , R100 and R0
Let R0 and R100 be the resistances of ice and steam respectively and Rt the resistance of the platinum at tpoC.
Tp = [(Rt - R0)/(R100 - R0)] X 1000c
Platinum resistance thermometer has the wide range noted for extreme accuracy over length of this range. Though, its chief drawback is long time required for it to suppose temperature of surroundings and time required for making the observation so that it can't follow rapidly changing temperatures. This is where thermo-electric thermometer is the good substitute.
The thermo-electric thermometer is otherwise known as thermo-couple. This kind of thermometer is made by using Seebeck effect. Seebeck effect just defines that if two dissimilar metals, like copper and iron are joined to make the complete circuit, then on heating one end of junctions, a current flows round circuit.
Presence of galvanometer is to notice and compute magnitude of the current which flows in circuit. This kind of arrangement is known as a thermo-couple. Emf established round circuits depends on nature of metals utilized to form couple and also on temperature e difference between hot and cold junctions. The cold junction is generally maintained at ice point. It has been illustrated experimentally which when other junction is at some temperature t, thermo-electric emf set up depends on temperature accordingly as
E = A + Bt + Ct2.................. Eq.1
Where A, B and C are constants depending on metals utilized.
Temperature where emf is maximum for two metals is known as neutral temperature tN.
It is though more suitable to observe thermo-electric behavior of two metals graphically by plotting the thermo-electric power (P) against temperature. Thermo-electric power is change in thermo-electric emf per degree Celsius in temperature between hot and cold junctions. From the Eq.1 if t is measure from ice point, then A is equal to zero. Then,
E = Bt + Ct2
But Power (P) is provided by
de/dt = B + 2Ct
Intercept on t - axis indicates where de/dt = 0 and that is the neutral temperature. It is possible to establish from graph the emf when junctions are kept at temperatures t1 and t2. It is equal to area under curve between t1 and t2.
E = t1∫t2Pdt
In theory, it is more acceptable to compute electromotive force (emf) developed instead of current. This is due to emf depends on total resistance of circuit that alters as temperatures changes. In industrial set up, the high resistance millivoltmeter is utilized. Thermo-electric couple is able to measure up to 1500oC.
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