PLATINUM RESISTANCE THERMOMETER
Introduction Platinum Resistance thermometers are temperature sensors that exploit the predictable change in electrical resistance of platinum with changing temperature. These are being 0 used in place of thermocouples in industrial applications at temperatures below 600 C. At temperatures higher than that it becomes difficult to prevent the platinum from being contaminated by impurities from the metal sheath of the thermometer.
Earlier we discussed the determination of a low resistance using a Carey Foster’s bridge. In this section we are going to use this bridge to calibrate a resistance temperature device namely Platinum resistance thermometer. Platinum resistance thermometer requires a small current to pass through it to determine its resistance at different temperatures. Platinum has a linear resistance-temperature relationship; we can use this method to find the resistance at different temperatures. Platinum Resistance thermometer consists of a fine platinum wire (platinum coil) wound in a non-inductive way on a mica frame M (Figure 1). The ends of this wire are soldered to points A and C from which two thick leads run along the length of the glass tube (that encloses the set up) and are connected to two terminals (P, P) fixed on the cap of the tube. These are the platinum wire leads.
Figure 1: Platinum Resistance Thermometer
Also, by the side of these leads, another set of leads run parallel and are connected to the terminals (C, C) fixed on the cap of the tube. These are called compensating leads and are joined together inside the glass tube. The compensating leads and the platinum wire are separated from each other by mica or porcelain separators (D, D). The electrical resistance of the (P, P) leads is same as that of the (C, C) leads.
Apparatus • • • • • • • • • • •
Carey Foster’s Bridge Two equal resistances of about 2 ohms each Thick copper strip A fractional resistance box A cell / battery Galvanometer Platinum Resistance Thermometer Water bath Thermometer One way key Connecting wires
Theory You have already determined the resistance per unit length of the Carey Foster’s bridge wire in Section A (Part 1).Now replace the unknown resistance X in Part II by Platinum Resistance thermometer. Place the PRT in a water bath and perform the experiment at different temperatures. The circuit for determination of resistance of PRT at different temperatures is given in Figure 2. The resistance of pure platinum wire increases with temperature according to the following relation: RT = R0 (1 + α T ) , o
o
where R 0 = resistance of the wire at 0 C , R T = resistance of wire at temperature T C and α is a constant. α is called the Temperature Coefficient of Resistance for Platinum. It is defined as the change in resistance of a platinum wire per unit resistance per unit change o -1 in temperature. Its units are per degree Centigrade ( C ). can be calculated by measuring the resistance of the platinum resistance thermometer at any two temperatures as described below. α
Let R1 and R2 be the resistances of the PRT at temperatures T 1 and T 2, then
R1 = R0
(1 + α T 1 )
and
(1 + α T 2 ) (1 + α T 2 ) = (1 + α T 1 )
R2 = R0 R2 R1
R2 (1 + α T 1 ) = R1 (1 + α T 2 )
( R2 − R1 ) = α (T 2 R1 − T 1 R2 ) ( R2 − R1 ) α = (T2 R1 − T 1 R2 )
(9)
Figure 2: Circuit for determination of resistance of PRT at different temperatures
Learning Outcomes This experiment will enable you to 1. describe the construction and applications of a Platinum resistance thermometer relate the variation of resistance with temperature using a platinum resistance 2. thermometer 3. determine the temperature coefficient of platinum.
Pre-lab Assessment
Answer the following questions
(1) (2) (3) (4) (5) (6) (7) (8) (9)
How does the resistance of semiconductors vary with temperature? How does the resistance of metals vary with temperature? What is meant by temperature coefficient of resistance (α) of a material? How is the resistance of a material expected to behave if α is positive? How is the resistance of a material expected to behave if α is negative? Explain the construction of platinum resistance thermometer. Why should the PRT be called a thermometer? Mention some materials having a negative temperature coefficient of resistance. Can a thermometer be constructed with Manganin or constantan?
Procedure The experiment is performed in two parts. Part I Determination of resistance per unit length ( ρ) of the Carey Foster’s wire
Bridge
The procedure to find the resistance per unit length of the bridge wire is explained in the experiment CAREY FOSTER BRIDGE. Part II Determination of the resistance of PRT at different temperatures (RT)
1. 2.
3. 4.
5.
6. 7. 8.
Connect the circuit as shown in Figure 2 above. Put the PRT in water bath and connect the PP leads in gap 1 and the compensating leads (C, C) in gap 4 in series with a fractional resistance box X. The wires used to connect the (P, P) and (C, C) leads should be cut from the same bunch and should be of equal length. Connect the two standard resistances P and Q in the inner gaps 2 and 3 and also, the galvanometer with a jockey as shown in Figure 2. Put some crushed ice in the water bath and note the temperature T1. Ensure that the PRT is surrounded by crushed ice with a little water so as to fill all air spaces between the ice pieces. This will ensure uniform temperature for the PRT. Introduce a suitable resistance from the fractional resistance box X (0.5 or 1.0 Ω) and note down the balancing length from one end as l 1. The balance point should be determined only after the PRT has acquired a steady temperature failing which, the position of balance point on the bridge wire will not be stable. Also record the same for reverse current by interchanging the terminals of the battery. Interchange the resistances in the outer arms (i.e. gaps 1 and 4) and note l 2 from the same end for direct as well as reverse current. Calculate R1 = X + ρ (l 2 – l 1); that is the resistance of the PRT at temperature T1. Now, remove the ice and put the PRT in water at room temperature, say T2. Note down and record T2 in the observation table
9.
Determine the resistance of PRT at T 2 (that is R2) by repeating the steps 5 – 7 as above. 10. Now, heat the water for some time till the PRT acquires a constant temperature T 3. Note down T 3 and repeat the steps 5 – 7 to determine the resistance at T3. 11. Repeat step 10 for at least five more temperatures.
Observations Table 1: Determination of ρ
S. No.
X (Ω)
Position of balance point with Cu strip in the Right gap (l' 1 in cm) Left gap (l' 2 in cm) Direct Reverse Mean Direct Reverse current current current current
Mean ρ = ---------
Mean
l' 2-l' 1
ρ= X /( l' 2-l' 1)
(cm)
(Ωcm )
-1
-1
Ωcm
Table 2: Determination of resistance of PRT at different temperatures -1
Resistance per unit length of the bridge wire, ρ = ------ Ω cm Fractional resistance = 0.5 or 1.0 Ω
S. No.
Temp. of water o ( C)
Position of balance point with PP leads in the Right gap (l 1 in cm) Left gap (l 2 in cm) Direct Reverse Mean Direct Reverse current current current current
l 2-l 1
Mean
(cm)
R T= X - ρ (l 2-l 1) (Ω)
Calculations o
Plot a graph (Figure 3) between the temperature (in C) and the resistance of PRT (in Ω) that is the calibration curve of the PRT.
RT
T
Figure 3: Graph showing change in resistance with temperature
From the graph, α can be calculated as
α 1
α
=
( R2 − R1 )
(T2 R1 − T 1 R2 ) ( R3 − R2 ) = 2 (T3 R2 − T 2 R3 ) ( R2 − R1 ) = 1 (T2 R1 − T 1 R2 )
α
One can see that a number of values of α (α1, α2, α3 -----) can be calculated by choosing any two points on the temperature scale and the corresponding resistances. Mean value of α can be taken as the mean of all the calculated values.
Result 1.
2. 3.
The variation of resistance of PRT as a function of temperature has been studied and calibration curve for the given resistance temperature device has been drawn. The resistance is found to vary linearly with temperature in the range of temperature investigated here and hence, the calibration curve is a straight line. The temperature coefficient of resistance for platinum using PRT is found to be -o -1 ---------- C . -4 o -1 Actual value of α is 37 X 10 C for platinum.
Probable sources of error • • • •
• •
The ends of connecting wires, thick copper strips and leads for the resistance box may not be clean, so there may be an additional contact resistance at the connections. The plugs of the fractional resistance box may be loose, again introducing undesirable contact resistance. The bridge wire may get heated up due to continuous passage of current for a long time. This will change its resistance. If the jockey is not pressed gently or if it is kept pressed on to the wire while being shifted from one point to another, that may alter the cross sectional area of the wire and make it non uniform. The bridge wire may not be uniform and hence the calibration curve will not be linear. The bulb of the PRT may not be properly immersed, which can give incorrect resistance for the particular temperature.
Post-lab Assessment Answer the following questions:
(1) (2) (3) (4) (5) (6)
What is the purpose of introducing compensatory leads in a PRT? What is the function of mica discs in a PRT? Why is platinum preferred in a resistance temperature device? PRT can be used as a temperature measuring device for what range of temperature. Why is the platinum wire wound in a non inductive way on a mica frame? What is the disadvantage of a PRT as a temperature measuring device.
(7) (8) (9) (10)
What is the ‘fundamental interval’ of a PRT? What approximate degree of purity of platinum is need ed to construct a PRT? What factors limit the value of battery current in your experiment? o The melting point of platinum is 1750 C. Why should the PRT be used to o measure temperatures up to 1200 C only?
Glossary Temperature coefficient of resistance (α) : Relative change in resistance of a substance per unit change in temperature is called temperature coefficient of resistance. All other words listed in the glossary for CAREY FOSTER’s BRIDGE.
Answers to Pre-lab Assessment 1. The resistance of semiconductors decreases with increase in temperature. 2. The resistance of the given material has to increase with increase in temperature. 3. It is defined as the change in resistance of the given material per unit resistance per unit change in temperature. 4. The resistance of metals increases with increase in temperature. 5. The resistance of the given material has to decrease with increase in temperature. o 6. At temperatures higher than 600 C it becomes difficult to prevent the platinum from being contaminated by impurities from the metal sheath of the thermometer. 7. As the temperature of platinum increases with increase in temperature, by studying the variation of its resistance with temperature and drawing the calibration curve, it can be used to determine an unknown temperature. 8. Carbon and electrolytes possess negative temperature coefficient of resistance. 9. Since the temperature coefficient of resistance for these alloys is very small; the change in resistance will not be measurable for a small change in temperature. Hence, the accuracy will be very poor.
Answers to Post-lab Assessment 1. They are used to compensate for the resistance of the platinum wire leads so that only the resistance of the platinum coil is obtained in the experiment. 2. The mica discs provide the insulation inside the glass enclosure of the PRT. 3. It is preferred because the temperature coefficient of resistance for platinum is large, positive and uniform over a large range of temperature. o o o o o 4. 200 C to 1200 C ; with an accuracy of 0.02 C in 0 C to 630 C range. 5. This is done to eliminate the effects due to self induction and eddy currents 6. The disadvantages are:
(a) As platinum has a large heat capacity, it takes appreciable time to attain the temperature to be measured. (b) It cannot measure rapidly changing temperatures and the temperature of a point o 7. It is defined as the increase in resistance of the PRT for 100 C rise in temperature and its value is generally equal to 1Ω. 8. The degree of purity of platinum should be such that R100 R 0
≥ 1.39
9. The current should be kept small to minimize the heating effects on the resistances used in the circuit and on the other hand, be sufficient for accurate determination of null point. About 0.5 mm of shift should produce a readable deflection. 10. It is because the resistance of platinum does not vary linearly with temperature o beyond 1200 C, hence, the formula that has been used here, cannot be used for o calculations beyond 1200 C without modifications.
