Temperature of water using the change of resistance of a metal Brief Undertake an individual instrumentation project, the project may involve one of: Building and testing a sensor Exploring the characteristics of a sensor Designing and assembling a measurement system, and using the sensor to make a measurement I have chosen to design and assemble a measurement system then use the sensor to make a measurement. The task is to use a sensing device to put together a system to make a measurement. The report of results should evaluate the measurement, giving evidence of how reliable it is likely to be.
Introduction I have decided to make a sensor to measure the temperature of water by measuring the change in resistance of a metal. I will look at how the resistance changes in a wire passed through a beaker of water at different temperatures. I can measure the resistance in two ways, one using the built in resistance calculator on the multimeter, or I can run a current through the wire and measure the potential difference across the wire and the current running through it to calculate the resistance using the formula: Resistance (? ) = Potential Difference (V)
Current (I) I would then calculate the resistivity at the temperature it was at using the formula: R=? L Resistance = resistivity x length/ area A I could then calculate the temperature using the formula: ? T2 =? T1 (1+? t1 t2 (T2-T1)) Where: T1 and T2 are the initial and final temperatures in ? C ?T1 is the resistivity at temperature T1 ?T2 is the resistivity at temperature T2 ?t1 t2 is the temperature coefficient of resistivity for the range of temperatures T1 to T2 I would use the value of resisitivity for the metal I was using at 20? C for ? T1.
From this I could then calculate the temperature the water was at. Considerations Before I prepare my plan I must consider the following: Length of wire to be used Material of wire Diameter of the wire Type of power supply and amount of current Anything else that may affect or effect the result The wire I will use must have a fairly large resistance that is affected by change in temperature as much as possible for me to get a reasonable reading. Looking at the equation, ‘Resistance = resistivity x length/ area’ it can be seen that the greater the value of the length, the higher the resistance.
However the more resistance there is, the more the wire will heat up due to the current flowing through it which would affect the resistance. This will only affect the temperature of the water though, as the heat energy given out will be spread along the whole wire and it will not affect the temperature of any individual part of the wire. I will decide a value when I have decided on the diameter of the wire and found the resistivity of the material I am using, I will also look at the amount of heat energy produced from the wire. I will want to use as long a length as possible that will comfortably fit in the beaker.
Again looking at the equation ‘Resistance = resistivity x length/ area’ it can be seen that as the area approaches 0, resistance goes to infinity. So the smaller the area the higher resistance I will have. The area is affected by the diameter of the wire in the equation ‘area of circle = ? r2’ therefore as radius decreases, area decreases. Another consideration is how quickly the wire is heated throughout. The thinner the wire the more quickly the heat will travel through the whole thickness of the wire, as it does not have to conduct as far to reach the centre of the wire.
Because of this I have decided to use as thin wire as I can get hold of, to increase the resistance. I will have to be careful though and make sure that it does not cause the wire to heat up too much. The material of the wire will be very important in my experiment as resistance in some materials is minimally affected by temperature whereas in some materials temperature greatly affects the resistance. I have done some research on the Internet and have found the following table from ‘http://hyperphysics. phy-astr. gsu. edu/hbase/Tables/rstiv. html#c1’ Material Resistivity (ohm m).
Temperature coefficient per degree C Silver 1. 59 x10^-8 .0061 Copper 1. 68 x10^-8 .0068 Aluminium 2. 65 x10^-8 .00429 Tungsten 5. 6 x10^-8 .0045 Iron 9. 71 x10^-8 .00651 Platinum 10. 6 x10^-8 .003927 Manganin 48. 2 x10^-8 .000002 Mercury 98 x10^-8 .0009 Nichrome (Ni, Fe, Cr alloy) 100 x10^-8 .0004 Carbon (graphite) 3-60 x10^-5 -. 0005 Germanium 1-500 x10^-3 -. 05 Silicon 0. 1-60 … -. 07 Glass 1-1000 x10^9 … Hard rubber 1-100 x10^13 … Assuming the data in the table is correct, it can be seen that the resistivity of copper is affected most by temperature with the exception of Germanium and Silicon.
As this is readily available within the lab I have decided that this would be the most suitable material to use. It also has low enough resistance so the wire will not be heated too much. After a preliminary experiment I have decided to measure the resistance by running a current through the wire and measuring the p. d across it as well as the current in the circuit. This will increase the accuracy by several decimal places, which will be very important on the scale I will be working with. I found that the built in resistance measure on the multimeter was not sensitive enough.