1.9 TEMPERATURE
• Temperature is of significant concern in most disciplines.
- fine temperature measurements 0-100 deg C - e.g. science/laboratory work
- high temperature measurements <3000 deg F - e.g. metal refining/processing
- low temperature measurement 0--60 deg C - e.g. freezers
- very low temperatures <-60 deg C - e.g. superconductors in MRI units
- very high temperatures > 2000 deg C - e.g. fusion research
1.9.1 Resistive Temperature Detectors (RTDs)
• These devices use the effects of temperature change on conductivity.
• Uses resistive wires made of materials such as,
- platinum
- nickel
- copper
- nickel-iron
• Typical material properties are, [Bryan]
• Wires are wound about an insulator, for support, and covered for protection.
• These devices typically have positive temperature coefficients that cause resistance to increase linearly with temperature.
• Typical changes are doubling of initial resistance value over range of use.
• Advantages,
- stable
- accurate
- more linear than thermocouples
• Disadvantages,
- expensive
- requires a power supply
- small absolute resistance and resistance change
- self heating
1.9.2 Thermocouples
• Uses a junction of dissimilar metals to generate a voltage proportional to temperature.
• The bimetal junction will generate an electrical potential between the metals as the temperature increases. The change in voltage is linearly proportional to the change in temperature.
• When using a thermocouple for precision measurement, a second thermocouple can be kept at a known temperature for reference.
• A series of thermocouples connected together in series produces a higher voltage and is called a thermopile.
• Basic thermocouple types are J, K, etc. These are designed for different temperature ranges.
• The basic calculations for thermocouples are given below,
• Advantages,
- self powered
- simple, rugged
- inexpensive and commonly available
- wide temperature ranges
• Disadvantages,
- nonlinear
- low voltage
- reference devices needed
- least stable and sensitive
1.9.3 Thermistors
• Non-linear devices that change conductivity with temperature. These are usually semiconductors.
• The resistance drops as the temperature rises. (Note: this is because the extra heat reduces electron mobility in the semiconductor.) This effect can change the resistance by more than 1000 times.
• The basic calculation for thermistors is given below,
• The circuit below can be used to amplify the output of these devices.
• These devices are normally small and come as beads or metallized surfaces.
• The advantages of these devices include,
- fast response
- small
- higher resistance reduces the effect of lead impedance
- inexpensive
• Disadvantages include,
- smaller temperature range
- signal is not linear
- very wide resistance range
- self heating
