## 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.

- stable

- accurate

- more linear than thermocouples

- 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,

- self powered

- simple, rugged

- inexpensive and commonly available

- wide temperature ranges

- 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