eNotes: Mechatronics and Controls

13.9 TEMPERATURE

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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

13.9.1 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

13.9.2 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

13.9.3 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