Filled system thermometer simply means that they are filled with any of the substitute. They generally come in two main classifications: the mercury type and the organic-liquid type. Since, mercury is considered an environmental hazard, so there are regulations governing the shipment of that type of devices that contain it. Now a day, there are filled system thermometers which employ gas instead of liquids.

The advantages of these types of devices are that they do not require any electric power, they do not pose any explosion hazard and they are stable even after repeated cycling. And the disadvantage of these types of devices is that they do not generate data that are easily recorded or can be transmitted and they do not make spot or point measurements.

1.3(a) THERMOCOUPLES:

Thermocouples essentially comprises of a thermo element (which is a junction of two dissimilar metals) and an appropriate two wire extension lead. A thermocouple operates on the basis of the junction located in the process producing a small voltage, which increases with temperature. It does so on a reasonably stable and repeatable basis.
1.3(b) RESISTANCE THERMOMETER :

Resistance thermometer utilizes a precision resistor, the resistance (Ohms) value of which increases with temperature. RTD has had positive temperature coefficient. Such variations are very stable and precisely repeatable.
1.3(c) THERMISTORS :

Thermistor is a semiconductor used as a temperature sensor. It is manufactured from a mixture of metal oxides pressed into a bead, wafer or other shape. The bead is heated under the pressure at high temperatures and then encapsulated with epoxy or glass. Beads can be very small, less than 1 mm in some cases.
The result of all these is a temperature sensing devices that displays a very distinct non linear resistance versus temperature relationship. The resistance of thermistor decreases with increase in the temperature; this is called as negative temperature coefficient of thermistor.
Thermistor exhibits a very large resistance changes for a small temperature change. This can be as large as 3 to 5% per °C. This makes them very sensitive to small temperature changes. They can detect temperature change as low as 0.1 °C or smaller. A thermistor element is significant smaller in size compared to RTDs. The sensitivity of thermistors to temperature change and their small size make it ideal for use in medical equipment.

Because they are semiconductors, thermistors are more susceptible to permanent decalibration at high temperatures than are RTD's or thermocouple. The use of thermistors is generally limited to a few hundred degree Celsius and manufacturers warn that extended exposures even well below maximum operating limits will cause the thermistor to drift out of its specified tolerance.

Thermistors can be made very small which means they will respond quickly to temperature changes. It also means that their small thermal mass makes them especially susceptible to self-heating errors. Thermistors are good deal more fragile than RTD's or thermocouple and they must be carefully mounted to avoid crushing or bond separation.

Infra red sensors & Pyrometer, now a day is the most common non contact temperature instrument in the industrial applications as it is easy to operate and use if the working principle is known to the user. Infra red sensor & Pyrometer measures the temperature of the object without being in the contact of the body but how is it possible? The answer is here, every object whose temperature is above absolute zero (-273.15K) emits radiation. This emission is heat radiation and its wavelength/frequency depends upon the temperature. So, this property of emission is used when the temperature is to be measured via non contact method.
The term infra red radiation is also in use because the wavelength of the majority of this radiation lies in the electromagnetic spectrum above the visible red light which is in the infra red region.

Similar to the radio broadcasting where emitted energy from a transmitter is captured by a receiver via an antenna and then transformed into sound waves, the emitted heat radiation of an object is received by detecting devices and transformed into electric signals, and in this way the temperature of an object is measured through non contact temperature measuring instruments.