What is an RTD?
If the following are key to your temperature measuring needs then the most effective temperature measuring device would be a Resistance temperature detector:
• accuracy and stability
• accuracy over a wide temperature range
• area rather than point sensing would improve control
• a high degree of standardization
RTDs are a type of very accurate and precise thermometer. They function by exploiting the predictable change in electrical resistance of certain materials with changing temperature.
The most commonly used metal in an RTD element is platinum, which has many characteristics that make it an almost perfect material for resistance temperature measurement. These characteristics include its stability, high temperature rating, a near linear relationship between resistance and temperature and its ability to be heated and cooled repeatedly without negative effects.
The resistance of platinum changes in a positive and virtually linear fashion thus for each increase in temperature there is a directly proportional increase in resistance.
This relationship is highlighted in the graph below:
Resistance Temperature Curve for 100Pt Platinum RTD (nominal α value): 0.003851°C¯¹
The most common types of platinum RTD found on the market are wire wound and thin film.
Wire wound RTDS
A 100Pt wire wound RTD is constructed by winding fine platinum wire into a coil until there is enough to provide a resistance of 100 Ω.
This coil is then inserted into a ceramic tube and threaded through 2 bore holes at the top of the ceramic (4 bore holes for a 2 coil, or Duplex, construction).
Standard RTD elements usually range from 1 to 5mm diameter and 10 to 50mm in length.
The construction, as shown above, allows the platinum coil to move freely within the ceramic sheath, helping give the RTD long term stability and resistance to vibration.
The boreholes are backfilled with bore glass once the coils have been inserted giving additional support to the coils.
RTD Products RTDs are capable of withstanding vibration and acceleration levels of up to 30g over the frequency range 10Hz to 1 kHz.
Wire wound RTDs are the most accurate RTDs and designed for use in a wide range of applications.
Platinum Thin Film RTD
The thin film style of RTD is now the most common type of RTD due to the fact that it can be mass produced thus giving these RTDs a relatively low cost.
Thin film elements are manufactured by depositing a thin layer of platinum on to a substrate. The metal is deposited in a specific pattern and then trimmed to a specific resistance. The element is then coated with either a thin layer of glass or epoxy for moisture resistance.
These platinum RTDs are the least expensive to manufacture, can be mass produced and are relatively hard wearing. They also allow greater resistances (e.g. 1000 Ω) to be placed in smaller areas.