Resistance Thermometer

Resistance thermometers are sensors that are used to measure temperature. Known as an RTD, resistance thermometers correlate the resistance of the RTD element with the temperature.  The thermometer is designed with a length of coiled wire that is wound around a ceramic or glass center.  There is an RTD element inside of the thermometer that is fragile, and made from pure metal. Some metals that are typically used in RTDs are platinum, nickel, or copper. These materials often have a predictable change in resistance as the temperature fluctuates, and this change is how the resistance thermometer measures temperature.

Essentially, a RTD sensor is based upon the notion that: as temperature of metal increases, the metal’s resistance to the flow of electricity decreases. An electrical current is passed through the RTD element in order for the device to register temperatures. For metals, the resistance increases with rising temperatures. If the relationship between temperature and resistance is well established, you can read temperatures by recording the change in resistance.

There are many advantages of having a platinum resistance thermometer. They are extremely accurate, have a wide operating range, are suitable for precision applications, and have low drift. The standard platinum resistance thermometer is the most accurate of all RTDs.  Known as the SPRT, the platinum thermometer is wound with high-grade platinum wire and has internal lead wire that is additionally made from platinum. The internal supports of the thermometer are made with quartz or fuse silica.  There are some less frequently used metals that can serve as RTD elements, but they are not as accurate.

RTD Elements are commonly recognized in three formats.

1) Thin Film RTD elements- thin film elements contain a platinum, or a metal glass slurry, that is deposited on to a small level ceramic substrate. This form is the most accurate and rugged of the three.
2) Wire Wound RTD elements- Wire wound elements are comprised of platinum and metal wire wrapped around a glass and or ceramic bobbin. The bobbin is then sealed with a special coating of molten glass.
3) Partially wound- partially wound RTDs have elements that are wound with a small coil of wire that has been inserted into a ceramic insulator.

The more wires that a Resistance Thermometer has the more accurate the device is.  Without wires, the RTD would not be able to get a stable and effective reading of resistance changes.

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