RESISTANCE TEMPERATURE DETECTOR ( RTD ) - PRINCIPLE - CONSTRUCTION - WORKING

RESISTANCE TEMPERATURE DETECTOR (RTD)

RTD stands for a resistance temperature detector. It also referred to resistance thermometer.

Principle 

• The basic principle of RTD is changing metal resistance with temperature

• Practically, An electrical current is transmitted through the metal resistor  ( RTD  element ).

• Located in the proximity area where temperature is to be measured. the resistance value measure by instrument and that resistance is correlated with temperature based upon the known resistance characteristics of the RTD element.

Construction 

• The RTD incorporates pure metals or certain alloys that increase in resistance as temperature increases and conversely decreases in resistance as temperature decreases. 

• Generally,RTD elements are constructed of platinum, copper,or nickel. these metals are best suited for RTD application because of their linear resistance - temperature characteristics and they are high coefficient of resistance  

Platinum RTDPlatinum RTDs are the most common type of RTD used in industrial applications. This is because platinum has excellent corrosion resistance, excellent long-term stability, and measures a wide range of temperature, (-200…+850°C).

Nickel RTDNickel RTDs are less expensive than platinum and have good corrosion resistance. However, nickel ages more rapidly over time and loses accuracy at higher temperatures. Nickel is limited to a measurement range of -80…+260°C. 

Copper RTDCopper RTDs have the best resistance to temperature linearity of the three RTD types, and copper is a low-cost material. However, copper oxidizes at higher temperatures. Copper is limited to a measurement range of -200…+260°C.

• RTD inner construction is shown in the below figure

• The platinum wire cover with porcelain insulator, that prevent short circuits between wires and metal sheath.

• Generally  RTD sheath constructed by nickel-iron-chromium alloy. because it is highly inherent corrosion resistance and also capture the temperature of gas or liquid very quickly.

• Thus a change in temperature of the sheath will cause platinum wire heat or cool.

• This change in resistance is then measured by a precision resistance measuring device that is calibrated to given the proper temperature reading. this device is normally a bridge circuit.

• RTD protective with well and terminal head . it protects the RTD from the damage the gas and liquid while measuring.

• Protecting wells are normally made of stainless steel carbon steel, Inconel or cast iron, and they used for temperature up to 1100°C

Working

• RTD working principle based on the correlation between metals resistance and temperature.

• As the temperature of a metal increases the metal's resistance to the flow of electricity increases. 

• similarly as the temperature of the RTD resistance element increases, the electrical resistance increases. 

• Electrical resistance measure in ohms(Ω)

• RTD elements are commonly specified according to their resistance in ohms at zero degree celsius

• The most common RTD specification in 100 Ω, which means that at 0◦ C the RTD element should demonstrate 100 Ω of resistance 

• RTD acts somewhat like an electrical transducer, converting changes in temperature to voltage signals by the measurement of resistance.