Instrumentation Knowlege

 The important reasons for temperature measurement are:

• To ensure the safety of personnel and equipment by maintaining temperatures within design limits.

• To ensure accurate process control.

• To ensure the product quality.

 RTD

RTD is based on the fact that a metal’s electrical resistance varies with temperature changes. If a metal wire is heated, according to the equation, its electrical resistance will increase:

Rt = Ro(1+At+Bt2+……)

Where,
C°Rt = resistance of the metal at T
C°Ro= resistance of the metal at 0
a, b= Constants

different temperature calibration devices:

• Decade resistance box

• Millivolt Calibrator

• Multi-loop Calibrator

• Precision temperature calibrator

• Fluidized calibration bath

Pt100:

 industry standard Platinum RTD with a value of 100 Ohms @0°C to IEC751; this is used in the vast majority of PRT assemblies in most countries

Why Use a Thermocouple?

• Temperature range: Most practical temperature ranges, from cryogenics to jet-engine exhaust, can be served using thermocouples. Depending on the metal wires used, a thermocouple is capable of measuring temperature in the range –200°C to +2500°C.

• Robust: Thermocouples are rugged devices that are immune to shock and vibration and are suitable for use in hazardous environments.

• Rapid response: Because they are small and have low thermal capacity, thermocouples respond rapidly to temperature changes, especially if the sensing junction is exposed. They can respond to rapidly changing temperatures within a few hundred milliseconds.

• No self heating: Because thermocouples require no excitation power, they are not prone to self heating and are intrinsically safe.