Application Guide catalog - Watlow - #49

RTDs generally have a positive temper- be taken into account. Ideally the

RTDs are temperature sensors utilizing metals known and predictable change in electrical resistance based on a rise or fall in temperature. The sensing element is a deposited film or coil of wire, usually platinum, nickel, copper, or nickel-iron.

ature coefficient. RTDs are powered devices. The instrument receiving their "signal" reads temperature as the change in voltage. Looked at another way, as temperature changes, so does the sensor's resistance.

Because these are resistance devices, their operation generates heat in addi­tion to the heat they're measuring. When specifying the use of an RTD, its mass and self-generated heat must

substance being measured will have enough mass and thermal conductivity so as to make the RTD's self-generated heat a negligible factor.

Interchangeability is a commonly cited factor of RTD accuracy. It tells how closely the sensing element of an RTD follows its nominal resistance/temperature curve, and the maximum variation that should exist in the readings of identical sensors, mounted side-by-side under identical conditions.

Interchangeability consists of both a tolerance at one reference tempera­ture, usually 0°C, and a tolerance on the slope, or Temperature Coefficient

of Resistance (TCR). Even a slight deviation in the TCR will cause a significant error to result at elevated

temperatures. Thus, it's important for the user to specify the TCR when ordering RTD probes.

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(D 3 (A O

*Thin film element -50 to 550°C (-58 to 1020°F).

DIN/IEC class A: ±(0.15 + 0.002 |t|°C DIN/IEC class B: ±(0.30 + 0.005 |t|°C

Where t is the actual temperature, in °C, of the platinum elements.

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