Application Guide catalog - Watlow - #51

The most notable advantage of platinum RTDs is their precise and predictable response to changes in temperature. Platinum is the most widely used RTD in military aerospace and nuclear and many other applications requiring a high degree of precision. Platinum also has the advantage of being relatively indifferent to its environment,

corrosion resistant and not easily oxidized. It can be drawn to a fine wire; uniformly deposited in films and can withstand extreme temperatures with its high melting point of approximately 2040°C (3700°F).

All resistance wire RTDs have a positive temperature coefficient—their resistance increases as temperature increases.

Continued

Since the amount of electrical resis­tance is a function of a material's temperature, resistance of RTDs can theoretically be made from any metallic element or alloy. Most often they're made from copper, nickel, nickeliron or platinum. The non­linear response of nickel and limited temperature range of copper makes platinum the most commonly used resistance material.

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Because an RTD is a resistance device, any resistance in the lead wires between sensor and instrument will add resistance to the circuit and alter the readings. Compensating for this extra resistance with adjustments at the instrument may be possible. However, variations in ambient temperature alter copper lead wire resistance, so this only works when lead wires are held at a constant temperature.

Gauge leads, 150 feet long:

Total resistance = 300 ft X 0.0165 ohm/ft

= 4.95 ohm Approx. error = 4.95 ohms/

(0.385 ohm/°C)

= 12.9°C

The table below contains resistance values for common copper lead wire gauges. To approximate the error in an uncompensated sensor circuit, multiply the length (in feet) of both extension leads by the approximate value in the table. Then divide it by the sensitivity of the RTD element to obtain an error value in °C. For example, assume a 100 ohm platinum element with 0.00385 TCR and 22 B & S

Depending on the length of run, lead wire error can be significant. Particularly so if the gauge is small, or connected to a low sensitivity ele­ment. Using a three-wire circuit will reduce errors in most applications to a negligible level.

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