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RTD Construction and Lead Wire Configurations

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Platinum RTD elements are available in two types of constructions:
(a) Thin film and

(b) Wire wound.

Thin Film
Thin-film RTD elements are produced by depositing a thin layer of platinum onto a substrate. A pattern is then created that provides an electrical circuit that is trimmed to provide a specific resistance. Lead wires are then attached and the element coated to protect the platinum film and wire connections.
Thin film elements are available in the.

European standard (0.00385 Ω/Ω/°C), and in a special version, used primarily in the appliance industry, that has a temperature coefficient of 0.00375 Ω/Ω/°C. Thin film elements are not available in the American standard.

Wire Wound:
RTD elements also come in wire-wound constructions. There are two types of wire-wound elements:

(a)Those with coils of wire packaged inside a ceramic or glass tube(the most commonly used wire-wound construction), and

(b)Those wound around a glass or ceramic core and covered with additional glass or ceramic material (used in more specialized applications).

Wiring Arrangement of RTDs:
In order to measure temperature, the RTD element must be connected to some sort of monitoring or control equipment. Since the temperature measurement is based on the element resistance, any other resistance (lead wire resistance, connections, etc.) added to the circuit will result in measurement error. The four basic RTD element wiring methods according to the IEC/ASTM color  codes are:
(a) 2 Wire configuration
(b) 3 Wire configuration
(c) 4 Wire configuration
(d) 2 Wire configuration with compensating loop.

2 Wire configuration RTD:

This wire configuration provides one connection to each end of the RTD sensor. This construction is suitable where the resistance of the run of lead wire may be considered as an additive constant in the circuit, and particularly where the changes in lead resistance due to ambient temperature changes can be ignored. This wire configuration is shown below:

Note that the resistance of probe and extension is added to the RTD resistance and will increase the measured value. This could be a source of error in applications where high accuracy is required.
3 Wire Configuration RTD:
This is the standard wire configuration for most RTDs. It provides one connection to one end and two to the other end of the RTD sensor. Connected to an instrument designed to accept three-wire input, compensation is achieved for lead resistance and temperature change in lead resistance. This is the most commonly used configuration.

4 Wire Configuration RTD:
This wire configuration provides two connections to each end of the RTD sensor. This construction is used for measurements of the highest precision.

2 Wire Configuration RTD with Compensating Loop:
This is similar to 4 wire configuration RTD except that a separate pair of wires is provided as a loop to provide compensation for lead resistance and ambient temperature changes in lead resistance. 

For more information on RTD Sensors, check out:

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