Leadwire Effects and Related Errors: Bondable Resistance Temperature Sensors

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Leadwire Effects and Related Errors
Leadwires are a source of error in all circuits using TG sensors , except three-wire null-balance and four-wire circuits. To minimize these errors, leadwires between the sensor and the readout device (or LST network) should be of low resistance and no longer than necessary. A total two-wire resistance of 0.5 ohm will introduce a shift or offset of about +4° F (+2° C) at room temperature. This leadwire resistance corresponds to 25 ft (7.5 m) of AWG No. 20 (0.8-mm diameter) copper double leads, or 100 ft (30 m) of AWG No. 14 (1.6-mm diameter) double leads.

Changes in leadwire temperature are normally a minor source of error. A change of 50° F (28° C) over the entire length of a 0.5-ohm copper leadwire circuit will create an offset error of approximately 0.4° F (0.2° C) when the sensor temperature is near +75° F (+24° C). This error decreases at higher sensor temperatures and increases at lower sensor temperatures. Accurate measurements in the cryogenic temperature region may require three-wire null-balance or four-wire circuits when long lengths of small diameter leadwire must be employed.

Initial "zero" errors or offsets due to the tolerances applicable to LST networks and the TG sensors themselves can be eliminated by stabilizing the sensor installation at any known temperature close to +75° F (+24° C), and then setting the instrument BALANCE dial so that the reading corresponds to this known temperature. This procedure also eliminates offset error caused by initial leadwire resistance.

In certain circumstances it may be necessary to locate the instrumentation at a long distance from the sensor installations. When LST networks are employed under these conditions, it is preferable to position the network close to its associated sensor and use a three-wire lead circuit between the network and the remote indicator (this should not be done if the test temperature exceeds the temperature capability of the LST network). This type of hookup will eliminate first-order offset errors due to leadwire resistance and leadwire temperature changes. Desensitization or slope-change error is greatly reduced and can be eliminated by setting the strain indicator GAGE FACTOR dial properly. The correct setting can be calculated on the basis of known leadwire resistance or directly determined by applying shunt calibration to the remote network terminals.

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