Campbell Scientific CR6 CR6 Measurement and Control System - Page 362
Error from Wire Resistance, Error from Wire Resistance - 1000 Ft Cable, Error from Wire, Resistance
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Section 8. Operation • Temperature coefficient of the bridge resistors • Thermistor interchangeability • Error of the Steinhart-Hart equation or RTD coefficients • Cable resistance Of these, only errors induced by cable resistance have practical remedies. Higher temperatures and longer cables increase cable resistance. While cable resistance has little effect on the vibrating-wire measurement, it can cause significant errors in the temperature measurement. The following suggestions can minimize cable resistance: • Calibrate the RTD with a single-point offset. • Keep cable length as short as possible. • Shield the cable from solar heating. Putting it in buried conduit is one way to do this. • Purchase sensors with larger wire gage. Cable resistance primarily induces offset error, so a single-point calibration in an ice bath prior to deployment will minimize the offset. To illustrate the effects of cable length and cable temperature, the four figures Error from Wire Resistance (p. 362), Error from Wire Resistance - 1000 Ft Cable (p. 363), Error from Wire Resistance - 3000 Ft Cable (p. 363), Error from Wire Resistance - 5000 Ft Cable (p. 364) characterize the response of the thermistor found in a Geokon 4500 piezometer. Figure 103. Error from thermistor-wire resistance. Computed for a two-wire thermistor embedded in a vibrating-wire sensor. Thermistor lead-wire resistance is 16 Ω per 1000 feet; size is 22 AWG. Shows error increasing with cable temperature and length. 362
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