Campbell Scientific CR10 CR10 Measurement and Control - Page 180

SECTION 13. CRlO MEASUREMENTS 0.5 0.4 3 0.3 _ac -(lLo 0.2 | 0.1 q, ofv a o-n1 O o-n1 L "'L LJ -0.4 -0.5 -40 -30 -20 -10 0 10 20 30 Actuol Temperoture (C) 50 60 FIGURE 13.4-1. Thermistor Polynomial Error When both junctions of a thermocouple are at the same temperature, there is no voltage produced (law of intermediate metals). A consequence of this is that a thermocouple cannot have an offset error; any deviation from a standard (assuming the wires are each homogeneous and no secondary junctions exist) is due to a deviation in slope. ln light of this, the fixed temperature limits of error (e.9., +1.0"C for type T as opposed to the slope error of 0.75o/o of the temperature) in the table above are probably greater than one would experience when considering temperatures in the environmental range. In other words, the reference junction, at 0"C, is relatively close to the temperature being measured, so the absolute error (the product of the temperature difference and the slope error) should be closer to the percentage error than the fixed error. Likewise, because thermocouple calibration error is a slope error, accuracy can be increased when the reference junction temperature is close to the measurement temperature. For the same reason, differential temperature measurements, over a small temperature gradient, can be extremely accurate. In order to quantitatively evaluate thermocouple error when the reference junction is not fixed at 0"C, one needs limits of error for the Seebeck coefficient (slope of thermocouple voltage vs. temperature curue) for the various thermocouples. Lacking this information, a reasonable approach is to apply the percentage errors, with perhaps O.25To added on, to the ditference in temperature being measured by the thermocouple. ACCURACY OF THE THERMOCOUPLE VOLTAGE MEASUREMENT The accuracy of a CR10 voltage measurement is specified as 0.2"h (0.1% 0 to 40'C) of the full scale range being used to make the measurement. The actualaccuracy may be better than this as it involves a slope error (the error is proportionalto the measurement being made though limited by the resolution). The error in the temperature due to inaccuracy in the measurement of the thermocouple voltage is worst at temperature extremes, where a relatively large scale is necessary to read the thermocouple output. For example, assume type K (chromel-alumel) thermocouples are used to measure temperatures at 600"C. The TC output is on the order of 24.9 mV, requiring the +25 mV input range. The accuracy specification ol 0.1"/" FSR is 25 uV which is a temperature error of about 0.60'C. In the environmental temperature range with voltage measured on an appropriate scale, error in temperature due to the voltage measurements is a few hundredths of a degree. 13-14