Question

Problem 3 (25 pts.) A J-type thermocouple is calibrated against an RTD standard to within 10.02°C between Oº and 200°C. The emf is measured with an A/D converter having a resolution of 0.075 mV. The reference junction temperature is provided by an ice bath. The calibration procedure yields the following linear equation: T(°C) = 0.11 + 19.2V where the units of voltage Vis mV. This first order curve fit has an error of 0.5°C. Part a) 15 pts. Find the uncertainty in the temperature measurement within the range of temperature values. Part b) 10 pts. Suppose the ice bath has melted and the reference junction is now at room temperature of 25°C. What is the temperature sensed by the thermocouple if the emf at the measuring junction is 4 mV?

Answer 1

1. The uncertainity of measurement is range of values in which the true value lies from the measured value.
It is the sure range in which the true reading lies. All precise measurement instruments are callibrated and measurement uncertainity is given. It is given in various different ways like measure of extreme possible error, measure of minimum readable value etc.

For uncertainity of temperature measurement, the RTD reading is ±0.02°C and curve fit has error of 0.5°C.

Hence, max possible error = +0.02 + 0.5 = 0.52°C
Which means if thermocouple reads value of 100°C, the actual temperature will be 100±0.52 °C.

Thus, the uncertainty is ±0.52 °C. .........Answer 1


2. T = 0.11 + 19.2V
Given that V = 4mV = 0.004 V

T = 0.11 + 19.2 * 0.004  = 0.1868 °C

Thus, the temperature sensed by thermocouple will be 0.1868°C ..........Answer 2


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