System and method for temperature measurement
View Patent ↗A method for precision thermal measurement and control, especially for bioreactors, as well as the correction of temperature sensitive probes such as pH and dissolved oxygen. Typical control requirements are +/−0.1° C. The thermal measurement circuit converts a sensor output to a high level voltage or current with great accuracy and provides noise immunity and sensor isolation. While digital outputs from sensor converters can have the greatest noise immunity, the noise associated with digital circuitry may contaminate low level sensor signals so in many cases an analog sensor converter is preferred because of low noise generation, especially if the converter is near the sensor. The circuit is low cost, reliable, generates minimal heat is immune to, and does not generate noise, and requires minimal calibration effort.
1. An analog temperature measurement circuit comprising:
i. a thermistor,
ii. a filtering capacitor in shunt with said thermistor,
iii. a linearizing resistor Rs in series with said thermistor, said linearizing resistor having a non-zero resistance which resistance is selected using a numerical optimization method,
iv. a reference voltage source which sets the maximum R T voltage across said thermistor
v. a chopper stabilized operational amplifier in series with said reference voltage source, and in parallel with
vi. a feedback resistor.
2. The circuit of claim 1 wherein the resistance of linearizing resistor Rs is selected to minimize self heating of the thermistor whose minimum resistance is R Tmin by limiting the voltage (V) across the thermistor such that V/R Tin is less than the maximum power dissipation of the thermistor.
3. A method of calibrating the circuit of claim 1 to correct measured deviations in the resistance of linearizing resistor Rs from its nominal value utilizing perturbation analysis in conjunction with resistors of known accuracy and the thermistor in said circuit is calibrated using a Steinhart-Hart curve fitting method using a three point calibration to correct determined errors in the Steinhart-Hart coefficients.