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Abstract

A measuring system for use in an emissions monitor and comprising a control mechanism (12) and a sensor mechanism (14). The control mechanism (12) means a differential current passing through an active resistive temperature device RTD (34) with respect to the current flowing through a reference RTD (36) when the sensor mechanism (14) is exposed to a particular gas of interest. The differential current is detected by control mechanism (12) via a four winding transformer (32) having an active winding (A), a reference winding (R), a sense winding (S), and a zero winding (Z). The current delivered to the active RTD (34) passes through the active winding (A) while the current passing through the reference RTD (36) passes through the reference winding (R) so as to produce a differential flux in the core of the transformer (32). The differential flux is detected by the sense winding (S) and is converted into a voltage signal by a current-to-voltage convertor (42). The voltage signal is subsequently converted by an analog-to-digital convertor (44) into a digital signal that is processed by a processor (48). The processor (48) applies a PID algorithm to the digital values in order to derive a control signal proportional to the concentration of the particular gas detected by the sensor mechanism (14). The control signal is combined with the excitation signal generated by the oscillator (20) at a multiplying DAC (28) in order to generate a precision zero signal which is sent to the zero winding (Z) in order to cancel the flux detected by sense winding (S).

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