Temperature measuring circuit in a flowmeter
A value of a line-to-line resistor ( 24 ) is determined by subtracting a resistance value determined as a product of a voltage ratio based on divided voltages and a value of a reference resistor in a non-conductive state between a second electric wire ( 26 ) and a third electric wire ( 27 ) from a resistance value determined as a product of a voltage ratio based on the divided voltages and the value of the reference resistor in a conductive state therebetween. Once the value of the line-to-line resistor ( 24 ) is determined, it becomes possible to determine a compensated resistance value related to temperature.
1. A temperature measuring circuit in a flowmeter, comprising:
first and second resistor-type temperature sensors which are provided at first and second measuring positions in the flowmeter and form a serial connection state when one ends of the sensors are connected to each other;
a first electric wire which includes a line-to-line resistor and is connected to another end of the first resistor-type temperature sensor;
a voltage reference connected to the first electric wire through a power supply resistor;
a second electric wire which includes a line-to-line resistor and is connected to the one ends of the first and second resistor-type temperature sensors;
a third electric wire which includes a line-to-line resistor and is connected to another end of the second resistor-type temperature sensor;
a reference resistor in which one end thereof is connected to the third electric wire and another end side thereof is grounded;
a switching device that is connected to both of the second electric wire and the third electric wire and turns those electric wires to a conductive state or a non-conductive state;
a multiplexer that selects any one of the first electric wire, the second electric wire, the third electric wire, and the grounded side of the reference resistor;
an A/D converter that performs A/D conversion on divided voltages selected by the multiplexer; and
an arithmetic operation/control device that controls the switching device and receives a signal from the A/D converter,
wherein the arithmetic operation/control device determines a value of the line-to-line resistor by subtracting a resistance value determined as a product of a voltage ratio based on the divided voltages and a value of the reference resistor in the non-conductive state between the second electric wire and the third electric wire from a resistance value determined as a product of a voltage ratio based on the divided voltages and a value of the reference resistor in the conductive state therebetween.
2. A temperature measuring circuit in a flowmeter according to claim 1 , wherein the determined value of the line-to-line resistor is corrected by a hyperbolic approximate expression related to the second resistor-type temperature sensor, to determine a corrected value of the line-to-line resistor.