Electric circuits
AssigneeRosemount Engineering Company Limited
InventorJames Stewart Johnston
Life of this patent
Estimating the term…
Abstract
An electric circuit includes three matched resistance thermometer sensors and is responsive to changes in rate of heat loss from at least one of them. The sensors are connected in a series-parallel arrangement to form one arm of a bridge circuit. An amplifier detects imbalance in the bridge and controls current flowing in the bridge to restore balance. Twice as much current flows in one sensor than in the other two, so that the one is maintained hotter. Environmental changes effecting a change in the rate of heat loss from the one sensor are detected as a change in current flowing in the bridge.
Claims15
An electric circuit responsive to changes in the rate of heat loss from a resistance thermometer sensor, the circuit comprising a bridge circuit having first and second arms connected in parallel, the first arm comprising a first resistance thermometer sensor and second and third resistance thermometer sensors connected in parallel with each other and connected at a first balance point in series with said first resistance thermometer sensor, the three resistance thermometer sensors having closely matched values, the second arm comprising first and second resistances connected in series at a second balance point, and a differential amplifier having its inputs connected between the first and second balance points and having its output connected to control current flow in the bridge circuit, the arrangement being such that the differential amplifier is operative to balance the bridge circuit with a finite current flowing in said resistance thermometer sensors.
Apparatus for measuring fluid flow rate comprising an electric responsive to changes in the rate of heat loss from a resistance thermometer sensor, the circuit comprising a bridge circuit having first and second arms connected in parallel, the first arm comprising a first resistance thermometer sensor and second and third resistance thermometer sensors connected in parallel with each other and connected at a first balance point in series with said first resistance thermometer sensor, the three resistance thermometer sensors having closely matched values, the second arm comprising first and second resistances connected in series at a second balance point, and a differential amplifier having its inputs connected between the first and second balance points and having its output connected to control current flow in the bridge circuit, the arrangement being such that the differential amplifier is operative to balance the bridge circuit with a finite current flowing in said resistance thermometer sensors, said first resistance having a value fractionally greater than twice the value of said second resistance, and in combination with said electric circuit, means for exposing the three resistance thermometer sensors correspondingly to a flowing fluid of which the rate of flow is to be measured.
Apparatus for determining changes in the thermal conductivity of a gas mixture due to changes in composition of the mixture, the apparatus comprising a circuit responsive to changes in the rate of heat loss from a resistance thermometer sensor, the circuit comprising a bridge circuit having first and second arms connected in parallel, the first arm comprising a first resistance thermometer sensor and second and third resistance thermometer sensors connected in parallel with each other and connected at a first balance point in series with said first resistance thermometer sensor, the three resistance thermometer sensors having closely matched values, the second arm comprising first and second resistances connected in series at a second balance point, and a differential amplifier having its inputs connected between the first and second balance points and having its output connected to control current flow in the bridge circuit, the arrangement being such that the differential amplifier is operative to balance the bridge circuit with a finite current flowing in said resistance thermometer sensors, and a third resistance in series with said parallel connected second and third resistance thermometer sensors, the value of said third resistance being a fraction of the ice point resistance value of said second and third resistance thermometer sensors connected in parallel, and in combination with said electric circuit, means for exposing the three resistance thermometer sensors correspondingly to the gas, the values of the first and third resistances being selected to minimise the effect of the temperature coefficient of the thermal conductivity of the gas mixture.
Apparatus for determining the thermal conductivity of non-gaseous materials comprising an electric circuit responsive to changes in the rate of heat loss from a resistance thermometer sensor, the circuit comprising a bridge circuit having first and second arms connected in parallel, the first arm comprising a first resistance thermometer sensor and second and third resistance thermometer sensors connected in parallel with each other and connected at a first balance point in series with said first resistance thermometer sensor, the three resistance thermometer sensors having closely matched values, the second arm comprising first and second resistances connected in series at a second balance point, and a differential amplifier having its inputs connected between the first and second balance points and having its output connected to control current flow in the bridge circuit, the arrangement being such that the differential amplifier is operative to balance the bridge circuit with a finite current flowing in said resistance thermometer sensors, and in combination with the electric circuit and elongate probe comprising a hand grip portion and a stem portion extending from the hand grip and having a pointed steel tip, the three resistance thermometer sensors being mounted flush with the surface of the stem near the tip.
Dependencies are inferred from the claim text.