Abstract A cell for measuring the power factor of oil including a guard electrode, a center electrode and an outer electrode, all of which are adapted to be immersed in a test sample of the oil. The center electrode and the outer electrode of the cell are adapted to be selectively connected in parallel with a variable impedance arm of a bridge that is energized by a suitable a.c. source. The output of the bridge is taken on two conductors which are connected to the dual inputs of a differential amplifier that is used to drive an output meter. The bridge is initially balanced with the cell disconnected. When the cell and sample are connected to the bridge the meter reading reflects the total current I IC + IX flowing between the cell electrodes. The bridge is then readjusted so that a null indication is provided at the meter. The ratio of the meter reading with the meter nulled to the reading I above is proportional to the power factor of the cell oil. In one embodiment the guard electrode of the cell is grounded, and in an alternative embodiment the guard electrode is driven at the same potential as the center electrode by an operational amplifier arranged in a voltage follower configuration. The connections between the cell and the bridge can be made by a triaxial cable so that power factor readings on samples can be made in a remote chamber at other than ambient temperature.
An arrangement for measuring power factor of an oil sample comprising a cell having a center electrode, a guard electrode and an outer electrode, said electrodes adapted to be immersed in the oil sample; a measuring circuit associated with the cell including a bridge having a pair of input terminals, adapted to be energized by an a.c. source, and a pair of output terminals electrically interconnected to the input terminals by a pair of variable impedance arms and a pair of fixed impedance arms, a dual input differential amplifier, the input of said amplifier electrically connected to the output terminals of said bridge, and a meter connected to the output terminal of said differential amplifier; and circuit means for selectively connecting the center electrode of the cell and the outer electrode in electrical parallel with one of the variable impedance arms of the bridge.
Claim 2
Dependent Claim
144 words
An arrangement for measuring power factor of an oil sample comprising a cell having a center electrode, a guard electrode and an outer electrode, said electrodes adapted to be immersed in the oil sample; a measuring circuit associated with the cell including a bridge having a pair of input terminals, adapted to be energized by an a.c. source, and a pair of output terminals electrically interconnected to the input terminals by a pair of variable impedance arms and a pair of fixed impedance arms, a dual input differential amplifier, the input of said amplifier electrically connected to the output terminals of said bridge, and a meter connected to the output terminal of said differential amplifier; and circuit means for selectively connecting the center electrode of the cell and the outer electrode in electrical parallel with one of the variable impedance arms of the bridge.
Claim 3
Dependent Claim
13 words
The arrangement of claim 1 wherein said circuit means comprises a triaxial cable.
Claim 4
Dependent Claim
13 words
The arrangement of claim 2 wherein the guard electrode is connected to ground.
Claim 5
Dependent Claim
57 words
The arrangement of claim 1 further including a dual input operational amplifier, a feedback resistor connected between the operational amplifier output and one input thereof, means connecting the other input of the operational amplifier to the center electrode of the cell and means connecting the output of the operational amplifier to the guard electrode of the cell.
Claim 6
Dependent Claim
15 words
The arrangement of claim 2 wherein the meter is directly calibrated in power factor units.