testing HP 4277A

[Physikfan]

To all electronics freaks

After using the capacitance meter HP 4277A not exactly successful
to measure the self-capacitance of three electrometers,
I have checked the HP 4277A with a high-voltage air rotary capacitor with two independent capacity ranges.
Here is the high-voltage air rotary capacitor, turned in or
turned out:






The HP 4277 A allows measurement frequencies from 10 kHz to 1 MHz.
I therefore have measured both capacity ranges at 10 kHz and 1 MHz,
fully turned in or fully turned out, all values ​​in pF.

10 kHz, large capacity range, fully turned in: 145.08
1 MHz, large capacity range, fully turned in: 145.49

10 kHz, large capacity range, fully turned: 43.30
1 MHz, large capacity range, fully turned out: 43.78

10 kHz, small capacity range, fully turned in: 54.38
1 MHz, small capacity range, fully turned in: 54.24

10 kHz, small capacity range, fully turned out: 22.68
1 MHz, small capacity range, fully turned out: 22.63

The HP 4277 A seems to work fine with the air condenser, the differences at 10 kHz and 1 MHz are very small.

[precaud]

Nice. I like the 4277A, and use it's sibling 4276A a lot. Wish I knew 6502 assembler, I'd love to remove the display range limitations. There are workarounds via GPIB, but still...

[Conrad Hoffman]

I don't know about directly measuring the capacitance of an electrometer. I've done it by charging a known capacitor (small pF size) to a known voltage and then measuring it with the electrometer. The voltage drop tells you the capacitance. Not sure how to do it at a high frequency.

[Physikfan]

This could work, but as the time constant is defined by
 Tau = R * C
you have to know the insulation resistance of the electrometer.

[TimFox]

A quick look at the manual for the HP 4277A shows that the DUT is connected to four coax terminals, in a popular configuration for measuring components in suitable test fixtures.  Grounding one side of the test circuit does not seem to be suitable for this unit.  Your air variable capacitors were presumably floating, but the electrometer input capacitance is probably grounded.

[Conrad Hoffman]

This could work, but as the time constant is defined by
 Tau = R * C
you have to know the insulation resistance of the electrometer.

Well, it's an electrometer so the insulation resistance better be impressively high!

[dl6lr]

This could work, but as the time constant is defined by
 Tau = R * C
you have to know the insulation resistance of the electrometer.

No, you have to make the measurement immediately (t approx. = 0). As you distribute a known charge Q1 from a known capacitor C1 with voltage U1 to an unknown capacitor Cx+C1, you can calculate Cx from the voltage Ux shown by the electrometer (assuming the electrometer is exact) with Cx+C1=Q1/Ux. You would perhaps use a spherical capacitor for this.
Example: Known capacitor with 5pF at 20kV: Q1=100nC. If you measure Ux=7.5kV when bringing it to the electrometer, the capacitance of the electrometer is Cx=Q/Ux-C1 = 8.333pF.

[jonpaul]

Confusing that you have this thread and " measuring small capacities around 1 pF" for the same topic.

The moderator should combine these two topics  into one eg " HV Electrometer capacitance and leakage"

Jon