measure the capacitance between a probe and a piece of metal

Does anyone know how to measure the capacitance between a probe and a piece of metal that is not connected to the measuring circuit?

can you be a bit more specific.

what probe are you using, the dimension of metal/metal sheet and the distance apart, this would give an idae of the capacitance range you are trying to measure

Re: measure the capacitance between a probe and a piece of m

Sorry for being so vague. The probe has an area of about 5.5 in^2. The sheet of metal is much larger than the probe so I guess it could be assumed as infinite. Below is a link to a device for a similar application.

https://www.hypertherm.com/languages/english/sensor_ohc.php

imho , the device in photo is connected to ground and and measures distance to the metal sheet which is also grounded or have much larger capacitance to ground .
so looks like it does make sensing of capacitance from probe to ground.

Re: measure the capacitance between a probe and a piece of m

I actually have this device at work and it does work with a piece of steel lying on carpet not connected to ground. When the device is on a machine the plate should be grounded threw the machine but the device itself does not connect to the plate otherwise.

Did you check its accuracy while sheet is on carpet ?
Anyway if it is pure capacitance measurement - it matter of physics - you need 2 sides to get capacitance between them . From other side - is the circle probe is one piece and is not split into 2 pieces ? I mean it is possible to have circle divided into 2 pieces of same size so sheet will be intermediate electrode of 2 capacitance formed by "first half circle - sheet" and "sheet - second half circle".

Re: measure the capacitance between a probe and a piece of m

Thanks for your help artem, you are probably right, I just figured that the plate being grounded threw the machine would be too noisy to be reliable. The probe is a one piece. Anyways does anyone know away to measure this capacitance?

I guess it is not simple capacitance measurement done there . The probe could be connected to ultrasound transducer to get differential vibrating capacitance which is function of distance between probe and surface. Just conventional measurement techniques wont work due to parasitic capacitance changing as instrument head will move . Look for Kelvin probe info on that .
http://www.kelvinprobe.info/technique/principles.html
http://en.wikipedia.org/wiki/Kelvin_probe_force_microscope

If you have a probe try to check whether i am right - put laser to its probing surface and amplify reflected wave by photo diode. If i am correct you should see modulated signal on photodiode amplifier output .

but i could be wrong as it says device uses capacitive ring measuring technique. I will try to find out what it is .

have a look to that :

http://www.analog.com/UploadedFiles/Application_Notes/52818682786599AN_833_0.pdf

though it is for different purpose but could give you idea . It could also be concentric rings which forms ring sensor .