On Nov 30, 3:16 pm, PeterD wrote:
On Fri, 30 Nov 2007 13:00:04 -0800 (PST), "



wrote:
On Nov 30, 10:28 am, (Gaetan Mailloux)
wrote:
Hello

Even without tests leads there is a .1 nf reading, I want to use it for
low value cap testing (5 to 50 pf), so it's kind of problematic.

Thank

Bye

Gaetan

"William Sommerwerck" )
writes:

Wen I use it to measure capacitors it do not do it's 0 before measuring,
even without any wires plug to it there is alway a .1 nf reading, there is
only two trimpots on the pc board, how can we reajust to have 0 ?

On the lower capacitance scales, it _shouldn't_ be zero. The test leads have
a certain amount of capacitance.

If you've removed the leads, a residual reading might be due to internal
capacitance, or the converter's LSB flopping.

In any case, if you're reading microfarads, 0.1nF (100pF) is hardly of any
concern.

Hi
For such low capacitances, there are is only one method that I've
found
that works well. Find or make a coil that when used with you caps to
be measure, will resonate in the 2 to 10 MHz range some place.
Use a few reference caps of known value and tolerance. These should
be close to the values being checked. Use a grid dip meter to measure
the frequency

A chain is only as strong as its weakest link... That grid dip meter
needs to be accurate, and you need to know the inductor's value
accurately as well.

As long as your reference parts are on the same scale of the grid dip
meter,
you should be able to ignore the coils actual value. One should be
able
to interpolate the grid dip scale. If more accuracy is needed, a
frequency
counter can be used. In which case, your readings would most likely be
as good as the reference parts.
Your not depending on the absolute accuracy of the meter or knowing
the
actual value of the coil, that is why you use reference parts.
Dwight



and back calculate the values of the caps.
Do remember that smaller values can be made by stacking ( or series
wiring ).
If you wind your own coil, try to leave at lease one or two wire
widths
between each turn. This will minimize the effects of internal
capacitance
of the coil. This is easiest done by winding wire two or three wires
and then removing the extras while a fixing glue is setting.
I do have a bridge that I get reasonable measurements for around 50
pf
but anything in the 5pf range is not reliable.