Hi,
I have a following multimeter
Specification Sheet:
http://site.ldbepurchase.com/Manuals/VC99.pdf
Voltmeter:
http://www.amazon.com/Digital-Multimeter-Capacitance-Temperature-Frequency/dp/B0073C9ELO
I am trying to measure the DC offset voltage of Electrodes. The process is to connect two electrodes together with gel between them and use a voltmeter with 10Mohm input impedance and 1mV resolution. The Voltmeter while measuring should apply less than 10nA of bias current to the electrodes.
I do not understand how can I measure 10nA of current during this test. or How do I know that this meter will apply less than 10nA of bias current?
Electrodes
_______________________
__|____ Voltmeter
______
|_______________________
melissa
Firstly, a DMM cannot and will not "apply" any bias.
Any voltage read by the meter would be from potential differences
between the electrodes in the gel; the DC offset voltage of Electrodes.
I suggest you start with the DMM set to 6V full scale; that is the
equivalent of 0.6uA or 600nA full scale.
The most sensitive scale, 600mV will allow 60nA full scale and thus
allow reasonable resolution and accuracy.
Make the reading(s) with the meter connected one way, and the same
readings with the meter connected the other way; log and document all
readings.
You might see +12uA one way and -8uA the other way for an apparent
error of 4uA; use two electrodes of exactly the same PH and usage
history (as internal chemistry changes during use due to ion migration).
Using the above "example" for real-life readings, this test setup
with equal electrodes should then give +2uA one way and -2uA the other
way and thus a consistent experience.
Well, that would put the lie to the ASS-u-ME-mption that "a DMM
cannot and will not apply any bias".
There are some DMMs that _DO_ apply a bias to determine if test leads
are correctly used for a given scale (but they are what i call "fancy").
The datasheet specs seem to never give a direct clue that this can
happen.
One of my DMMs is a Protek 6500 that uses the equivalent of 40nA for
sensing meter probe use/misuse; that makes for a reading of +50nA on the
500uA scale with the leads plugged in and open - obviously ZERO current
is flowing from external source(s).
So..make some measurement in KNOWN conditions and document them.
For example, for your actual needs, set the meter to the 600mV scale
and NO leads plugged in; make a few dozen readings and log them. Better
be zero, darn close or average of zero with max + and max - readings
darn close to zero.
Next short the leads and repeat the measurements; you may have to
twist the leads together or get the meter away from AC wiring.
That gives you a "base" of confidence and a better knowledge of meter
limitations.
If you are an "electron pusher", use a fresh carbon-zinc AA cell and
build a voltage divider; say 100K in series with 1K and read voltage
across the 1K both polarities - should be close to 15.3mV which is a
reasonable reference point.
Again, make a log; draw the circuit with values.
If you want to get more picky, use the meter to measure the resistors
and the battery; use those values in the diagram; this makes for a
"calibration" scheme.
Know your equipment; draw the FULL circuit (eeach electrode as a
half-cell of X mV and resistance), gel resistance (even if initially
unknown; it ain't zero nor infinite), meter resistance, etc.
Have fun; you will wind up knowing more than the majority of us
electron pushers.