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A G R E AT E R M E A S U R E O F C O N F I D E N C E B. Keep the insulator clean--Oils and salts
from the skin can degrade insulator per-
formance, and contaminants in the air
can be deposited on the insulator surface,
reducing its resistance. Clean the insula-
tor with a cotton swab dipped in metha-
nol. After cleaning allow the insulator to
dry for several hours at low humidity be-
fore use, or dry it with dry nitrogen gas.
Problem: Errors C. Guard the insulator--Use a driven guard
that surrounds the insulator; a guard that
in Low Current
surrounds the entire high impedance area
is even better.
Measurements Potential Cause:
Meter Bias Current
Ammeters have some small current that
flows when the input is open.
One of the most common causes of error 3. Water Absorption--Leakage depen- Remedy
when measuring low currents (<1nA) is off- dent on the amount of water that has Choose an electrometer or picoammeter
set current, which can come from the test been absorbed by the insulator. to minimize meter bias current. Meter bias
setup or the measuring instrument. 4. Piezoelectric or stored charge ef- currents can be greatly reduced by enabling
fects--The creation of charge unbal- the instrument's Current Suppress feature
Potential Cause: ances (and thus current flow) or volt- with the input terminals disconnected and
Insulating Material age shift due to mechanical stress. the Zero Check switch open.
Current can leak through an insulating 5. Triboelectric effects--The creation of
material or over its surface. The insulating charge unbalance due to frictional ef-
material may itself store or generate charge. fects when materials rub against each Potential Cause:
other. Detector Dark Current
Remedies 6. Dielectric Absorption--The tendency Photomultiplier tubes usually have a
A. Choose a good insulator--Several prop- of an insulator to store/release charge small current flowing even when they are not
erties are important when evaluating an over long periods of time. illuminated.
insulator material:
1. Volume Resistivity--Leakage of cur- For a listing of common insulating ma- Remedy
rent directly through the material. terials and their characteristics, see the This current can be cancelled out from
2. Surface Resistivity--Leakage across Keithley Low Level Measurements hand- the measurement by using the built-in zero
the surface, a function primarily of book, Section 2.2.2. suppression feature of the picoammeter or
surface contaminants. electrometer.
Specifications are subject to change without notice.
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