Hioki DM7275 Series - Page 186

Appx.6
Causes of Error in Voltage Measurement
Thermal electromotive force relative to copper
Metal
Thermal electromotive
force(μV/°C)
Nickel -22.4
Platinum -7.6
Aluminum -3.4
Lead -3.2
Brass -1.6
Carbon -0.6
Silver -0.2
Zinc 0
Copper 0
Gold 0.2
Iron 12.2
Use a metal with a positive value for contacts facing copper and a metal with a negative value for the opposite
side. (Chronological Scientic Table, 2006 Edition)
Effects of input resistance
When the measurement target has a large output resistance, measured values will be attenuated
by the instrument’s input resistance. Caution is particularly warranted when selecting the 100 V
range or the 1000 V range, or when xing the input resistance to
10M
Ω
for the 100 mV range to
the 10 V range.
Example: Measuring a coin battery with an open voltage of 3 V with the input resistance set to
10M
Ω
and a measurement target output resistance of 1 k
Ω
V
9997.23
k1M10
M10
=×
Ω+Ω
Ω
V
R
OUT
R
IN
V
V
RR
R
INOUT
IN
+
Output resistance
Voltage
Input resistance
Figure. Effects of Input Resistance
Effects of bias current
A miniscule current ows to the instrument’s input terminal. This current, which is needed in order
to drive the instrument’s measurement circuit, is known as a bias current. When the measurement
target has a large output resistance, the measurement error caused by the bias current will
increase in magnitude.
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