Campbell CR800 Series - Thermocouple Measurements - Details

Section 8. Operation
331
where
percent-of-reading = 2500 mV • ±0.06%
= ±1.5 mV
and
offset = (1.5 • 667 µV) + 1 µV
= 1.00 mV
Therefore,
accuracy = ±1.5 mV + 1.00 mV
= ±2.5 mV
Electronic Noise
Electronic "noise" can cause significant error in a voltage measurement,
especially when measuring voltages less than 200 mV. So long as input
limitations are observed, the PGIA ignores voltages, including noise, that are
common to each side of a differential-input pair. This is the common-mode
voltage. Ignoring (rejecting or canceling) the common-mode voltage is an
essential feature of the differential input configuration that improves voltage
measurements.
Figure PGIA with Input Signal Decomposition
(p. 348), illustrates the common-
mode component (V
cm
) and the differential-mode component (V
dm
) of a voltage
signal. V
cm
is the average of the voltages on the V+ and V– inputs. So, V
cm
=
(V+ + V–)/2 or the voltage remaining on the inputs when V
dm
= 0. The total
voltage on the V+ and V– inputs is given as V+ = V
cm
+ V
dm
/2, and V
L
= V
cm
–
V
dm
/2, respectively.
8.1.2.2 Thermocouple Measurements — Details
Thermocouple measurements are special case voltage measurements.
Note Thermocouples are inexpensive and easy to use. However, despite
the use of a thermocouple in Quickstart (p. 35), the CR800 is not designed
for accurate measurement when thermocouples are attached directly to
the wiring panel.
CR800 design features that cause thermocouple measurement inaccuracy include:
• lack of an insulating wiring-terminal cover.
• no high-thermal mass element incorporated in the wiring panel.