Campbell CR3000 Micrologger, Operator'S Manual

Page: 281 / 644

Section 7. Installation
281
Notes
•
Why use four-wire half-bridge?
Use a four-wire half-bridge when lead resistance is more than a few
thousandths of an ohm, such as occurs with long lead lengths.
•
Why use 10 kΩ series resistor
?
Referring to figure PT100 BrHalf4W() Four-Wire Half-Bridge Schematic
(p.
277),
the 10 kΩ series resistor allows the use of a higher
-excitation voltage and
a low analog voltage input range.
•
Why use high excitation and low range?
High excitation and low range minimize the effects of signal noise.
•
Why use a bridge resistor near value of PT100?
By using a bridge resistor (Rf) that is close in value to that of the PT100
(RS), the differential measurement of V2 (voltage drop across PRT) can be
made on the same range as the differential measurement of V1 (voltage drop
across Rf). Using the same range eliminates range translation errors that can
arise from variances in the 0.01% range translation resistors internal to the
CR3000.
7.7.17.4
Example: 100 Ω PRT in Three
-Wire Half Bridge with Voltage
Excitation (PT100 / BrHalf3W() )
FIGURE 68: PT100 BrHalf3W() Three-Wire Half-Bridge Schematic
Procedure Information
BrHalf3W() Three-Wire Half-Bridge Equations
X = RS / Rf
RS = Rf • X
VX = VS/(RS/(Rf + RS))