Campbell CR6 Series, Operator'S Manual

Page: 338 / 690

Section 7. Installation
338
Notes
•
The three-wire half-bridge compensates for lead-wire resistance by
assuming that the resistance of wire a is the same as the resistance of
wire b (see FIGURE: PT100 BrHalf3W() Three-Wire Half-Bridge
Schematic
(p. 334))
. The maximum difference expected in wire resistance
is 2%, but is more likely to be on the order of 1%.
• The average resistance of 22 AWG wire is 16.5 Ω per 1000 feet, which
would give 500 ft lead wires (for example) a nominal resistance of 8.3 Ω.
Two percent of 8.3 Ω is 0.17 Ω. Assuming that the greater resistance is
in wire
b
, the resistance measured for the PRT in the ice bath (RS0) is
100.17 Ω, and the resistance at 40 °C (RS) is 115.71 Ω.
• At 40 °C, because of the error from wire b , the measured ratio RS/RS0 is
1.1551 while the ratio without the error would be 115.54/100 = 1.1554.
As a result, the temperature computed by PRTCalc() from the ratio with
the error is about 0.43 °C higher than the temperature measured without
the error from wire b . This source of error does not exist in a four-wire
half-bridge configuration.
7.7.17.5 Example: 100 Ω PRT in Four-Wire Full Bridge with Voltage
Excitation (PT100 / BrFull() )
FIGURE 73: PT100 BrFull() Four-Wire Full-Bridge Schematic
Procedure
1. Build circuit
1
:
a. Use FIGURE: PT100 BrFull() Four-Wire Full-Bridge Schematic
(p. 338)
as a
template.
b. Choose a 1%, 10 ppm/°C, 5000000 Ω (5 kΩ) resistors for R1 and R4
c. Balance the bridge.
i. Find the midpoint of the temperature range. The range of –40 to 60
°C is selected for this procedure, so the midpoint is 10 °C.