Campbell CR3000 Micrologger, Руководство оператора

Страница: 297 / 644

Section 7. Installation
297
Resistance() Four-Wire Full-Bridge Bridge-Resistance (RB) Values
RS
–40
RS
0
RS
10
RS
60
°C –40 0 10 60
RB 2551036 2554990 2555969 2560810
Procedure
1. Build circuit
1
:
a. Use FIGURE: PT100 Resistance() Four-Wire Full-Bridge Schematic
(p. 296)
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.
ii. Select a 1%, 10 ppm/°C resistor for R2 with a resistance that is
approximately equal to the resistance of the PRT at 10 °C. See
Procedure Information (PT100 Resistance() Full Bridge)
(p. 296).
Since a
103.9 Ω resistor is hard to find, use a 100 Ω resistor. It is close enough.
2. Wire circuit to datalogger:
Use FIGURE: PT100 Resistance() Four-Wire Full-Bridge Schematic as the
wiring diagram. Wire the current return wire to the current return terminal.
3. Calculate excitation current:
Use the following equation to calculate the best excitation current (IX) for the
measurement range –40 to 60 °C. The equation reduces the absolute result
by 1% to allow for resistor inaccuracy:
IX
max
= (VS
max
/ RB
min
) • 0.99
where,
VS
max
= 50 mV (maximum voltage in the ±
50 input range)
RB
min
= 2551036 mΩ (PT100 bridge at
–40 °C)
2
so,
IX
max
= 0.00196 mA