Campbell CR3000 Micrologger, Руководство оператора

Страница: 278 / 644

Section 7. Installation
278
Procedure
1. Build circuit
1
:
a. Use FIGURE: PT100 BrHalf4W() Four-Wire Half-Bridge Schematic
(p. 277)
as a template.
b. Rf should approximately equal the resistance of the PT100 at 0 °C. Use a
1%, 10 ppm/°C resistor.
2. Wire circuit to datalogger:
Use FIGURE: PT100 BrHalf4W() Four-Wire Half-Bridge Schematic
(p. 277)
as
the wiring diagram.
3. Calculate excitation voltage
2
:
Use the following equation to calculate the best excitation voltage (VX) for
the measurement range –40 to 60 °C. The equation reduces the absolute
result by 1% to allow for resistor inaccuracy:
VX
max
= (VS
max
• (Rf + RS
max
+ R1) / RS
max
) • 0.99
where,
VS
max
= 50 mV (maximum voltage in the ±
50 mV input range)
Rf = 100000 mΩ (100 Ω)
R1 = 10000000 mΩ (10 kΩ)
RS
max
= 123240 mΩ (PT100 at 60 °C)
3
so,
VX
max
= 4160 mV
4. Calibrate the PT100:
If the PRT accuracy specification is good enough, and you trust it, assume
RS
0
= 100000 mΩ. Otherwise, do the following procedure:
a. Enter CRBasic EXAMPLE: PT100 BrHalf4W() Four-Wire Half-Bridge
Calibration
(p. 280)
into the CR3000. It is already programmed with the
excitation voltage from step 3.
b. Place the PRT in an ice bath (0 °C).
c. Measure the PRT. If you are doing a dry run, assume the result of
BrHalf4W()
= X
0
= 0.01000.