Section 7. Installation
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. 262) 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
= 25 mV (maximum voltage in the ±25 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
= 2053 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. 264) into the CR800. 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.
d. Calculate RS
0
RS
0
= X
0
• Rf = 100000 mΩ
Wow! We are lucky to have a perfect PRT! In the real world, PRT
resistance at 0 °C will probably land on either side of 100 Ω.
5. Measure the sensor:
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