7.9.15.2.3 Choosing Rf
The fixed 100 Ω resistor must be thermally stable. Its precision is not important
because the exact resistance is incorporated, along with that of the PRT, into the
calibrated multiplier. The 10 ppm/°C temperature coefficient of the fixed resistor
will limit the error due to its change in resistance with temperature to less than
0.15 °C over the –10° to 40 °C temperature range. Because the measurement is
ratiometric (R
S
/R
f
), the properties of the 10 kΩ resistor do not affect the result.
A terminal-input module (TIM) can be used to complete the circuit shown in
figure PT100 in Four-Wire Half-Bridge
(p. 240). Refer to the appendix Signal
Conditioners
(p. 647) for information concerning available TIM modules.
Figure 63. PT100 in Four-Wire Half-Bridge
CRBasic Example 50. PT100 in Four-Wire Half-Bridge
'This program example demonstrates the measurement of a 100-ohm PRT using a four-wire half
'bridge. See FIGURE. PT100 in Four-Wire Half-Bridge
(p. 240) for the wiring diagram
Public Rs_Ro
Public Deg_C
BeginProg
Scan(1,Sec,0,0)
'BrHalf4W(Dest,Reps,Range1,Range2,DiffChan1,ExChan,MPS,Ex_mV,RevEx,RevDiff,
' Settling, Integration,Mult,Offset)
BrHalf4W(Rs_Ro,1,mV25,mV25,1,Vx1,1,2200,True,True,0,250,1.0111,0)
'PRTCalc(Destination,Reps,Source,PRTType,Mult,Offset)
PRTCalc(Deg_C,1,Rs_Ro,1,1.0,0) 'PRTType sets alpha
NextScan
240
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