Arroyo Instruments TECPak 585 Series, User Manual

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TECPak User’s Manual · Page 21
Working With RTDs
Like thermistors, RTDs also function by converting temperature into resistance,
but unlike thermistors, RTDs increase in resistance as temperature increases.
RTDs are also a fairly linear device, meaning they can be used across a much
broader temperature control range.
According to IEC751, the resistance/temperature relationship is determined
using one of two equations, dependent on the temperature or resistance value
being measured. For resistances above the R
0
value (resistance at 0°C, typically
100
Ω
) of the RTD, the following equation is used:
) 1 (
2
0
BT AT R R
  
Below R
0
, an additional term is added to the equation:
] ) 100 ( 1 [
3 2
0
T T C BT AT R R
    
In both of these equations, R
0
is the resistance of the RTD at 0°C, and A, B, and
C are the coefficients as defined by IEC751, through regression analysis, or by
using the Callendar-van Dusen method. By default, the TECSource uses the
Laboratory standard coefficients, which are for a 0.003926
Ω / Ω /°C curve (A =
3.9848x10 -3 , B = -0.58700x10 -6 , C = 4.0000x10 -12 , and R
0
= 100).
The
TECPak automatically determines which equation to use based on the
conditions at the time of the measurement.