Arroyo Instruments TECSource 5305, User Manual

Page: 24 / 44

Page 24 · 5300 Series TECSource User’s Manual
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 TECSource automatically determines which equation to use based on the
conditions at the time of the measurement.