Arroyo Instruments TECSource 5400 Series - Working With RTD Sensors; RTD Resistance Equations

5400 Series TECSource User’s Manual · Page 35
temperature range. The typical setting is 100μA, but your application will
determine the actual needs.
The Steinhart-Hart Equation
As can be seen from the temperature versus resistance graph above, resistance
varies inversely with temperature in a non-linear fashion. This relationship can be
accurately modeled by polynomial equations, and one such being the Steinhart-
Hart equation:
3
)ln(*)ln(*
1
RCRBA
T

The coefficients A, B, and C can usually be obtained from the thermistor
manufacturer. The TECSource defaults to the coefficients for the BetaTHERM
10K3A1 thermistor (A = 1.12924x10
-3
, B = 2.34108x10
-4
, C = 0.87755x10
-7
). You
can change the coefficients under the Main Sensor Menu and Aux Sensor
Menu.
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
100Ω) of the RTD, the following equation is used:
)1(
2
0
BTATRR 
Below R
0
, an additional term is added to the equation:
])100(1[
32
0
TTCBTATRR 
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 0.003926Ω/ Ω/°C curve (A =
3.9848x10
-3
, B = -0.58700x10
-6
, C = 4.0000x10
-12
, and R
0
= 100).