Arroyo Instruments ComboSource 6300 Series - Working with Rtds

6300 Series ComboSource User’s Manual · Page 39
application temperature, which varies greatly by temperature and thermistor
types.
The ComboSource supports operation using a 10μA or 100μA constant current
source, which limits the upper control range to 450kΩ or 45kΩ, respectively. To
minimize noise and maximize stability, you should select highest current while
still allowing you full operation across your required 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 ComboSource 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 Sensor Coeffs 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