Arroyo Instruments TECPak 586 Series, User Manual, Page: 23 / 40

586 TECPak User’s Manual · Page 23
The TECPak supports operation using a 10
μ
A or 100
μ
A constant current
source, which limits the upper control range to 450k
Ω and
45k
Ω,
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 100
μ A, but your application will determine the actual
needs.
The Steinhart-Hart Equation
As can 6be 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
R C R B A
T
  
The coefficients A, B, and C can usually be obtained from the thermistor
manufacturer. The
TECPak 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 using the TEC:CONST command, or through the
Settings… dialog within Arroyo Control .
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