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.
