Intab PC-Logger 3100i, Как пользоваться инструкцией, Страница: 38 / 49

INTAB Interface-Teknik AB
38
1.1 Temperature Measurements with Pt-100 Sensors
A Pt-100 is a resistance temperature sensor or RTD (Resistance Temperature
Detector). Its resistance changes with temperature. By measuring resistance
using the four-wire technique described above, it is easily converted to
temperature. The conversion can be either a linear transform (y=kx+l) or for
more accurate conversions a polynomial.
One linear transformation can be defined as the line that passes through 0°C at
100
Ω
and f. ex. 266,4°C at 200
Ω
. In other words: 50mV corresponds to 0°C and
100mV corresponds to 266.4°C
(I=0.5mA)
If you want to transform resistance into temperature you can of course use this
equation: Rt=Ro(1+At+Bt
2
)
where Ro ~ 100
A ~ 3.9083 x 10
-3
B ~ -4.183 x 10
-7
Concerning accuracy of the ”3100i” when performing Pt-100
measurements, the following should be noted:
In four-wire techniques total uncertainty is the sum of ”3100i” analog channel
uncertainty and constant current uncertainty. The analog inputs have an
uncertainty of +/- 300ppm. Using the 100mV range and reading 50mV, this
translates to about 15
µ
V. The constant current sources have an uncertainty of +/-
0.5
µ
A. At a resistance of 100
Ω
this corresponds to 100
Ω
* 0.5
µ
A = 0.050 mV.
A Pt-100 sensor changes its resistance from 100.0
Ω
to 100.39
Ω
as a result of a
temperature change from 0°C to 1°C.
As the constant current is 0.5mA this corresponds to:
0.39
Ω
* 0.5mA = 0.195mV. Therefore:
•
uncertainty due to input is 0.015/0.195 = 0.08°C.
•
uncertainty due to constant current is 0.050 / 0.195 = 0.26°C.
Total uncertainty can therefore, if you are unlucky, add up to 0.34°C. ( ”3100i”
@ 25°C +/- 10°C)
Don’t forget that Pt-100 sensors by no means are perfect.
A class B Pt-100 has an uncertainty of +/-0.8°C at 100°C and
a class A Pt-100 has an uncertainty of +/-0.35°C at 100°C.
These possible errors have to be added to the ”3100i” uncertainty to get a
reliable estimate of total system uncertainty.