Appendix A Sensor calibration
Nominal Resistance at 25˚C
All temperature sensors used by Heatmaster are NTC temperature sensors (NTC resistors –
N
egative
T
emperature
C
oefficient). The sensors are simply made of one resistor, which alters under
the influence of measuring physical values (in this case the temperature). The value is given in Ohm
and can be adapted to specific manufacturer’s instructions. Frequently the value is given by 25˚C or
can be taken out from the tables of data specifications or the curve chart.
Sensor calibration
The more accurate you want to measure, more operative points to be considered. In this case it is
suggestive to calibrate sensors individually.
The calibration can be done by electrical Ohm-meters, by which the Rn is calculated at 25˚C itself.
For that You need a calibrated reference thermometer and a medium like water in a container with
25˚ C of temperature. If the reference thermometer shows the 25˚ C water temperature, dip the
sensor, which has to be calibrated, in the water and notice the indicated value (in Ohm) after 30
seconds. Enter this value under the variable “Nominal Resistance at 25˚ C”. The input of the real
Resistance in Ohm enables the accurate measuring relatively within the scope of possibilities of
NTCs. 25˚ C is therefore a good average value, as this is close to the room temperature and
general measuring range. Rather distant “Reference points” from that could mean higher deviations
from the real temperature.
Reference thermometer calibration
If you have an accurate digital thermometer (more accurate than the normal NTC thermo sensors),
which allows calibration, you can follow these instructions:
Fill a container with ice cubes. Fill cold water in and stir it for about 1 minute. The water temperature
is now 0°C. Dip the digital thermometer, wait 30 seconds and setup 0°C in the digital thermometer.
Qualitative first-class sensors can be identified with the availability of respective information in data
specification. Further important values are tolerance data for nominal resistance, which can have
abso/- 1°C as well as tolerance of beta-value. Sensors with very good beta-tolerance-values
and tolerances provide more often higher accuracy and reliability in the whole working temperature-
scope.
Using a calibrated thermometer (e.g. bulb thermometer from laboratory) is more precise but also
more expensive.
Self calculating of the Beta-value
Normal NTC-sensors (for measuring air- and water temperature) of the PC-field don’t posses data
for beta-value. To intercept this disadvantage, some standard sensors are adjusted with different
measurements and formulas to counteract the nonlinearity of NTC-sensors and to enable accurate
measuring from the beginning. This leads to the default value of 3435 in the property “
Constant b
”.
If component-specific Beta value should be calculated, the Resistance at two different temperatures
should be measured and the beta-value can be calculated with the use of a formula. Three different
temperatures are much better.
Example 2: Temperature points measuring.
For the beta-value in the desired temperature range for e.g. from 20°C - 100°C, one reading should
be done at 20°C and the other at 100°C. With that other ranges such as 0°C and 80°C or more
precise closer ranges are viable.
NTC thermal resistors are nonlinear resistors, which change their resistance with the temperature.
The resistance decrease with the heat. How exact this nonlinear gradient looks is described in a
constant, which is popular as beta (b)-value in the thermal resistor industry. The value is calculated
as follows based on the formula given below.
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