Chapter 3 Functional descriptions
48 SMSO-2-02.03- 15.07.03
3.2.13 Temperature sensor
The temperature sensor is an active measurement system to
control the temperature of the instant heater and the steam
generator. It measures the temperature of these elements
almost at its highest point.
Function
The temperature sensor is based on the principle of a change
in resistance of a current-conducting wire when the tempera-
ture changes. It is an active measurement circuit.
The resistance of the measurement loop decreases as the
temperature increases; this is known as NTC resistance (NTC
= negative temperature coefficient).
This change in resistance is constantly analysed by the unit
control system and when the limit values set in the heater
control system are reached, is converted into switch on or
switch off commands to the heaters (relays). The conductor
loop is embedded, insulated, into a stainless steel tube sealed
into a union nut at the end. The 2-wire measurement lead has
a connector that is plugged into the cable loom with the corre-
sponding counter-piece.
Temperature sensor
1 Sensor tube (stainless steel)
2 Measurement loop (NTC resistance wire)
3 Sealing compound (epoxy resin)
4 Clamping ring
5 Union nut
REPAIR
After a safety thermostat has been trig-
gered, the temperature sensor must be re-
placed.
Technical data
Maintenance
Most frequent faults
Malfunction as a result of too high transition resistance in
plug connectors etc.
Calcium deposit: Prevents heat conduction and causes
the system to overheat, thus causing a safety thermostat
to trigger.
Check
Service technicians are not able to test the temperature sen-
sor with a sufficient degree of accuracy. Therefore, if you
suspect that the temperature sensor is malfunctioning, you
should replace it. Check for calcium deposits: dismantle and
decalcify if necessary.
Service intervals
When you are making a visual check of the interior of the
instant heater and the steam generator you need to dismantle
the temperature sensor. If it is not water-proof when you refit
it, you can insert a copper seal (part no. 068423).
Drawing symbol
Illustration
Temperature [
o
C ]
Resistance [kΩ]
20 12.565
25 10.000
80 1.242
84 1.095
90 0.910
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