Channel for
heating element
Insert location for RTD
probe (same as on
opposite side)
Explosion-proof plugs
(shown enlarged)
Figure 1-7
Location of Temperature Probes and Heaters in Airless Oven
The heating elements are enclosed in hollow channels running diagonally down the side of
the aluminum casing. The heating elements are inserted in the channels from the lower front,
and the wires protrude out the top. The probes are inserted from the top as shown in the photo
above. During operation, the open ends of the heating element channels are equipped with
screw-in, explosion-proof plugs. These plugs must be fully screwed in so that the threads are
fully engaged.
The temperature control path of the airless oven is shown below.
PECM
Airless Oven
Heater
Temp cntl
PWM
I
2
C
Overtemperature Shutdown
AC
Line
SSR
a
SSR
b
SYSC
O
N
OT = Overtemperature shutdown
PWM = Pulse-width modulation
OT
Setpoint
Resistor
Compare
Analog
Overtemp RTD
Temp Control RTD
Analog
Oven
setpoint
in
software
Figure 1-8
Airless Oven Heater Control Loop
Two relays are used to control the power to the heater. One relay powers the heater to maintain
the set point temperature of the heater (oven temperature) in the software. The second relay
is connected in series with the first relay and is used for overtemperature shutdown. Because
the overtemperature shutdown sensor should not activate unless there is a mechanical failure
of the set point temperature, its related control circuit or its relay, the power to the heater cannot
be reactivated without powering down and restarting the analyzer. Note that the
overtemperature shutdown can be triggered by incorrectly entering a setpoint temperature that
is too high, such as 250°C when the OT setpoint is 200°C.
Oven Types
1.3 Isothermal Airless Oven Heater System
Maxum II Valves and Oven Components
14
Service Manual, 10/2018, A5E42019844001
