Chapter 7 – Maintenance
Page 7-10
TestEquity 1007H Temperature Chamber
Theory of Operation
Overview
The chamber is heated by an open element nichrome heater. Cooling is accomplished by a
cascade refrigeration system, which consists of two compressors. The air is circulated by a
propeller fan. The heater, evaporator (cooling coil), and fan are located within an air plenum
which is on the back wall of the chamber interior.
The heater, compressors, and circulator fan motor operate directly from the 208 or 230 VAC
input line. All line branch circuits are individually protected with Class CC fuses. A step-down
transformer provides 115 VAC for all instrumentation and control elements.
Refer to the electrical and refrigeration drawings to identify the referenced items described
below. The electrical items are referenced with a letter(s) and number. The refrigeration items
are referenced with a number only or number followed by a single letter.
Air Heating System
The chamber is heated by an open-element nichrome heater (HT1). The heater is located in the
air plenum. The F4T/F4 Controller provides a time-proportioned output to a solid state relay
(SSR1). This turns the heater on/off as required to maintain the temperature set point. Pilot light
PL1 provides an indication on the front panel when the heater is on.
A fusible heat limiter (HL) provides failsafe protection against a catastrophic failure by opening
the heater circuit at +240°C. The master heat contactor C1 provides a power interlock for the
heaters, circulator fan motor, and the control system. C1 is controlled by both the Master Switch
and safety relay (CR3). CR3 is controlled by the temperature limit controller (TCR2). If either
the high or low temperature safety limits are exceeded, TCR2 turns off CR3, which turns off C1.
Humidity System
Water is heated by an immersion-type heater (HT2) to generate vapor. The heater is located in
the humidifier tank. The F4T/F4 Controller provides a time-proportioned output to a solid state
relay (SSR2). This turns the heater on/off as required to maintain the humidity set point. Pilot
light PL4 provides an indication on the front panel when the humidifier heater is on.
Water level in the humidifier is maintained by a float valve assembly which is external to the
humidifier tank.
The immersion heater has an embedded thermocouple which is connected to Input 3 of the F4
Controller to control its Alarm 1 output. This provides failsafe protection against a heater failure
in the event of a low water condition by opening the humidifier heater contactor C4 if the heater
r125°C. The alarm will self-reset when the heater r100°C.
