Chapter 9 – Maintenance
TestEquity 101H Temperature/Humidity Chamber
Page 81
Theory of Operation
Overview
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 in the air
plenum. The F4T Controller provides a time-proportioned output to solid state relay SSR1. This
turns the heater ON/OFF as required to maintain the temperature set point. A fusible heat limiter
(HL) provides failsafe protection against a catastrophic failure by opening the heater circuit at
+240°C.
Humidity System
A Mass Transfer Vapor Generator or MTVG (patent-pending) provides both humidification and
dehumidification. Humidity conditioning is external to the chamber. The MTVG includes an
inlet tube on the bottom which is below the evaporator coil and an outlet tube on the top above
the evaporator coil. The pressure difference between the two forces a portion of the air in the
chamber to flow from the MTVG inlet to the MTVG outlet. A column between the MTVG inlet
and outlet contains refrigeration tubing for dehumidification. The bottom of the MTVG has a
small area which contains water and embedded heater for humidification.
Water flows from the built-in reservoir to the Float Valve Assembly through Solenoid Valve
SV4. Solenoid Valve SV4 allows water to flow from the Float Valve Assembly to the MTVG
when the Humidity mode is ON. Water level in the MTVG is then maintained by the Float Valve
Assembly. Water in the MTVG is drained through Solenoid Valve SV3 when the Humidity
mode is OFF. Solenoid Valve SV3 will open for 10 seconds when the Humidity mode is first
turned OFF, then will close to prevent moisture from inadvertently migrating from the drain to
the chamber interior through the MTVG inlet and outlet tubes.
Water is heated by an immersion heater (HT2) in the MTVG to generate vapor. The F4T
Controller provides a time-proportioned output to solid state relay (SSR2). This turns the HT2
ON/OFF as required to maintain the humidity set point.
The immersion heater HT2 has an embedded thermocouple which is connected to Input 3 of the
F4T Controller to control an Alarm function. This provides failsafe protection against a heater
failure in the event of a low water condition by disabling the Humidification output if the
immersion heater temperature r125°C. The alarm will self-reset when the heater reaches
+100°C.
Refrigeration System
The F4T Controller turns the compressor ON based on the demand for cooling. Dry-bulb cooling
is accomplished through the Capillary Tubes (9) and the finned evaporator coil (15) inside the
chamber. The F4T Controller cycles the liquid-line solenoid valve (SV1 or 7/8) ON/OFF as
required to maintain the temperature set point.
