ABOVE: A modern. well-constructed test chamber from Thermotron.
|Chamber construction is one of the most important considerations which must be
assessed before committing to purchasing a chamber. Most chambers
are painted on the outside and lined with stainless steel on the
inside. But neither the exterior nor interior "looks" convey true construction quality. When selecting, examine the specification
details closely -- you will see differences that can significantly
affect the chamber's long-term reliability and durability. The critical
differences pertain to the manner in which seams are connected for
the stainless steel liner. Are the seams welded, pop-riveted, or
screwed together? In what way are the ports fastened to the stainless
steel liner and the outer cabinet?
When leaks occur in the stainless steel liner,
a route is created for moisture to travel in and out of the chamber,
especially when the chamber is used for temperature and humidity
testing. The humid air in the chamber finds the leak and condenses
in the insulated area.
Most manufacturers of chambers use fiberglass
insulation much like that which is found in the walls and attics
of homes. When moisture condenses on the insulation, it becomes
saturated like a sponge and loses its capability to insulate. The
walls of the chamber then have less insulation, and that can affect
the temperature and humidity performance. The water in the insulated
space eventually will corrode (rust) the outer sheet metal (which
is not stainless steel) and allow water to leak onto the floor.
A chamber with continuously-welded seams is much less likely to
develop leaks than a chamber assembled with other construction
Pay careful attention to the design of the access
ports. As the pressure in the chamber fluctuates up and down during
temperature changes, the walls will deflect in and out.
The ports connect the inside chamber to the external
cabinet. As the inside walls move from the expansion and contraction
of air, the port transfers that movement to the outer cabinet.
Therefore, the connection between the port and the chamber must
be extremely durable to withstand the repeated deflection.
Chamber manufacturers utilize various methods
to install ports, ranging from pop-riveted and caulked to fully
welded. As noted earlier, a welded port is much more durable than
other fastening methods.
After a chamber is used for certain period of
time, another design or construction compromise may be noted may
develop in the areas where the refrigeration lines penetrate the
stainless steel chamber. The refrigeration lines (tubing) are normally
made of copper. If any sharp edges come in contact the tubing,
a refrigerant leak may develop. A proven method for eliminating
this friction is to install the tubing in sleeves where it penetrates
the chamber. As movement occurs, the lines will not come in contact
to sharp edges.