Jung, Marketing Manager, ESPEC North America
Unfortunately ‘thermal shock tests’ are
something that many people ask for without knowing that there
are several different interpretations of what that is. And there
are folks talking about ‘thermal cycling tests’ that
actually mean ‘thermal shock’. A clear definition is in
Possible Definitions of ‘Thermal Shock’:
1. Alternately dipping the product in hot
and cold liquids. More
precisely, this should be referred to as “liquid-to-liquid
thermal shock”. Because this is not a common test, this
possible definition is often disregarded.
2. Changing the air temperature as quickly
as possible in a single
should be called “thermal cycling” or “stress
screening”. Because of the limited temperature change possible
in a single chamber, this is not truly a shock condition.
3. Moving the product from a hot to a cold
chamber or other methods for sudden change of the air
temperature. This is
“air-to-air thermal shock” or “two-zone thermal shock”,
and is what is most likely to be assumed what you are talking
about ‘Thermal Shock’.
With thermal shock tests, there are also
several ways to define the performance.
Thermal shock tests focus on “recovery time”, or how
long it takes to stabilize after the switch.
definitions of ‘recovery time’:
1. The time it takes for the product to
move from zone to zone. This
should really be called: “transfer time”, and does not
measure performance at all.
2. The time for the air
temperature to recover in the new zone.
This can be measured in the air stream before
or after the test
load. This can be
called “upstream recovery time” or “downstream recovery
3. The time for the physical
product to recover, called “product recovery”.
This time is dependent on where the sensor is placed on
the load. Mil-Std
883 method 1010.7 defines their requirements as “worst-case”
product recovery, which is for a sensor embedded in a sample
buried among other samples.
a thermal shock chamber is:
A thermal shock chamber is essentially two
connected test chambers, usually sitting on top of each other.
While the hot-zone chamber is nothing more than a convection
oven, the cold-zone is more elaborate.
The cold-zone is cooled by cascade
refrigeration, which drives the temperature to extremes as low
as -75°C. Because it stays cold all the time, it must be
designed to keep moisture out. Even so, additional steps such as
a nitrogen or dry-air purge may be necessary to limit the number
of defrost cycles. (A defrost cycle holds the test load in the
hot-zone while the cold-zone is warmed to defrost.)
During the hot-zone exposure, the cold-zone
may be run to an even lower temperature than the setpoint. This
allows the system to ‘store’ additional cooling capability
by chilling aluminum plates which are included in the
cold-zone—we call this a pre-chill function. Some systems may
use a liquid nitrogen (LN2) ‘boost’ instead of a pre-chill,
so watch for utility requirements of LN2.
The product carrier is moved from zone to
zone by way of an elevator system. The mechanism used by the
manufacturer should be of special interest to the buyer, as this
is where most problems are seen with thermal shock chambers. We
recommend direct-drive lifts and avoiding cables & pulleys.
how is the system controlled?
Older designs are operated as two separate
chambers, each with their own controller, which is very
limiting. Modern systems control both zones and the lift system
in one integrated controller to maximize the performance and
functionality of the system. Diagnostic alarms are also a
significant aid in such a complex system.
A great example of comprehensive air-to-air thermal shock
chamber is our TSE-11 (www.espec.com/digest/tse10.htm).
It is meets strict Mil-Std requirements with only 208V power (no
cooling water or LN2 needed).