CSZ EZT-570S, Руководство пользователя

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EZT-570S User Manual
61
Product Placement
Typically, the most detrimental affects
are caused when shelves are used in
a chamber and not enough free space
around the product is provided for
proper air flow. It is more beneficial to
use multiple shelves with staggered
product placement than to overload a
shelf. Regardless of the number of
shelves, staggering product is a must.
WRONG CORRECT
Once product is correctly positioned in the chamber, the need for product control depends on how the
product responds to the test program and if the test requirements demand that the product be at a
specific temperature. Small product placed inside a chamber with adequate airflow over them should
follow the air temperature relatively close. However, large and/or dense product may lag behind air
temperature.
This may require the test cycle to be extended in order to allow longer soak periods to provide
enough time for the product to reach air temperature. Again, due to the need for a temperature
difference to force the heat to move between the product and air, the product may never reach air
temperature. If the test requirements specify that the product must be at the designated temperature,
using product control is the only effective way to do this in the shortest cycle time possible.
Since speed for product to market is always desired, the need for faster test cycles is a necessity.
Thus, if your testing needs are leaning towards the use of product control, it is important to implement
it correctly for your product. Since the response that every product will have to a test program is an
unknown, the exact settings for every situation cannot be predetermined. However, the following
sections provide guidance on where to start. They provide assistance for the two “known” types of
product loads that can be placed within a chamber; a dead load or a live load.
5.5.1.2 Implementing Product Control on “Dead Loads”
A “dead load” is a term describing a product that does not generate heat. Any product can be a dead
load, even if it is an electrical product, as long as it is not powered during the test. Dead loads lag
behind the air temperature when increasing or decreasing temperature. Since they lag in both
directions, the product control settings need to be adjusted in order to allow the air temperature to go
above and below the desired product temperature in order to drive it to set point.
In determining the upper and lower set point limits for product control, it is important to consider the
maximum and minimum air temperatures that your product can be exposed to. Since the air
temperature will exceed the product temperature in order to get it to set point, the air temperature
could exceed the maximum or minimum air temperature that the product should be exposed to at any
given time. Therefore, it should be considered when selecting the product control mode and set
points.
Deviation Control
Deviation control uses the lower and upper set points as minimum and maximum offsets from the
product set point. In other words, these values are subtracted and added to the product set point to
create an air temperature set point range. For example, if you enter a product loop set point of 50
degrees, with a lower product control set point of -20.0 and an upper product control set point of 20.0,
the air temperature control loop will be given a set point from the product loop in the range of 30.0 to
70.0 degrees as required to get the product to set point.