Page 90
FCO770 Users Guide Issue 1
OFFSET & AUTO TRACKING
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The normal method of testing items is to pressurise them to the required test pressure then to
allow them to stabilise to eliminate any pressure changes caused by adiabatic heating,
component movement, and temperature differences between the test object and ambient. This
means that when the test phase is entered, only leaks in the test item will cause a pressure
change and leak tight items will give zero pressure change. Although this gives the most
repeatable and accurate results, the total cycle time can be quite long with some plastic test
items taking over five minutes to totally stabilise.
To reduce the time required to test, the test phase can be entered before the test item has fully
stabilised. If the pressure offset due to instability is repeatable from one test item to the next it
can be deducted from the total pressure change leaving just the pressure change due to a leak
using the OFFSET parameter.
Auto offset tracking allows this method to be used even when a slow drift in the transient
occurs due to variations in the surrounding temperature or production rate etc. The FCO770
tracks any variation in the offset over a number of tests by digitally filtering all leak readings
below the fail limit. This allows the instrument to determine whether any changes in readings
are due to drift or leaks.
As the rate of drift experienced at any test site is dependent on the test item, its surroundings
and the rate of production, the digital filter can be varied to allow the leak detector to be
optimised for any application. Each leak test can have the tracking function disabled or
adjusted to compensate each step of a product independently for drift.
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Although the auto offset tracking system can significantly reduce the time required to test
components, the technique is only suitable for components that have repeatable stabilisation
characteristics from one test item to the next. It is important to establish the suitability of the
product for the technique and to obtain a master leak tight component that can be used to
learn the offset value for the test system on a regular basis.
Initially the instrument should be set up as standard with both Offset and Track Filter set to
zero in the Test Data menu to disable them. The fill stabilisation and test times for the test
item must first be established. Even if the instrument is to be used with cc/m or cc/h etc, it is
necessary to initially commission the instrument using Pascals, as this will ensure that a test
time is used that will provide enough resolution for the leak.
Use a leak tight component, and vent it between successive tests. Reduce the stabilisation
time until an offset of approximately 5% of full scale (100Pa on a 2kPa instrument) is
displayed at the end of the test time. This will probably cause the instrument to fail, but this is
OK as the next stage is to remove the offset. The test item must be allowed to recover
between tests and hence should be vented for at least a minute between every test cycle.
Summary of Contents for FCO770
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