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Scientific Industries, ESI, as a durable, easy-to-use LCR
meter intended for technicians working in production or
in the field. The meter is capable of making accurate
measurements of resistance, inductance, capacitance,
dissipation factor or conductance. It is available as a
standard 120/240 VAC @50/60Hz powered device or
may be purchased with an optional battery pack,
252/SP2596, which allows portable use in the field such
as in aircraft hangars.
The 252 measures at a test frequency of 1 kHz and
provides a signal amplitude that will not damage Fenwal
sensing elements.
Tegam Model 252, is the Recommended
LCR Meter for Testing Aircraft Overheat
Detection Systems
Fenwal Safety Systems, a division of Kidde Aerospace,
is a trusted name in the aerospace community. Today,
many commercial and military aircraft companies like
Boeing, Airbus, and Lockheed Martin depend on Fenwal
sensors for detection of overheat conditions in specific
aircraft zones such as wings, anti-ice, or bleed air ducts.
A number of years ago, Fenwal Safety Systems
evaluated several LCR meters and selected the Tegam
Model 252 and battery powered 252/SP2596 as their
LCR, meters-of-choice, for the testing of their
temperature sensing elements. This article will provide a
general overview of the Fenwal sensing elements and
the test methodologies used in conjunction with the
Tegam Model 252 in accurately determining the
condition of the sensors. In particular the testing of the
35XXX-X-XXX continuous loop series will be
discussed in detail.
General Overview of Fenwal Sensing
Elements for Aircraft
The Fenwal sensing element consists of a coaxial
assembly, with a center conductor. Surrounding the
center conductor is a eutectic salt compound embedded
in a porous ceramic material, which at normal ambient
Using the Tegam Model 252 LCR Meter to Verify
Fenwal Temperature Sensing Elements
YOUR GLOBAL SOURCE FOR TEST AND
MEASUREMENT SOLUTIONS
YOUR GLOBAL SOURCE FOR TEST AND
MEASUREMENT SOLUTIONS
Using the Tegam Model 252 LCR Meter to Verify
Fenwal Temperature Sensing Elements
There are a number of test devices that produce a test
signal that could produce permanent damage to
temperature-sensing elements so care must be taken when
selecting a test instrument for this application.
In addition to its simplicity, portability and reliability, the
design of the 252 allows accurate measurement of the
conductivity of the sensing element despite the large
capacitive component of the sensor.
Optimizing Test Conditions
Before performing a conductance verification test on any
of the sensing elements, it is good practice to have a
general knowledge of the sensors and the precautions,
which must be taken to avoid erroneous test results or
serious damage to the sensor. Meggers or HiPot testers
must not be used for testing these sensing elements in any
way. Meters that use a DC source should also be avoided
except for measuring resistance or continuity of the center
conductor and then only for short durations. The sensors
should not be tested if they are within 100°F (38°C) of the
temperature alarm point of the sensor. Ideally, all
temperature sensor testing should be performed with the
aircraft power disabled. This will minimize the likelihood
of obtaining an erroneous reading due to stray electrical
interference.
Failure in taking any of these precautions could cause
irreversible damage to the sensors or produce invalid test
results.
With reference to the Fenwal Safety Systems Abbreviated
Component Maintenance Manual for Continuous Sensing
Elements, Publication Number 512, the Model 252 LCR
Meter is used to measure the conductance between the
center conductor and outer shell. 1 Because the sensor
could be damaged by improper test voltages, special care
must be taken to control the electrical test signal applied
across these test points. It must be limited to 1-volt rms, ac
and at a frequency of 1 kHz. Excessive current between
these points could do permanent damage to the sensor. The
Model 252 has a single test frequency and a limited output
voltage, which prevents accidentally exposing the sensing
element to damaging test signals.
For the conductance test, the ideal temperature of the
sensing element must be kept as close to room
temperature, (77°F, 25°C), as possible. Humidity must also
be kept to a minimum as condensation could cause leakage
to occur across the terminal connections.
When making electrical connections to the female end of
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