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the temperature sensors, it recommended that an adapter,
Fenwal P/N 06-134724-000, be used to avoid damaging
the socket with the meter leads.
Verifying the Sensing Element
Due to the critical role that temperature sensing elements
have in alerting the flight crew of an over temperature
condition, efficient troubleshooting of these systems is
essential to a quick return to service. A central control unit
monitors all sensor loops installed throughout the airframe
and engine cowls. If the controller senses a fault condition,
either a short to ground or an open circuit, it outputs a
maintenance alarm and a fault code. The fault code notifies
the maintenance technician of the airframe zone in need of
inspection. When troubleshooting overheat sensors the
proper use and interpretation of test instrumentation is
important in order to obtain reliable test results.
In the Fenwal CMM test procedure, the temperature
sensors are categorized into two types. The first type
includes all temperature sensors that have an alarm point
of 310°F, (154°C), The second category includes
temperature sensors of all other types or alarm points.
Depending upon the active length and the type of sensor
being tested, a high resistance, (R), or low conductivity,
(G), limit can be associated with each type of sensing
element. This value may be measured between the center
conductor and outer tubing of the sensing element. The
“Sensing Element Conductance/Resistance” table on the
next page summarizes these allowable characteristics.
When making a determination of the maximum allowable
conductance for a temperature sensor the first step is to
define the sensors active length. The active length of a
sensor is found by taking the sensors overall length and
subtracting the length of the two end connectors, 2 inches,
from it.
YOUR GLOBAL SOURCE FOR TEST AND
MEASUREMENT SOLUTIONS
Using the Tegam Model 252 LCR Meter to Verify
Fenwal Temperature Sensing Elements
Table 1: Sensing Element Conductance/Resistance 1
MAXIMUM
CONDUCTANCE
MINIMUM
RESISTANCE
SENSING
ELEMENT ALARM
POINT μS/in μS/cm MΩ•in MΩ•cm
Type 310°F
(154°C)
0.050 0.0197 20 50
All Other
Types
0.010 0.0039 100 250
For example, an 18”, 310°F, sensor would have an active
length of 16”. Once this active length of a sensor is
determined the maximum allowable conductance or the
minimum allowable resistance between the center
conductor and the outer tubing may be determined. To
calculate the maximum allowable conductance, the
constant value in Table 1 is multiplied by the active length
of the sensor.
To find the maximum allowable conductance of a 310°F
sensor that is 90” long, we multiply the effective length of
the sensor, 88 in. times the constant .050μS/inch. The
resulting conductance limit is 4.4 μS. Thus, any sensor
having a conductance greater than this value would be
characterized as being a bad sensor.
LCR meters possess various design schemes, which may
have an effect on their operation and how they are used to
make a measurement. The 252 has a single range switch
that is shared for each of the L, C, R, G, & D functions.
As typically done when measuring unknown values, it is
recommended that the test leads are attached to the sensor
with the meter in its highest conductance range.
Then the range selection is lowered until a reading with the
most significant digits is displayed.
In some instances, an over range condition, (indicated by a
blank display), will be shown on the meter when
measuring a conductance that is known to be within the
selected range. This happens because the 252 also
measures the capacitive, component of a device while
reading the conductance.2 If the overall capacitive
component of the temperature sensor exceeds the
capacitance range of the meter, then an over range
condition is displayed by the meter. Specifically, the
capacitive component of the sensor may prevent the
technician from using the absolute lowest conductance
range of the meter. This is normal operation of the meter
and the event is addressed by simply switching to the next
higher range.
Fenwal sensors are used in virtually all military and
civilian aircraft. Many aircraft manufacturers choose
Fenwal sensors because of their high reliability. Regular
maintenance of aircraft requires the cyclic testing of these
safety systems. During these routine checks it is crucial
that test practices are executed properly and that all
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