panels in the immediate area. To provide remote
at the other (fig. 5-5). The open end of the tube
indications at a central location, you use electrical
is fixed in position, and the sealed end is free to
temperature measuring devices in conjunction
move. The tube is more or less elliptical in cross
with signal conditioners. The devices we will
section; it does not form a true circle. It
discuss in this section are the resistance
becomes less circular when there is an increase
in the volume or in the internal pressure of the
temperature elements (RTEs), and thermocouples.
contained fluid; this tends to straighten the tube.
These devices sense variable temperatures at a
Opposing this action, the spring action of the tube
given point in the system and transmit the signals
metal tends to coil the tube. Since the open end
to a remotely located indicator.
of the Bourdon tube is rigidly fastened, the sealed
end moves as the volume or pressure of the
contained fluid changes. A pointer is attached to
the sealed end of the tube through appropriate
The RTDs operate on the principle that
linkages; the assembly is placed over an
electrical resistance changes in a predictable
appropriately calibrated dial. The result is a
manner with temperature changes. The elements
Bourdon-tube gauge that may be used for
of RTDs are made of nickel, copper, or platinum.
measuring temperature or pressure, depending
Nickel and copper are used for temperatures of
600°F or lower. Platinum elements are used for
of the scale.
temperatures of 600°F or greater. Figure 5-6
Bourdon tubes are made in several shapes for
shows two typical types of RTDs.
various applications. The C-shaped Bourdon tube
As with the bimetallic thermometers, you will
shown in figure 5-5 is perhaps the most commonly
usually find RTDs mounted in thermowells.
used type; spiral and helical Bourdon tubes are
Thermowells protect sensors from physical
used where design requirements include the need
damage by keeping them isolated from the
for a longer length Bourdon tube.
medium being measured. This arrangement also
lets you change the RTD without securing the
system in which it is mounted. This makes your
maintenance easier and less messy.
As temperature increases around an RTD, the
On the gas turbine propulsion plant, you will
corresponding resistance will also increase at a
have to monitor temperature readings at remote
proportional value. The temperature applied to
locations. Expansion thermometers provide
an RTD, if known, will give you a known
indications at the machinery locations or on gauge
values listed in tables in the manufacturers'
technical manuals. Normally, only a few
resistance values are given.
To test an RTD, you will have to heat it to
a specific temperature. At this temperature the
shown in the table. The most common method
of heating an RTD is to use a pan of hot water
and a calibrated thermometer. Some newer ships
and repair activities test RTDs using a thermobulb
tester. This method is more accurate and easier
to use. For specific instructions, refer to the
manufacturers' technical manuals supplied with
The most common fault you will find with an
RTD will be either a short circuit or an open
circuit. You can quickly diagnose these faults by
using digital display readings or data log print-
outs. By observing the reading or the printout,
you may find that the indication is either zero or
a very low value. A malfunction of this type
Figure 5-5.--C-shaped Bourdon tube.