handbook of modern sensors

294 7 Position, Displacement, and Level
(A)
(B)
(C)
Fig. 7.46. Breakwire concept with thermocouples consisting of metals x and y (A); light pipe
concept (B); and measurement of thin-film liquid by a capacitive method (C).
namic heating often rely on ablating thermal protection systems (TPSs) to keep the
internal structure and equipment below critical operating temperatures. An ablating
TPS undergoes chemical decomposition or phase change (or both) below the internal
structure’s critical temperature. Incident thermal energy is then channeled into
melting, subliming, or decomposing the ablator. The ablator recession rate is directly
proportional to the flux at the surface. A measure of ablator thickness is required to
estimate surface heat flux. Thus, an ablation sensor is a kind of position sensor that
detects position of the ablation layer’s outer surface and provides a measure of the remaining
thickness. The ablation sensors can be built into the ablation layer (intrusive
sensors) or be noninvasive.
The intrusive sensors include the breakwire ablation gauge, radiation transducer
(RAT) sensor, and light pipe [13].The breakwire ablation gauge consists of several thin
wires implanted at various known levels in an ablator. As the material progressively
erodes, each successive wire is broken and results in an open circuit. Figure 7.46A
illustrates this concept. In some cases [14] breakwire doubles as a thermocouple (TC)
and each is situated so that no breakwire TC is directly above another. This arrangement
allows an unobstructed conduction path through the ablator to each breakwire
TC, including those at lower levels. Although the breakwire method provides temperature
time histories until the last TC is exposed and destroyed, this method only
provides recession data at a few distinct points.