This is essentially a pulse-echo reflectometer
(McClements & Fairley, 1992). Pulsed ultrasonic measurements
can be made either with a single transducer
used to generate an ultrasonic pulse and detect its return
from a reflecting surface, or transmission mode, where
separate transducers generate and detect the pulse, respectively.
The former method is used here. An important
aspect of the cell design in the present work is that a
reflectance measurement made when freezing the sample
from one side is acoustically equivalent to a transmission
measurement made during freezing from both sides.
In the former case (Fig. 3a), the sound pulse travels from
a transducer through a progressively cooler sample, reflects
at the cell surface, and then travels back through
the (same) sample to the same transducer. In the latter
case (Fig. 3b), the pulse first travels through a progressively
warmer sample until it reaches the thermal core,
then through a (different) progressively cooler sample
until it reaches the second transducer. By using the reflectance
geometry we are able to reliably and consistently
position the thermocouple in the thermal core without disrupting the sample, yet at the same time
demonstrate applicability to a realistic freezing process.