Critical Point Drying (CPD) - Intermediate Stage

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4. Intermediate Stage

Table of Contents:

1. Introduction
2. The Critical Phenomena
3. Critical Point Drying
4. Intermediate Stage
5. Transitional Stage

As mentioned previously this involves Dehydration and Intermediate Fluid, the following indicating a possible schedule.

(Wet Specimen) H₂0 → Acetone → CO₂ → CPD (Dry Specimen)

The specimen is usually processed through varying concentrations of Dehydration Fluid, culminating in complete replacement of the water with this Intermediate Fluid, because it has a low surface tension the specimen is less likely to experience damage due to evaporation while transferring to the chamber and being miscible with CO₂ (The Transitional Fluid) ensures satisfactory conditions after the flushing (purge) for the CPD process to commence.

(Wet Specimen) H₂0 → Acetone → 30%* ---- 100% → CO₂** → C.P.D. (Dry Specimen).

* 50/60/70/80/90 Typically 10 minutes each.
** Flush Typically 3 times.

The table Figure 3 gives an indication of some Intermediate Fluids. (Water is 73 Dynes/cm.)

Figure 3.

Having transferred the specimen to the chamber in the Intermediate Fluid, the chamber is flushed several times to replace it with the Transitional Fluid. The process from which the complete technique derives its name C.P.D. can now be initiated.

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