Module 4 · Evidence-Based Diagnosis
4.2 Gravimetric and Calcium Carbide Sampling
10 min read
Gravimetric analysis is the reference method: take material, weigh it, dry it, weigh it again, and the difference is the water. Calcium carbide is its field cousin, trading some precision for a result in ten minutes on site. Both are straightforward in principle and routinely botched in practice, and almost every way of botching them is a procedural habit rather than a misunderstanding.
Learning objectives
By the end of this lesson you will be able to:
- Carry out gravimetric sampling to a standard that would survive scrutiny.
- Use the calcium carbide method correctly and state its limits.
- Identify the procedural errors that invalidate a result.
- Report a moisture content with the context that makes it meaningful.
Key terms
- Gravimetric moisture content
- Mass of water as a percentage of dry mass, from weighing before and after oven drying. The reference method against which others are compared.
- Calcium carbide method
- A field test where a weighed sample is reacted with calcium carbide in a sealed pressure vessel; the acetylene generated registers as a pressure proportional to the water present.
- Oven-dry mass
- Constant mass after drying at a controlled temperature. Reaching constant mass, rather than drying for a set period, is what defines it.
- Sample integrity
- Whether the sample still holds the water it held in the wall. Lost to open air, to frictional heat, and to delay.
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