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We recently received the following tip from one of our readers:
When sampling using a vacuum-type pump, hot oil or exceptionally viscous oil can result in the plastic sampling bottle collapsing, making it difficult if not impossible to pull sufficient vacuum to draw out the oil sample.
To prevent this, try getting a short piece of clear rigid PVC pipe whose internal diameter closely matches the outer diameter of the plastic sampling bottle. Slide this over the outside of the bottle before drawing a vacuum with the hand pump. The rigid plastic sleeve prevents the bottle’s collapse and the clear plastic enables the sampler to see when the bottle is full.
The fit or gap between the sleeve’s inner diameter and the sample bottle’s outer diameter does not need to be snug. However, the larger the gap, the less effective the sleeve is in preventing the bottle’s collapse.
Stephen French, Metallurgist PPL
Q: So, how does a PVC sleeve reduce vacuum bottle collapse in vacuum samples?
A: By looking closely at failure modes of collapsed tubes and bottles, the ovality of the structure plays a major role in determining which pressure will cause a collapse.As with many manufactured components, the bottles and tubing are not always perfectly round. A bottle that is misshaped (out of round) has an increased chance of buckling (warping) inward. The physical parameter that generally remains the same is the perimeter of the bottle. When a bottle begins to collapse, a small section folds inward. For an instant, this small section becomes straight while the areas around the fold will be forced out, allowing the fold to become straight. As soon as the fold in the bottle breaks over the straight position, a collapse will occur.
By placing a piece of PVC snugly around the bottle, the PVC will restrict the bottle’s tendency to initially bulge out, which in turn restricts a section of the bottle from folding inward. This, however, only reduces the tendency of the sample bottle from collapsing because this is just one of many ways a sample bottle may collapse.
An explanation of bottle expansion can also be illustrated mathematically. The following assumptions are made: a section of bottle will become flat, the perimeter of the bottle remains constant and the section of the bottle wall that is not in the flat zone will uniformly expand. Note, in a real bottle, the expansion would localize around the flat zone, however, by assuming uniform expansion, the mathematical equation is simpler:
Equation: pR = pr – 2sin-1(L/2r) + L
Consider the following example which assumes that R equals one inch and L equals one inch. Using the information above, it can be calculated that r would be 1.015 inches and the bottle would have to expand 30 thousandths to allow a collapse to occur. In a real bottle, the expansion would be larger because the bottle would not uniformally expand.
Andy Coverdell, Noria Corporation
A: Here’s a tip for those who are math-challenged: roll a piece of paper into a tube, cup your hand around it and give it a poke - you can see and feel it working. Obviously, the better the fit the better the result, and in the case of high-temperature sampling, the added layer of insulation when handling is a benefit, as long as the bottle comes out alright. This would support the need for the looser fit noted in the original tip.
Mark Smith, Analysts, Inc.