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that contains on the last page a simple approximate (the actual fluid mechanics is more complicated) equation for the true sink rate of a tip or line in terms of its diameter and density. The actual equation is

U = 37.5 (S-1)^0.563 D^0.689

where U is the sink rate in ips, S is the specific gravity of the section of the line or tip (the ratio of the density of the line to the density of the water it is in) in question, and D is the diameter in inches. For those that may be unfamiliar with this use of the carrot symbol (^) in this context this just means “to the power of”. These can be computed using any calculator with more than the most basic functions, though you may have to find a key you have not used much before. But S and D can be determined by anyone with a ruler, a scale, and a micrometer. So for example if you have a box full of unidentified sink tips you can use this formula to figure out the sink rates.

The equation is a first order approximation, but is accurate to %1 for lines and tips.

One of the “problems” is that different companies use different methods to determine the labeled sink rates and so those rates cannot be reliably compared across brands. The other, related issue is that the actual sink rates on the label are not strictly correct, and tend to overestimate the sink rates of a real line. The latter issue may not be that practically important since the way tips are used tends to be more like shifting gears in a manual transmission, which you do by going up and down according to need and not typically with reference to the precise gear ratios.

The Havstad equation provides a reliable way to calculate the sink rate of any tip, or section of line. If the companies that make sink tips would simply post the Havstad sink rates then all of them could be put on the same footing and compared. That is exactly what he proposed in the paper referenced above. But companies are probably more concerned, in general, with marketing considerations. If they then determine a sink rate using a certain method and it comes out to be a number that sells, even if the method is inaccurate, they are covered. They probably feel off the hook on accuracy considerations. They will rightly point out that their competition is using similar methods so they have no choice, etc.

Anyway, this is the answer to a lot of questions about tips.