Anyway, back to density measurement. All objects when immersed in fluid possess buoyancy. That may sound like an odd statement but if you drop, for example, a cannon ball into a lake it will exhibit a measureable buoyancy, this obviously won’t stop it sinking, well erm …like a cannonball, but it will ever so slightly oppose gravity. This opposition to gravity, or up-thrust, is directly proportional to the volume of water displaced and thus the volume of the object immersed. Therefore if you can measure the up-thrust you can directly deduce volume, then you only need to weigh the object to calculate its density. The good news is that it’s incredibly easy to measure up-thrust.
At this point I thought I’d go through a set of measurements that I made recently on two very different fly lines; a T38 very high-density (fast sinking) competition casting line and a floating Scientific Anglers MED (because I had a new one nicely coiled-up).
Starting with the T38 line, the first thing needed is a dry weight of the line – unsurprisingly this came out at 38g. I should point out at this stage that I used our kitchen scales for this, these only weigh to the nearest gram. Much better results will be achieved with a balance that differentiates tenths of grams or better, however I wanted to show what could be done with cheap kitchenware rather than a £4000 Sartorius that I would use at work.
Next I tared a beaker of water of sufficient size to be able to suspend a coiled fly line in it without touching the bottom or the sides – that’s a very important bit. I then needed to rig a way of suspending the fly line within the beaker – I used two copies of ‘The Art of Fly Casting for Stillwater Trout’* by Mike Marshall and a wooden spoon balanced between them, to which I tied the fly line with the thinnest nylon I could find. Other books can be used but they won’t be nearly as effective.
* This is the finest book on fly casting ever written and signed copies can be purchased from the BFCC for £5. It’s also printed on lighter-than-air paper. **
** One of these statements is typical marketing bullshit, the other is factually incorrect and designed to hoodwink the unsuspecting (mug) punter.
I digress. In order to measure the up-thrust the fly line is immersed in the beaker whilst being suspended by the rig described above. I’m sure Mark would take issue with me calling this a ‘rig’ however, being as anyone who makes a technical statement on the internet these days is a scientist, I’m confident I can consider myself a rigger.
Anyway, something remarkable happens when the fly line is immersed – a weight will appear on the scales. It’s important to remember that this is not the weight of the fly line, this is still supported by my wooden spoon and two marvellous books. Due to every action having an equal, and opposite reaction (good old Newton’s 3rd law) the up-thrust is perfectly balanced by a downward force which registers on the balance – in this case I measured 17g.
As stated above, this up-thrust is directly proportional to volume, thus to measure relative density you simply divide the dry weight (38g) by the up-thrust (17g). The astute amongst you will question how can you divide a mass by a mass and end up with a density? Well you don’t, what you actually get is a ratio of the density of the line being tested to the medium in which it is suspended i.e. water with a density of 1.0g/cc. Therefore the density of the T38 line is (38 divided by 17) multiplied by 1.0g/cc = 2.2 g/cc.
Now perhaps you can see a problem with trying to measure a floating line? It’s not actually going to be possible to get it to immerse as, hopefully, it’s going to want to float. There’s a couple of ways around this. The first is to change the suspension fluid from water to something with a lower density than the thing you’re trying to measure e.g. oil or other hydrocarbon. The problem with this is such low density liquids are often volatile, making getting a steady weight difficult, and can pose a significant fire hazard. I’m not going to risk setting my kitchen ablaze in the name of researching a SL front page, so the good news is that there’s a simple solution – attach something heavy to the fly line to make it sink!
For the SA MED line I measured the dry weight as 27g (pretty standard for a 120ft MED). In deviation from the above method, used for the T38, I next measured the up-thrust from an empty metal spool upon which I was going to wind the line (actually the coils of the new line were sufficiently wide to slip over the arbor so no winding was required). The up-thrust from the metal spool alone was 24g. The up-thrust I subsequently measured from the combined metal spool and fly line was 56g. Therefore the up-thrust associated with the fly-line alone was 56g – 24g = 32g. Then, exactly the same as above, the density can be calculated as (27 / 32) x 1 = 0.84g/cc. Now that looks like exactly the sort of figure I’d expect from a floating line – obviously the precision would be improved with a better balance, but I’m pretty happy with the result.
So there we have it – this is how to measure the density of your fly lines without having to cut them up or make errors due to inaccurate measurement of the cross sectional area from either the line not being perfectly circular or by deforming it slightly in the jaws of a micrometre.
If you’re that way inclined you can use this for myth busting pseudo-scientific bullshit marketing claims. Alternatively, it may have a place in scrutineering at casting competitions where the rules state the lines must float – once familiar with the set-up and method it really is as simple as making two weight measurements, if the answer comes out above 1.0 g/cc then the caster gets a DQF.
Have a great weekend, James