How did you determine ratios?  Obviously at a given temperature, the volume will have a certain mass, the problem arises, though when you're building over a long period of time or significant temperature swings.  Then your weight (actually mass unless you're using a balance) will change from one mix to the next and you're not going to have consistent results with curing time or quality.

Of course if you've figured out a way to make it consistent, I'm very willing to give it a try because I hate mixing by volume.

FrankP

On some occasions, I have found the density of the two parts (which is generally a bit different) by doing an accurate volumetric measurement and then weighing them. I write down the percentage on the bottle.  I also write the actual grams (for my stuff, it is 26.6g of resin, 11.4g hardener even though  the stuff is 2:1 by volume) for my most common batch size (about an ounce and a half, or "one pump"), so that I usually save the effort of calculating.

One forum chap avoids calculator altogether by printing off a cheat sheet with the gram amounts for the whole useful range of batch sizes.  That's smarter than the way I do it to be honest.  But I tend to lose papers.

Be ready for a cheerful scolding (but also great tips on how to measure by volume quickly and accurately) by adherents of the other two main religions: the Volume-measurers and the Pumpers.  But if you are a forgetty klutz like me, stick to your guns and be an unrepentant Weigher.  First, the experienced builders know from experience that their boats never fell apart in spite of slight variations in the mixture.  BUT WE DON'T because what? we aren't experienced...duh!  Also, weighing is idiot-proof and works for any amount tiny or large or in-between.  The skilled folks always mix up huge batches which in our hands would result in GCE's.  (Garage Combustion Events.)  Even when I use the pumps, I use the scale to recover, in case of (a) the pump has an attack of flatulence, or (b) user error, or (c) forgetting how many pumps I did, etc. You can't unpump nor unmix the stuff, but you can always weigh it and add just the right amount to save the batch.

I have also looked up the densities for different brands on the web.  I think I've found them by googling (it is a common question) or in the case of West Systems, I think it is on their techsupport site as well.

Temperature changes can be ignored because don't affect the accuracy.  I'm AR but this is where even I must draw the line.

The other theory (which some have put forth here--maybe even CLC but don't quote me) is that the density of the two parts is close enough that you can use 2:1 by mass (or 5:1 for West).  I am not comfortable with that because it isn't all that close in my view (see above).

Best of all, I have found something else to worry about (as is my wont).  If the mix is off, what happens to the excess?  Excess resin we hope, but just as likely that nasty hardener with its volatile amines?  It is still in there unreacted, just waiting to leach out and cause an epidemic, and on the surface.  At least it is in my fevered mind.

I pump, but respect those who weigh. The best way to get the questions answered is to go to the manufacturers. For example, if you're using System Three products, here's some excerpts from the System Three Epoxy Book regarding mixing by weight instead of by volume:

"If you aren’t prepared to spend the time to properly maintain these mechanical pumps, then consider measuring by weight. This is the method we use and find it faster and more accurate than measuring by volume."

"A digital scale with a capacity of 500 grams with 0.1 gram divisions can weigh batches of less than one-half fluid ounce reproducibly with confidence that it is dead on each and every time."

"Don’t make the mistake of using the volume ratio when measuring by weight. We formulate products to be used by simple volume ratios (2:1, for example) as much for packaging purposes as any other reason. Weight ratios are usually expressed in parts of hardener to 100 parts of resin. Volume ratios of 2:1 are often equivalent to weight ratios of 43/100 because the hardener is less dense than the resin. Check the Product Data Sheet to get the correct weight ratio if you measure by weight."

Laszlo

I have to agree with the mixing by weight. It's more accurate and less wasteful. Anyone in the resin business will tell you that weight is the way to go. They can, and usually do, give you both ratios so that you can use volume or weight. Laszlo you're bang on the money, don't use the volume ratio when measuring by weight. I work in a large commercial composites shop and we use weight ratios for everything.

For what it's worth, I'm using West epoxy and their pumps. I started on a non-boat project the year before last. The resin (4L) and hardener sat over winter with the pumps still in place. When I went to use them last summer, the ratios weren't right for the first several squirts. I dumped the first batch. By the second or third batch, the ratio was OK again. I've gone through several 4L containers, and each time, I ran out of resin and hardener at the same time, within a few squirts.

After not using any epoxy over the winter months, I plan to pump some resin and hardener into separate containers when I start up again in a few weeks, and pour it back into the cans to make sure the pumps are flowing properly before mixing the first batch. I have read of some people having trouble with pumps, but they have worked OK for me so far.

 I bought some West GFlex a while back. It comes with a scale for measuring small batches. If I ever need a small quantity of 105 / 205, I do have that scale available.