Just a couple of quick comments on volume vs. mass measurements: It is well known among ammunition reloaders that the precision and accuracy of volume measurements (with respect to delivering a specified mass) can be quite sufficient for routine measurement of powder charges for reloading of ammunition. Interestingly, the precision required for powder measurements for reloading of ammunition are far more demanding than those required to make film developers, or at least that is my assertion. (The reason for this assertion is that chamber pressures are a very non-linear function of the powder charge, and both accuracy of the load and the safety of the load are strongly dependent on chamber pressures.)

It is well known that a volume measurement of a powder or granular material may give different masses if the physical properties of two different samples of material are different. However, as long as the powder or granular material is fairly uniform within a given sample and from batch to batch the uniformity of the measurement can be quite good. Many household, pharmaceutical or nutraceutical materials are quite uniform from batch to batch, and can be expected to give quite uniform results (even on a batch to batch basis) when measured by volume, as long as no serious changes take place during transport and storage, such as clumping or serious settling. Therefore, once these are calibrated by someone with a scale then someone else with only volume measurement devices (teaspoons, etc.) do a pretty good job of accurate and precise measuring. In some cases (e.g. settling) one can even manipulate a material (e.g. sift of fluff it up) to bring it back to a uniform and reproducible condition, as all home bakers know.

One person in the photo community (I forgot who) has pointed out that volume measurements may even be superior to mass measurements in certain cases. In particular, if a compound absorbs water without a large change in volume then mass measurements will often give a poorer result than volume measurements with regard to measuring the amount of active material in a sample. This is particularly the case for a hygroscopic material that has had significant exposure to atmospheric humidity.

Most scientists or engineers will agree that for a well characterized material a mass measurement using an accurate scale will almost always be more accurate and precise than a volume measurement, even using high quality volumetric devices. However, it is often the case that volumetric devices are "fit for purpose" (i.e. good enough) and are almost always more convenient than mass measurements. This is particularly the case for applications that do not require the utmost precision and accuracy. Most photographic applications probably fit into this category. The trick, of course, is to only use volume measurements on materials that are well suited to this type of measurement. Most uniform powders, granular, or liquid materials would fall into this category, but this certainly does not apply to all materials.

A final note: just in case someone is concerned about whether I am qualified to have an opinion on this matter, I am a professional chemist. I make my living by overseeing research and development of analytical methods and by overseeing some aspects of a routine testing laboratory.