The most accurate is hydro-static measuring, VERY low margin of error.
Some companies have a mobile apparatus for this. You can save a big investment and save space, by checking into this option. I’m guessing the costs are designed for the club or individual who coordinates this to make $ too.
Otherwise next most accurate, are calipers. Again a lot less, but the margin of error here is based on the expertise of the “measurer.”
All the other devises have margins of error based on the person hydration. Were they more or less hydrated the first time or this time… Which is why these margin of error are higher.
Hope this helps,