How to measure the inner diameter of thin tube (0.5mm to 2mm) Does anybody know what method or equipment could be used to measure the inner diameter of capillary tubes? They would be in the region of 0.5-2mm in diameter.
Edit: the tube is made of plastic and I have standard lab equipment available. 
 A: The simplest method is probably to use a travelling microscope to measure the wall thickness at the ends of the tube, and vernier calliper or micrometer to measure outer diameter at a few points along the tube. The drawback is that  this does not give an average value for wall thickness.
This article entitled "The Exact Measurement of Capillary Holes" describes a method using hydraulic resistance applying Poiseuille's Formula. The apparatus is a little complicated but fairly basic, and could probably be further simplified. It claims an accuracy of at least an order of magnitude better than the microscope method. 
(Item #3 on page 1 of a Google Search "measure diameter capillary tube". The article cited in J Shupperd's answer was #1 in the same search. Hence my down-vote for lack of research effort.)
A: An easily applied lab technique would be to measure a length of tubing, and
find its outer diameter with a micrometer, then weigh it.  Knowing the density of
stainless steel or glass (or whatever) completes the equation.   
mass = density * length * (D_outer **2 - D_inner **2)* pi/4
A: I'm not a physicist so what I'm saying may be senseless. Since you used the tag home-experiment I thought of something you could do at home. What if you fill with a syringe containing water the tube until it's full? By knowing how long the tube is and how much water the syringe had initially you could calculate the diameter (I think at least). To block the possible backdraught you could pierce first a pencil eraser(the built-in to the pencil). And yes, you can find syringe needle smaller than 0.5mm.
A: Some reasonable methods have already been proposed, but you might just measure the height of water column in the capillary (at least if the capillary is transparent) and calculate the diameter (https://en.wikipedia.org/wiki/Capillary_action#Height_of_a_meniscus)
