There are several approaches to determining the amount of water added to the spheres. For these smaller spheres you can either try to determine the amount from directly reading before and after filling syringe volumes. If you use this approach, here are some hints for relatively accurate measuring: A. Use a TB type syringe, whose total volume is 1 ml or less. B. Be certain to get all bubbles out of the syringe, including the needle. C. Before filling the sphere, squirt out 1 or 2 drops of water into a waste receptacle; this helps ensure no air was left in the needle. Write down the volume shown on the side of the syringe. D. Subtract the post-filling amount of syringe contents from the pre-filling amount to calculate the water volume inserted into the sphere.The other method involves the weighing of the sphere before and after filling, calculating the weight, and converting to volume. The weights will be on the order of grams, and so you will need a fairly sensitive device. One that works well for this application is the Mettler PE 360 (Mettler-Toledo, Inc., 1900 Polaris Parkway Columbus, OH, 43240, Phone: +1 614 438 4511). This method assumes that a gram of water corresponds to 1 ml of volume. Weigh the sphere before and after filling, and subtract the weights to determine the water weight. Convert grams to ml to get the volume. Example: A sphere weighs 20.0 and 25.1 grams before and after filling, respectively. The weight of the water is 5.1 grams, and thus the volume is 5.1 mls. Note that this assumption may not hold true for solutions of water and other components, like CT contrast media. In this case, you can determine the weight of the solution (put a known number of mls in a container that you have weighed before and after putting the solution in) and then determine ml per gram. Unknown volumes can be determined by calculations based on the weight of the test solution. Example: A 5 ml solution weighs 5.8 grams. So the solution is 1.16 grams/ml. Now if the sample whose volume is unknown has a weight of 2.32 grams, we divide 2.32 by 1.16, for a volume of 2.0 ml. Note that these are just some “made up” example numbers and may not be similar to your solutions.Note that sensitive scales are, well, sensitive. This means that you should use the shields that come with the scales to seal the measuring chamber against air currents that will produce erroneous measurements. Other tips to using (electronic) scales: A. Allow sufficient time for the instrument to warm up after turning it on. The various components will drift over time until they stabilize, which might take 15-30 minutes or so. B. Make certain that the scales are on a flat, level surface. “Level” the adjustable feet on the instrument, if it has this type. Some instruments have a built-in bubble level. C. Some scales have “tare” capability. This means that you can, after allowing the machine to warm up and stabilize, place the empty sphere on the scales and “zero” the reading on the display. After you fill the sphere and weigh it again, you can then read the weight of the water in the sphere directly off the display without having to perform any arithmetic calculations. D. Avoid making the measurements if the scales are placed in a location that experiences a lot of vibrations on the table-top. These can come from HVAC systems, large passing trucks, nearby construction, etc.