The following instructions are to help you properly prepare the tube to prevent leaking which will skew your test results:
Use the short tube for block only. It may not provide you with adequate pressure for appropriate testing on more dense substrates. This tube is graduated in mph rather than the ml marks that are on the tall tube. There is a protruding lip around the inner opening which aids in getting a good seal with the putty.
The tall tube may be used on all substrates. The tube is graduated in mililiters, with “0” at the top and representing the 98 mph mark. There is no lip around the inner opening on this tube.
Please note: On short tube, be sure that the putty makes complete contact with the protruding lip.
While pushing the putty against the base, pinch in an outward manner to form a "pie crust" appearance

Step 6: The resulting outward angle will help prevent the putty from obscuring the opening when attaching the tube to the test area
RILEM How To prepare a RILEM tube for testing