How important is the size of the inside bore of the cartridge tube in mercurial blood pressure instruments?

It is critical. When the inside bore is too narrow two problems can result; mercury lag and mercury separation. Mercury lag is the phenomenon that occurs when the air deflates from the cuff faster than the mercury column can respond. When the forces of friction (of the mercury against the inner walls of the cartridge tube) that prevent the mercury from dropping exceed the gravitational forces, this causes the mercury to drop and results in mercury lag. That same friction can actually cause the column of liquid mercury to split - forcing an air pocket that will distort readings. This is known as mercury separation.

ANSI/AAMI/ISO 81060 requires that the inside bore of the cartridge tube be a minimum of 3.9 mm in diameter. Believe it or not, the inside bore on the cartridge tube of many low cost imports is just 3.2mm - well below government standards. On the other hand, most ADC mercury instruments feature a cartridge tube with an inside bore of 5mm - more than 25% larger than government requirements. (The inside bore on the 922 series cartridge tube is 4.2 mm.)