I believe you are giving this design much more credit at sophistication than it really deserves. The principle here is to measure the flow rate. All the drip tube needs to accomplish is a seal at the top of the injector and a tube that makes a 150 to 180 degree bend that you can easily see the drips per unit time. The diameter of the tube could be anywhere from 1/8" to 1/2"...I think mine is about 3/8". The length and diameter of the tube is inconsequential; the longer/bigger it is the longer it takes to fill up and start dripping...but after its full, the the drip rate tells you the flow rate. You literally could watch the fuel drip off the bottom of the IP and establish the drip rate without a tube, but the tube is neat and more predictable.

My $.02 worth.

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1976 240D
1987 560SL
2007 E320 Bluetec
1998 C280 (now son’s car)
1982 240D Manual - Sold