Yeah, constant-volume would mean that the cross-section (round in the shank, flat and rectangular at the button) area of the airway is the same at all points. So you are moving the shape of the tube from being a 5/32" round hole to an equivalent size of rectangle (kind of like the ducts in your house being 3x10 rectangles as the terminal points of a 5" round tube, right?).
One way to do this is to not slot at all - just bang a 1/8" hole all the way through the stem - this is essentially what Wiley pipes look like. Nice draw but a thick stem for it.
A venturi is a tube that tapers, and the flow rate has to increase through the taper. Why would this be good? Bernoulli's laws suggest that the pressure of a liquid on the sides of the pipe it's flowing through varies inversely with the speed - if the gas is zipping through the line there's hardly any interaction with the walls of the pipe, and that ought to mean there's less time/inclination for condensation. So to build a pipe this way, you would use whatever drilling you like, 11/64" I think was pretty near Rick Newcombe's spec, and you taper that down through the stem to the bit, where you'd have the point of greatest restriction somewhere just inside the end of the stem. You could terminate it there, P-lip stems do just that, or you could make a slot for greater comfort after that. But at this point, the hot smoke is zipping along, hardly interacting with the walls of the tube (the inside of the stem) and not condensing and gurgling.
One way to do this is to not slot at all - just bang a 1/8" hole all the way through the stem - this is essentially what Wiley pipes look like. Nice draw but a thick stem for it.
A venturi is a tube that tapers, and the flow rate has to increase through the taper. Why would this be good? Bernoulli's laws suggest that the pressure of a liquid on the sides of the pipe it's flowing through varies inversely with the speed - if the gas is zipping through the line there's hardly any interaction with the walls of the pipe, and that ought to mean there's less time/inclination for condensation. So to build a pipe this way, you would use whatever drilling you like, 11/64" I think was pretty near Rick Newcombe's spec, and you taper that down through the stem to the bit, where you'd have the point of greatest restriction somewhere just inside the end of the stem. You could terminate it there, P-lip stems do just that, or you could make a slot for greater comfort after that. But at this point, the hot smoke is zipping along, hardly interacting with the walls of the tube (the inside of the stem) and not condensing and gurgling.