How do "automatic" fire hose nozzles work? I'm hoping someone can explain to me how "automatic" fire hose nozzles work. As in TFTs (Task Force Tips). I have a background in physics and since becoming a volunteer firefighter for my own interest have been working through some of the physics behind firefighting. I am happy with fixed-diameter nozzles, which I understand as a device that converts (static) pressure to velocity (dynamic pressure) -- Bernoulli's principle. But these selectable-flow-rate nozzles baffle me (pun intended). TFT have long descriptive article: http://www.tft.com/literature/library/files/ltt-010.pdf but it doesn't make much sense to me. It talks as if pumps are constant flow devices, not constant pressure devices (as operators usually regulate them to be). It also talks about producing a constant "nozzle pressure" of 100 psi and seems to indicate that this is the pressure of the ejected stream (Fig 13) when I would have thought the stream's static pressure was atmospheric (0 gauge) and neglecting friction loss the difference between this and pressure immediately upstream of the nozzle is equal to the constant pump pressure. So, is the above document very confused, or am I, or both? Is the nozzle a lossy device, that simply dissipates power through turbulence around a sharp obstruction (Fig 12) so as to uphold the promise of a "constant nozzle pressure"? If so, this doesn't really live up to the hype of providing the best stream possible for the available water. Or is it a conservative device, that regulates flow past a nearly-lossless spring-loaded obstruction (fig 16a)? If so, how does that work out-- what is the relationship between pressure and flow rate? I am struggling with ram pressure vs static pressure and the effects of angled faces and pressure gradients. Hoping someone can help :).