Water drop results of STC 2P solenoid valve

(These improved results were updated 6/29/10, using a longer 75ms Toff time)

When photographing water drop collisions, three things affect drop volume:

Valve pulse duration, which we tweak with wild abandon
Head pressure, which is just as important, and just as big a factor
Orifice diameter, a choice of different valves

Pressure through an orifice area for a duration produces a certain volume of flow.

Then of course, to form good clean drops without scatter, the nozzle size must be appropriate for this volume created, not too big, not too small.

This is about the STC 2P 025 solenoid valve (Specs and Source). Yes, the valve in the photo is the STC 2P, it just has the Shako DIN connector on it for power (DIN is simply a connector). The STC 2P is apparently the valve now provided by Stopshot, said to use a 5/32 inch ID nozzle. I have the 24V version. Note that 12 or 24 volts does not change the flow - the solenoid merely opens the valve, and then the flow runs through it, determined by the pressure and orifice size and pulse duration. I use the 1/8 inch NPT fitting size, believing a smaller water reservoir inside the nozzle is a plus (but that size fitting is harder to find locally).

The problem being discussed here is that the STC 2P valve has the larger 2mm orifice, which is double the orifice area of the 1.5mm Shako valve ( picture of disassembly and orifice ). Timing the purge to empty the Mariotte bottle (3 ounces from clear visible bottle area), I measure that the STC takes less than 1/3 the time the Shako takes. It outputs a LOT of water! This volume produces some messy water drops, so to be able to compensate, it begs for less head pressure than necessary for smaller valves. The 2P becomes quite decent with lower pressure heights and drop sizes. The setup at right was used to create these results. Head height was measured here from the bottom of the top Mariotte tube (the Mariotte works by maintaining constant atmospheric pressure at that bottom end of top vent tube), to the center line of the solenoid plunger (at the orifice), shown in the photo. Since the STC 2P has the 2mm orifice (large for water drops), then the drops seem greatly cleaner when the Mariotte bottle is mounted lower than the valve, to compensate larger orifice size with less pressure. This then needs a siphon hose to connect upward. The Mariotte bottle must be easily removable to all low filling and draining the tubing. The elbow on the valve may not be a good plan, as it can retain a big air bubble, if the valve cannot be tilted to purge it when filling the tube. The elbow on the bottle can be tilted and purged. Make sure there is no air between valve and bottle. The valve and nozzle must be very carefully aligned plumb, vertically straight, else it can mess up the drops. One big point though, is when the drop pulse isn't doing it for you, then also try changing the head pressure.

Two pressure cases are shown with drop sample results (are marked with a colored left border):

4 inches head First case is much better, with a 3/32 or 1/8 inch ID nozzle at 7 to 16 ms pulse

7 inches head Messy. Comparable head as a direct connection of Mariotte to valve

All cases shown here are Two drops, spaced 75 ms Toff with Stopshot, The Shako seems less sensitive to Toff, but CTS drops seem to need more than 40 ms. See the Shako results (same nozzles, etc). The sensor is at 18 inches on the yard stick here. A set of drill bits is handy to measure nozzle ID. The 2P is actually pretty good for water drops, if at a lower height. The cases with two and only two clean and consistently spaced drops is the goal. Some combinations which do not work are also intentionally included here (at the ends of the usable range), to show the idea of why the proper combinations of head and nozzle are important.

Valve	Orifice mm	Orifice Inches	Area mm²
STC 2V 035	1	0.039	0.78
Shako PU220AR	1.5	0.059	1.77
STC 2P 025	2	0.079	3.14
Orifices are about half the diameter of these nozzles

Nozzle Description	Nozzle ID inches		mm	ID % Increase
Drilled Pipe plug	0.0625	1/16"	1.59	-
for 1/8" tubing	0.09375	3/32"	2.38	50.0%
for 3/16" tubing	0.125	1/8"	3.18	33.3%
for 1/4" tubing	0.1875	3/16"	6.35	50.0%

1/8" pipe plug, drilled 1/16 inch. Easier to drill straight from inside.

4 inches head (measured by yellow marking above - lower than a direct connection)

Above: 4 ms pulse, 1/16 inch ID nozzle.

Above: 7 ms pulse, 1/16 inch ID nozzle.

Above: 9 ms pulse, 1/16 inch ID nozzle. (too much water through too small a nozzle)

Nozzle change

Above: 7 ms pulse, 3/32 inch ID nozzle. All right! This is beauty! Here we go!

Above: 9 ms pulse, 3/32 inch ID nozzle.

Above: 11 ms pulse, 3/32 inch ID nozzle.

Above: 13 ms pulse, 3/32 inch ID nozzle.

Above: 16 ms pulse, 3/32 inch ID nozzle.

Above: 20 ms pulse, 3/32 inch ID nozzle.

Above: 25 ms pulse, 3/32 inch ID nozzle.

Above: 30 ms pulse, 3/32 inch ID nozzle.

Above: 36 ms pulse, 3/32 inch ID nozzle.

Nozzle change

Above: 4 ms pulse, 1/8 inch ID nozzle.

Above: 7 ms pulse, 1/8 inch ID nozzle.

Above: 9 ms pulse, 1/8 inch ID nozzle.

Above: 11 ms pulse, 1/8 inch ID nozzle.

Above: 13 ms pulse, 1/8 inch ID nozzle.

Above: 16 ms pulse, 1/8 inch ID nozzle.

Above: 20 ms pulse, 1/8 inch ID nozzle.

Above: 25 ms pulse, 1/8 inch ID nozzle.

Above: 30 ms pulse, 1/8 inch ID nozzle.

Above: 35 ms pulse, 1/8 inch ID nozzle.

Above: 40 ms pulse, 1/8 inch ID nozzle.

Above: 50 ms pulse, 1/8 inch ID nozzle.

Nozzle change

Above: 4 ms pulse, 9/64 inch ID nozzle.

Above: 7 ms pulse, 9/64 inch ID nozzle.

Above: 9 ms pulse, 9/64 inch ID nozzle (nozzle is only 1/64 larger, but it needs more water)

Above: 11 ms pulse, 9/64 inch ID nozzle.

Above: 13 ms pulse, 9/64 inch ID nozzle.

Above: 16 ms pulse, 9/64 inch ID nozzle.

Above: 20 ms pulse, 9/64 inch ID nozzle.

Above: 25 ms pulse, 9/64 inch ID nozzle.

Above: 30 ms pulse, 9/64 inch ID nozzle.

Above: 36 ms pulse, 9/64 inch ID nozzle.

Above: 45 ms pulse, 9/64 inch ID nozzle.

Above: 60 ms pulse, 9/64 inch ID nozzle.

Nozzle change

Above: 7 ms pulse, 3/16 inch ID nozzle.

Above: 9 ms pulse, 3/16 inch ID nozzle.

Above: 11 ms pulse, 3/16 inch ID nozzle.

Above: 13 ms pulse, 3/16 inch ID nozzle.

Above: 16 ms pulse, 3/16 inch ID nozzle.

Above: 20 ms pulse, 3/16 inch ID nozzle.

7 inches head (measured by yellow marking above - similar to a direct connection)

Above: 4 ms pulse, 1/16 inch ID nozzle.

Above: 7 ms pulse, 1/16 inch ID nozzle.

Above: 9 ms pulse, 1/16 inch ID nozzle.

Nozzle change

Above: 7 ms pulse, 3/32 inch ID nozzle. (lots of water, needs this larger nozzle)

Above: 9 ms pulse, 3/32 inch ID nozzle.

Above: 11 ms pulse, 3/32 inch ID nozzle.

Above: 13 ms pulse, 3/32 inch ID nozzle.

Above: 16 ms pulse, 3/32 inch ID nozzle.

Above: 20 ms pulse, 3/32 inch ID nozzle.

Above: 25 ms pulse, 3/32 inch ID nozzle.

Nozzle change

Above: 7 ms pulse, 1/8 inch ID nozzle.

Above: 9 ms pulse, 1/8 inch ID nozzle.

Above: 11 ms pulse, 1/8 inch ID nozzle.

Above: 13 ms pulse, 1/8 inch ID nozzle.

Above: 16 ms pulse, 1/8 inch ID nozzle.

Above: 20 ms pulse, 1/8 inch ID nozzle.

Above: 25 ms pulse, 1/8 inch ID nozzle.

Nozzle change

Above: 11 ms pulse, 3/16 inch ID nozzle.

Above: 13 ms pulse, 3/16 inch ID nozzle.

Above: 16 ms pulse, 3/16 inch ID nozzle.

Above: 20 ms pulse, 3/16 inch ID nozzle.

Above: 25 ms pulse, 3/16 inch ID nozzle.

Above: 30 ms pulse, 3/16 inch ID nozzle.

Above: 36 ms pulse, 3/16 inch ID nozzle.

Above: 45 ms pulse, 3/16 inch ID nozzle.

Above: 60 ms pulse, 3/16 inch ID nozzle.