Sciencemadness Discussion Board

mechanical vacuum cutoff device

Panache - 21-8-2010 at 09:05

i use a large propane tank (1000mm x 300mm) as a vacuum reservoir for an aspirator system that is turned on/off from two locations via a standard 240v solenoid valve.
I would like to supplement this system with a device that switches the solenoid on or off depending upon the vacuum in the tank without having to use an electronic device as i find them quite unreliable for course vacuum work, as they get contaminated quickly. It should also be noted that i rely on the reservoir as my safety bottle.
I have a float switch that i used to use on my large stil that reliably turns off the power when a certain height is reached, it is a commercial float switch mechanism.
Does anyone have any ideas as to how could i use this existing mechanism to render a course but reliable vacuum cutoff/cuton?
if photos would help of the whole shebang as it currently exists i will gladly whack some up.

Magpie - 21-8-2010 at 10:39

Here's something that would likely work, but it's probably more than you want to spend. Maybe it will give you some ideas.

http://www.dwyer-inst.com/Products/Product.cfm?Group_ID=268&...

watson.fawkes - 21-8-2010 at 11:20

Quote: Originally posted by Panache  
Does anyone have any ideas as to how could i use this existing mechanism to render a course but reliable vacuum cutoff/cuton?
One of the earliest electric manostats, possibly the first, uses a mercury barometer, one electrode in the reservoir, and one at a target pressure height.

densest - 21-8-2010 at 11:44

There are ways to protect a vacuum switch from contamination: absorbent traps, liquid U-tubes, diaphragms, filters, pinhole blocks, etc. Just putting 5 or 10 feet of capillary tubing between the tank and the switch might solve your problem. It would help if you knew what the contaminants were? Gummy, corrosive, solvents, ??? Many of the switches use buna-N rubber diaphragms or O-rings so blocking substances that degrade rubber would probably fix your problem. If it's something gummy then an inline trap filled with glass wool or equivalent would work. If it's vapor,, an absorbent trap and a long line could help. It all depends on what the exact problem(s) are.

Panache - 22-8-2010 at 06:05

Quote: Originally posted by watson.fawkes  
Quote: Originally posted by Panache  
Does anyone have any ideas as to how could i use this existing mechanism to render a course but reliable vacuum cutoff/cuton?
One of the earliest electric manostats, possibly the first, uses a mercury barometer, one electrode in the reservoir, and one at a target pressure height.


ah like this piece i have been saving for reference! this one is for temperature but the same deal applies i guess, i'll try that first, because its ridiculously simple.

PICT0001.JPG - 54kB

aonomus - 22-8-2010 at 07:21

As terrible as this sounds, could you not open up a vacuum gauge, and add in a small microswitch to trip at a certain vacuum level?

Intergalactic_Captain - 22-8-2010 at 10:15

From what I gather, you want to control your current solenoid shutoffs electronicly via an electonic sensor? Sorry for the rambling explanation to come, but I'm sure some members here are NOT renters and may appreciate it...

About 18 months ago, my water heater (gas) died and I had to figure out what was up. The last plumber that was called (while I was away, or I'd have done the job myself) ended up charging us for $600 for what equated to babysitting a space-heater for 4 hours and replacing $12 worth of parts (and "upgrading" a custom-made, milled, aluminum filter bracket with a piece of sheatmetal shit) - So, I said fuck it and decided to fix it myself. Checked the valves and relays, everything in the controller circuitry, the powervent blower, all the tubing, and everything checked out fine....After a day or two of head-scratching, I realized I missed only one thing - The pressure switch. You see, after pondering the schematics, I saw that when the temp sensor kicked "on," the powervent turned on, which (should have) tripped the pressure switch, which then tripped a relay which opened the main burner valve - Everything else was fine, so the weak link had to be the pressure switch... I pulled the tubing between the blower and the switch, blowed into it, nothing... In a flash of insight, I sucked on it, and the main blower kicked on - Pulled the switch, disassembled it, and weakened the spring (heated it with a lighter to kill the temper in the middle third of it)...Been working fine since then....

...So, that said, I'd look into something along the lines of a water-heater pressure switch. They are made in both positive and negative pressure varieties, and can be modified incredibly easily by selectively weakening or replacing the spring. In this way, you'll have a robust, easy to install switch (you'll still need a relay to your solenoid) that can be found relatively cheap on the second-hand market. It's electromechanical, but none of the electrical circuitry will be "naked" in your setup - Just make sure the diaphragm material is compatible with your needs, though it shouldn't be hard to diy one after you take a look at how they work...

peach - 6-10-2010 at 10:42

Here yar go, found this one years ago, enjoy me hearty.... ARRRRr.... can I get an arrrr on that arrr?

{edit}Incidentally, connecting a vacuum gauge to a 60ml syringe, I have managed to pull a100mBar and below vacuum with one pull of the plunger. And produce over 100psi of positive pressure in the reverse direction. Actually, I managed 150psi, before the hose connecting the two looked like a balloon.


[Edited on 6-10-2010 by peach]