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rsterne
10-29-2010, 04:05 PM
I am going to be getting a 4500 psi compressor and will also be using it to top up a 3000 psi SCUBA tank.... McMaster Carr have a pressure switch # 3427K83:

http://www.mcmaster.com/#standard-pressure-switches/=9hjwlt

It is adjustable from 400 to 4,700 psi and is rated at 8,000 psi safe working load.... It's less than 3" high, with a 1/4" NPT fitting.... It is rated at 5 amps at 120VAC....

I know that the motor is rated at 400 watts, which is less than 4 amps.... but it would likely draw more than that on startup.... My question is:

If this switch is closed (ie low pressure) when the compressor starts.... or is switched into the circuit after the motor starts.... Would it work reliably to shut the motor off?

Bob

TomKaye
10-29-2010, 05:28 PM
Bob,

That's a good question. The motor does draw more amps on startup especially if your already at pressure. We tried a 6 amp rated switch in the prototypes but it welded itself in the on position every once and a while. We then went to the more expensive 15 amp switch and tested it 1000 times to make sure it wouldn't fail. For legal reasons we don't recommend you mess with the switch. I have seen people use a high amp relay between the switch and the motor in these situations. Remember that your safety burst disk is at 4500 psi so filling a 3000 psi tank and having a failure could potentially over pressure it by 50%.

Tom

rsterne
10-30-2010, 03:18 AM
I figured if I bypassed the (3000 psi cutoff) pressure switch on startup (two switches in parallel, one a spring loaded (off) 15A toggle or pushbutton) and then once the motor was running opened the bypass switch it would eliminate the load problem.... Then all the pressure switch would have to do is open with the 400 watt load on it once it gets to 3000 psi.... I would simply mount the pressure switch and its associated bypass switch in a box with a hose to the compressor and another to the SCUBA tank.... The ShoeBox cord would be plugged into that box.... It would only be used when I need to do a 3000 psi fill.... I am aware of the potential problem if there was a switch failure and it pumped the SCUBA tank to 4500 psi and blew the burst disc.... but if I purchased a 3000 psi ShoeBox (which also has a 4.5K burst disc) and the shutoff switch failed on that the same problem would exist.... I certainly understand the legalities of why you wouldn't want to recommend that, however....

Bob

TomKaye
10-30-2010, 04:05 AM
OOPS my mistake, the burst disk in the 4500 ShoeBox actually blows at 7500 psi. If your system failed and the compressor pumped all the way to 7500 while hooked to a 3000 psi tank you would be over pressuring it by 2.5 times service pressure which gets near its failure point. It could explode and kill you, and your kids and your dog. For what your going to spend on the pressure switch, accessory switch and hassle your better off buying two compressors, one for each rating.

Tom

rsterne
10-30-2010, 05:54 AM
Except for the fact that I won't be changing the switch arrangement on the ShoeBox at all.... It would still shut off at 4500 psi.... My switch would cut the power to the Shoebox (at 3000 psi) before the power plug.... Both switches would have to fail for the pressure to go over 4500 psi.... Plus the SCUBA tank has a 5K burst disc as well.... which is the hydro test pressure.... Let's not get tooooooooo paranoid.... *grin*....

Bob

4u2catch
10-31-2010, 10:26 PM
what will the burst disk in the 3000 psi compressor blow at?

rsterne
10-31-2010, 11:21 PM
AFAIK, it's 4500 psi.... I could also put a 4500 psi burst disc in parallel with my pressure switch for safety.... probably a good idea....

Bob

Zorro
07-22-2011, 05:08 PM
Hi Tom,

I haven't gotten my compressor yet but when I do I plan to put an electric pressure switch after the main on off switch along with a 12 volt power supply to power some added cooking fans. I plan to set the pressure switch to cut off at 3,000 psi because I only have a 3,000 psi Air Force Talon SS at this time. (I plan to purchase a 4,500 psi rig soon in the future but really don't want to purchase 2 compressors so it's going to be a 4,500 psi Shoe Box.) I plan to come out of the block with a nipple then to a Y fitting for the electric switch then another nipple out the front of the cabinet for the air hose. What size threads do I need to screw into the block? I noticed that North Shore compressor has a high pressure fitting with male threads on one leg of the long direction that could screw directly into the block thereby shortening the overall length so that the electric switch will fit inside the closed cabinet. Do you think this will work? Are there any other fittings that you think I might need? I used to have a working FX Compressor--it broke and they won't talk to me about fixing it but I do always monitor the high pressure fill process full time and fill the bottle in an explosion cage.

Thanks--and looking forward to getting my shoebox soon!

Zorro

TomKaye
07-22-2011, 09:41 PM
Well for legal liability reasons I can't comment on using a 3k pressure switch on a 4500 compressor. The female thread on the air block is 1/8" NPT pipe thread so you should be able to get a stock fitting to go in there. We have a system prototyped with an electronic switch in the housing and will eventually offer that as an option.

Tom