On-board air project for my 4WD

by Kam

Hi Bill

Appreciate you expertise and the hard work you have done explaining all about the air on the web. It’s just amazing.

I would be grateful for your opinion about the following.

To benefit from instant compressed air I am about to use my portable compressor 150psi/1034 kPa, 12V, 45AMP max, 150 LPM, which is only used occasionally to top up tyre pressure from sometimes low 16psi to 44psi max. during our off road trips, to an air tank.

I’ve come across a 10.56 gallon (40liter) air tank rated to 150 psi, work pressure 8 bar, tested to 20 bar.

Do you think that the tank is too big for the compressor?

I also found the Pressure Switch with Built-in Relay bellow. Do you think this could be suitable for the task? I am a bit confused about the psi On and OFFs stated in the description.

Tolerance is 5± of specified pressures
For 12 Volt systems only, up to 40 amps
1/8" NPT Male port fitting
Moisture resistant and tested up to 10,000 cycles.
Leads are pre-fitted with insulated 1/4" male push-to-connect terminals that are clearly marked
Dimensions: 108mm L x 57mm W x 57mm H

Part Number Description
90110 85 psi ON, 105 psi OFF
90111 110 psi ON, 150 psi OFF
90113 90 psi ON, 125 psi OFF
90118 165 psi ON, 200 psi OFF

Also the power cable will eventually be longer than the current 2.4m one as it will need to be mounted at the back of the ute.

I know that the positive wire should be as short as possible to minimise the amp. draw. Also motor should run to maintain enough supply voltage.

Is that an issue for this application set up Bill?

Also considering using the following for the setup:
1/4" FILTER REGULATOR which has:
•1/4" BSP inlet/outlet
•Maximum working pressure 150 p.s.i.
•Automatic water drain
•Polycarbonate bowl with metal bowl guard
•Robust metal construction

•Self retracting hose fitted with Jamec Pem 1/4" BSP male swivel fittings
•Recommended maximum working length: 90% of total hose length
•Compact diameter of coil
•Intended for compressed air applications only
•Excellent coil "memory"
Hose Length Hose Size O.D. Hose Size I.D. Max Working Pressure Fitting Material
6 m 12 mm 8 mm 860 kPa (125 psi) Brass


Do you think that could work?

Many thanks

Hello Kam.

Using a 12 VDC air compressor that is designed for sporadic, short duration use (such a refilling a tyre) to provide air to a tank may exceed the duty cycle of that compressor.

You need to contact the compressor manufacturer to determine if it can handle the long run time necessary, or, simply flip it on for a 5 minute run, then off for 10-15 minutes, then back on for 5 minutes and so on until the tank is full.

My compressor, which sounds similar to yours, cost me nothing, as I got it as a promo for service work done on my vehicle at the dealership. Retail, they typically sell here for under $20 so, if you use one up in your application, and it takes a while to completely wear it out, does it really matter?

You don't indicate what the air tank will be supplying - at least I didn't see that - so I don't know the frequency of filling it. That might be an issue.

The specs for your system, in terms of pressure rating, seem OK.

It were me, I wouldn't have the compressor running with a pressure switch, but a manual ON/OFF switch so that I could control the run time.


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air compressor tank - transport full of air

Hey Bill. I have to put an air compressor with a decent size tank in the back of a box truck.

My question is can I leave the tank full of air while I'm driving from stop to stop since I would rather not have to refill the air tank at every stop to get pressure?

It would be about a 20 gal tank at up to 150psi. Thanks in advance.
My personal thought is that if the compressor is well secured so that it cannot roll around and perhaps have a tank puncture, I can see no reason why you cannot travel with a compressor in a truck and with pressure in the tank.

As long as the tank is rated for the 150 PSI, that shouldn't be a problem either, as far as I'm concerned.

Every big rig on the road has one, if not more than one, compressed air tank full of compressed air, so the air is available to operate the air brakes and air horns. What you are trying to do is no different than that, as far as I'm concerned.

What are you using the air for each time you make a stop, if you don't mind me asking?



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Adding a smaller tank to my existing tank

by Stephen
(Pretoria, South Africa)

Ryobi 150 lt air compressor

Ryobi 150 lt air compressor

Hi there Bill.

I have a 150lt single pump domestic air compressor.
I wou;ld like to add a smaller tank to it (about 5kg).

The air would be transfer from the big tank to the smaller tank...

By doing this, would the pressure in the smaller tank be higher than the bigger tank?

If the two tanks are plumbed in series Stephen, the main compressor tank and then compressed air flows from the main tank into the smaller tank, the air compressor will run until the air pressure in both tanks reaches the compressor cut out setting and the compressor stops.

Both tanks will be at the same pressure, and the air gauge on the compressor tank will tell you what the air pressure in both tanks is.

As you use air, as long as there is nothing in the line between the two tanks but the air line itself, the air pressure in both tanks will reduce at the same rate until the air compressor cut in pressure is reached and the air compressor starts, and the cycle repeats.

You do want to read the page on Duty Cycle though.



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air tank size

by Harold
(China, MI, USA)

I am building a machine that will have 6 pneumatic Rock-drills requiring 80CFM each at 90psi.

I have a compressor that is rated At 375 CFM at 90 psi and they need to run for about 30-60 seconds at a time but, lets say 40 seconds to limit variables.

1. How big of an air tank would I need?

2. I have about 35' of 2" hose between the rig and compressor. the tank will be mounted in this air line about half way. Do I need a larger hose diameter?

Bill says...

Harold, this is more a question for an engineer than a compressed air circuit designer and tool user, but let's have a look at it. And, anyone else have any comments for Harold, please chime in.

To run your rock drills you will need 480 CFM of compressed air at 90 PSI. Your compressor will discharge 375 CFM at 90 PSI, a shortfall of 105 CF for every minute your drills run.

The page Gallons to Cubic Feet conversion, linked from the tank section on the site map, will give you info about how big a tank you might need.

Yet, what is the tank size on the compressor now? You may have enough, given that you want to run the drills for 40 seconds, meaning that your supply shortfall is really only 68 CFM.

By the way, in my opinion, the production figures for air compressors are akin to those mileage figures that car companies give us! Theoretical, unless you are in laboratory conditions. But, the numbers they give is a place to start.

As to the flow through a hose, also linked from the site map page, in section Plumbing Compressed Air, follow the link to the Pressure Drop pages for info.

Hope this helps. Any engineer with hard data is welcome to add to this post.



Comments for air tank size

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Mar 21, 2014
tank size
by: Doug in s.d.ca

(Bill: I was looking for something else when I ran across this, if you were wondering...)

I find it useful to think in terms of bars, where 1 bar is atmospheric pressure - about 15psig.

You are right that any tank at just 90psi is just a bulge in the line when you need 90psi flow.

90psi is about 6 bar, 150psi is a bit more than 10 bar.

If you put the receiver pressure at 7 bar and time how long before it drops to 6 bar, then for every bar over 7 you'll extend the run time by that much.

So, say 7 to 6 bar takes (at 90psi pressure through a regulator) 10 seconds.

In this case, 150psi (over 10 bar) is 4 bar more than 6 bar, so it would take more than 40 seconds to drop to where you had less than the target 90psi.

Also, all else being equal, doubling the volume of the receiver would double the otherwise unpowered run time as well as recovery time.

Hope that helps.

Jan 14, 2012
by: Anonymous

Thanks for the reply,

Seems I was trying to keep the question short and simple but wanted the complicated answer

First off, I haven't had much luck talking to engineers.

The compressor is a rotary screw type with no tank. It is capable of 150psi. (contractor type compressor).

I had already found the link to gallons to cubic feet. the problem is, if you are putting out 90psi and requiring 90 psi. a tank with only 90psi is just a bulge in the line which means if my compressor is only capable of 375cfm@90psi (or whatever you derate it to) all you can get out of a line off the tank is 375@90

So, let me try to explain my question. what I need to know is, how many 90psi cubic feet of air are in a 100 gallon (or whatever size I use) tank that is pressurized at 150psi



And you are welcome. I'm sorry that engineers haven't been able to help. I'm afraid my experience is more in the practical use of, and not in the theoretical area, of compressed air. I hope someone out there can post a response for you.

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