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Portable air compressors

By Dean Mellor, 16 Jun 2011 Electronics & Gadgets

Tech: Portable Air Compressors

An air compressor is one of the most useful bits of kit you can carry and, as they’re not that expensive, there’s really no excuse for not having one handy.

An air compressor is one of the most useful bits of kit you can carry and, as they’re not that expensive, there’s really no excuse for not having one handy.

A portable air compressor is a must-have accessory for any four-wheel driver, and the more often you drive off-road, the more use it’s going to get.

Compact, lightweight and usually housed in a sturdy and easy to pack container, a compressor should be carried in your vehicle at all times, because you never know when you might need it, whether it’s for pumping up a tyre after a repair or adjusting your tyre pressures to suit different driving conditions.

With the right attachments, air compressors are also handy for inflating camping accessories, such as mattresses, or even kid’s toys, such as beach balls.

When you’re in the market for an air compressor, don’t be tempted to buy a cheapie at your local supermarket or auto-accessories store, because it will simply not be up to the task of inflating large 4X4 tyres.

A few years ago, we compared all of the popular portable air compressors on the market and thought we’d throw a $50 cheapie into the mix. After three-and-a-half minutes of pumping, the cheapie compressor managed to inflate a relatively small 205R16 tyre from 15psi to just 24psi, at which point it overheated and expired completely.

There are a number of features you should look for when searching for the right compressor, and not all of them are related to performance but, having said this, you don’t want a slouch that will have you waiting around for an hour as it attempts to reinflate four tyres to road pressures after a day of sand driving; especially if you’re running big off-road rubber on your rig.

Most manufacturers will state some very important information relating to the performance potential of their compressor in a variety of conditions, and it’s this information that will dictate how long it could take to inflate your tyres.

Some things to look for when deciding on the right compressor are claimed airflow capacity, maximum operating pressure, maximum ambient operating temperature, duty cycle and whether it is fitted with a thermal protection switch.

The airflow will relate to how fast the compressor can inflate a tyre and is usually listed as litres of air per minute at a specified pressure. For example, a compressor may be capable of pumping 40 litres of air per minute at 0 psi and 30 litres of air per minute at 30psi.

To get an idea of how compressors compare with each other in performance, have a look at the airflow claims and see which one comes out on top.

But bear in mind that some compressors will be designed specifically to deliver a high volume of air at relatively low pressures, such as for tyre inflation, while others will be designed to deliver a low volume of air at high pressures for operating equipment such as air lockers and air tools.

Maximum operating pressure is fairly straightforward; if a compressor is claimed to be able to operate at 70psi, then it should be easily able to inflate a tyre to 70psi, and most likely beyond.

The maximum ambient operating temperature is also straightforward; it relates to a compressor’s ability to operate at a certain ambient temperature before it overheats and potentially ceases to function. Most good quality compressors are rated at around 70 degrees C, so they will easily operate in any extreme temperatures you’re likely to experience, even in the desert at the height of summer.

The duty cycle relates to how long a compressor can operate before it needs a rest. Duty cycle is usually listed in minutes at a certain ambient temperature. For example, 45 minutes at 45 degrees C means a compressor will operate for a minimum of this amount of time in this ambient temperature before it requires a rest. And if the duty cycle is 50 percent, for example, then the compressor will need as much cool-down time as running time, which in this case will be 45 minutes.

Some compressors have a continuous duty cycle, which means that they can operate indefinitely up to their maximum ambient operating temperature. These compressors will usually be fan-cooled units, but not always. Some units will also have obvious cooling fins that are designed to dissipate heat.

A thermal protection switch is a feature that will ensure a compressor switches itself off if it gets too hot, thus preventing damage to the compressor. Most compressor manufacturers will list the temperature at which the thermal protection switch is activated.

Maximum current draw is another figure that you should be aware of. The higher the current draw, the faster an air compressor will drain your battery. Most compressors will have a maximum current draw of around 20 amps, but some of the more powerful units on the market will draw more than 30 amps, in which case you should run your engine when you inflate your tyres, or you’ll want a dual battery set-up in your vehicle.

In addition to the compressor’s claimed performance, there are a number of other important features that you should look for. The packaging is one of the most obvious. Many portable air compressors are packaged in plastic cases that look like fishing-tackle boxes.

These are generally very sturdy, have a strong carry handle and make it easy to stow an air compressor’s related accessories such as wiring, air hoses and pressure gauges. Other units come with a built-in base and will be supplied in zip-up bags with extra pockets for accessories, and others still will be supplied in steel toolboxes.

My preference is for the sturdy plastic case, as it is easy to pack without damaging the inside of your vehicle and affords the best protection for your compressor, but another alternative is to hard-mount the compressor to your vehicle. This offers a couple of advantages: you’ll never be without your compressor and, without the carry-case, the retail price of the compressor will generally be lower.

If you do opt to hard-mount the compressor to your vehicle, make sure it’s not located where it’s going to be subjected to extreme temperatures, or too much dust or water ingress. And make sure it’s located where the hose can easily reach all four wheels.

The quality of the fittings and accessories supplied are just as important as the compressor itself because, if they fail, your compressor will be rendered useless. The wiring should be substantial and of good quality and the alligator clips for attaching the compressor to your vehicle’s battery should be decent-sized units.

You can usually pick quality via a visual inspection; make sure you check out the connections between the wires and the attachments. Oh, and a 12V power outlet connector will not suffice, as most decent four-wheel drive compressors will have too high a current draw for your vehicle’s ciggie lighter.

A heavy-duty air hose is equally important and should have quality fittings at both the compressor end and where you attach it to your tyre’s valve stem. Make sure there’s enough length in the wiring so that you can attach the compressor to your vehicle’s battery and still firmly place the unit on the ground.

You should also have enough hose length to reach all four tyres on your vehicle without having to move the compressor unit. And if you tow a trailer, make sure the hose is long enough to reach the trailer’s tyres as well.

Other features to look for include an easily accessible on/off switch, so you don’t have to disconnect the compressor from your battery to switch it off; a washable air filter so you can keep your compressor running at its best; and an air-pressure gauge, either a separate item supplied with the unit, one fitted on the compressor itself or, preferably, one fitted to the air hose.

Most decent portable air compressors start at around $200 and top-shelf units can exceed $500. To make sure you end up with the right air compressor to suit your requirements, check out the next issue of 4X4 Australia for a comparison test of the most popular units currently on the market.

Finally, you generally get what you pay for. Without taking a compressor apart, it’s difficult to judge the quality of materials used in its construction, but you can tell a lot by simply listening to it. If it’s a smooth-running unit then it’s unlikely to shake itself to bits after just a few uses, and if it has a thermal protection switch and is fan-cooled, then it’s probably been designed to have a service life of thousands of hours, so it’s going to last you a number of years. If in doubt, ask the experts at your local four-wheel drive accessories retailer.