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June 2006 Willie's Workbench Onboard Air

Posted in How To: Tech Qa on June 1, 2006
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When it comes to refilling tires after a day on the trail, I've used a variety of methods-from a hand pump (yes, the same type used to pump up your bike tires), to modified A/C compressors that are engine driven. I've used the "engine air" pumps that had this "thingamajig" that replaced a spark plug. The compression pulses moved a diaphragm in a small housing to pump air down the hose. Cheap and simple but darn slow. Over the years I have gone through about a dozen small 12-volt compressors, ones that were originally designed to pump up your basketballs and air mattresses but some marketing hype had convinced me that they would work on tires. Again slow, but even worse, a life expectancy about that of a grasshopper. I finally switched to packing a Thomas 12-volt compressor mounted in a GI ammo can. This compressor from Extreme Outback (www.extremeoutback.com, 866/447-7711) is compact, sports a quality coil-spring type air hose, has a built-in thermal overload safety, along with an on/off switch. It has served me well over the years. It seems to be a good compromise between a modified A/C compressor and the lower volume 12-volt ons.

Now, however, "everyone" keeps telling me how great CO2 is and that I just had to switch. OK, maybe it is time to try something new. I went to the originator of the now-almost generic name "Power Tank," Advance Air Systems Inc. (www.power tank.com 209/366-2163) and spoke to the owner Steve Sasaki, about my needs. Steve suggested their bestseller, the PT10BR. This is a 10-pound cylinder, powdercoated gloss black, with a specially designed Super Flow regulator, and a carrying handle designed to protect the regulator. Also included was a special hose that won't be damaged by the low temperatures produced by CO2 when it expands during delivery. I also ordered up the tank mounting bracket.

OK, but why CO2 instead of compressed air? Well maybe because it's better? CO2 is an odorless, tasteless, colorless, inert gas with molecules that are larger than those of nitrogen. Nitrogen, for those of you that failed high school chemistry, is the gas that makes up most of the "air" we breathe and fill our tires with, so this means CO2, having larger molecules, won't bleed out through the pores in the rubber as fast. In a gas state it has about the same thermo expansion ratio as air. That means for every 17 degrees of temperature change, the pressure will change one pound. When stored in the Power Tank, it's under between 400 and 1,000 psi and the majority of it turns into a semi-liquid state. This pressure stays pretty much the same as the liquid turns into a gas when there is a "pressure change" and gas movement, as in filling a tire or running an airtool. What this means is that even up until the Power Tank is nearly empty, the pressure within the tank is still high, unlike stored compressed air.

These figures may not be exactly right, but let's use them for an example to compare CO2 to air. Let's say you compress air to scuba tank pressures. Compressed CO2 in the same size tank will offer three times more expanded volume without the worry of extreme high pressure. Or if you compressed the air to 125 psi in the same size tank, the expanded volume of CO2 would be 50 times greater.

When it comes to filling tires, let's say it takes three to five minutes per tire to bring my 36 inchers up to highway pressure with my 12-volt compressor. With the Power Tank it's never more than 60 seconds!

Pop a tire bead off its seat when on the trail, no problem, a quick blast from the Power Tank and it's reseated. Heck, I even sometimes use it in the shop when mounting new tires on rims.

The amount of time it takes to fill a tire and the number of tires one can fill off the Power Tank naturally will be different depending on the size of the tire and the air pressure change. Sasaki says, for example: "a 10-pound bottle brings a 33x12.50 x15 tire up 10 psi in about 20 seconds, for 40 times." Pressure changes and number of fills are in direct proportion, so a 20psi increase will take 40 seconds and fill 20 tires. I found it hard to accurately measure filling time using a single valve stem but I do know that I can refill all four tires in about five minutes total time.

This all sounds good and great, and it is. I couldn't be more happy with my Power Tank. But there are some drawbacks. The tank is big, bulky, and heavy, which means it takes up a lot of room in my "flatfender" Jeep. When you're out of air-CO2-you're out until you have the bottle refilled. It costs about $10 to $12 to refill. That means every time you refill a tire it's costing you 25 to 50 cents. Think about this when all your buddies are lined up waiting to air up their tires from your bottle. To some people time is more important than money so the small expense is of no matter.

The initial cost of a Power Tank and a quality 12-volt compressor are about equal. The air may be free but you generally have to have the engine running when using the 12-volt compressor and over time it will wear out. CO2 refills cost you money but the Power Tank should last virtually forever if reasonable care is taken. Ah, decisions, decisions.

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