By Mark Polk, RV Education 101
Most motorhome owners understand the importance of maintaining proper tire inflation pressure. The problem is if you wait until you're on the road to check tire pressure, the tires are too hot for correct evaluation.
Checking the inflation pressure when you stop to refuel doesn’t make sense, either, because you will get higher pressure readings. And if you let air out, the tires will be underinflated when they are cold.
Another problem is when you are at the campground and want to check the tire pressure, but there is no available air support to do it.
For those reasons, you might want to consider purchasing a portable air compressor.
Air compressors can be confusing to understand, especially if you’re trying to find one that can increase the inflation pressure in large motorhome tires. Here are my 7 tips for choosing the right portable air compressor for the job.
1. Air compressors have three ratings you need to be concerned with: horsepower (HP), Cubic Feet per Minute (CFM) and Pounds per Square Inch (PSI). Understanding these three functions of an air compressor can help in selecting the right portable air compressor for the job.
2. Horsepower. Unfortunately, when it comes to air compressors manufacturers don’t do a great job explaining how the HP rating works, almost to the point some would consider false advertising. Manufacturers tend to overinflate HP ratings. I believe the reason is if 6 HP is advertised, rather than 2 HP, the customer would purchase the higher-HP rated compressor. This is true in many situations. If you’re a muscle car enthusiast, you would definitely consider the HP rating of a vehicle before buying it, right? But if we take a closer look at horsepower as it relates to air compressors, we can seek out the truth.
Horsepower was invented by an engineer named James Watt. The same watt you think about when buying a light bulb. Watts are used as a measure of electrical and mechanical power. When we apply watts to HP, 1 HP is equivalent to 746 watts. So if you buy an air compressor rated at 6 HP and it operates on 120 volts, it is, in a sense, a false rating. Here’s why. If you plug the compressor into a standard 15-amp, 120-volt circuit it would produce only about 2 HP (120 volts X 15 amps = 1,800 watts). This is slightly over a 2 HP rating.
3. The higher 6 HP rating advertised by the compressor manufacturer is commonly referred to as peak HP, similar to a vehicle’s engine HP rating. The peak HP rating is accurate only when the engine is running at a higher RPM range (usually 5,000 to 6,000 RPMs), but rarely do you drive your vehicle at 5,500 RPMs. As for the air compressor, it would require more amps, from a 240-volt circuit, to produce this peak HP rating.
4. Cubic Feet per Minute. CFM is an important rating when it comes to portable air compressors. CFM is basically a measurement of the rate at which a compressor can deliver a volume of air. CFM varies with atmospheric pressure. It also varies with temperature and humidity. Air compressor manufacturers calculate Standard Cubic Feet per Minute (SCFM) as CFM at sea level with 68 degrees F, at 36% relative humidity. Note: CFM ratings can be skewed by the manufacturer, too.
5. Pounds per Square Inch. PSI can help put things in perspective. For air compressors the CFM ratings are given at a specific PSI. In other words, an air compressor rated at 3 CFM @ 90 PSI should be capable of delivering the 3 CFM at 90 PSI. The problem is air compressor manufacturers can advertise a higher CFM at a specific pressure because the air compressor is capable of delivering the rated CFM as it goes from 0 to 90 PSI.
For example, let’s say you have tires that run 85 PSI. You purchase a portable compressor rated or 3 CFM @ 90 PSI, thinking it will do the job. The problem is the compressor might be able to supply the 3 CFM of air flow at 45 PSI, but as the pressure increases, the volume of air flow decreases. It can supply 3 CFM of air flow at, say, 0 to 45 PSI, but at 90 PSI the air flow decreases, so it labors trying to add a few pounds of air to the tire.
6. So the real question is, what is the true CFM delivered at 90 PSI? If your air tools or RV tires require a specific CFM rating at 90 PSI, you more than likely won’t get it based on the advertised CFM @ a specific pressure. What this translates to is that it will take a long time to inflate some larger RV tires, if the compressor is, in fact, capable of inflating the tires at all.
7. When shopping for a portable air compressor the key is to try and find one that can deliver about 4 TRUE CFM based on the TRUE HP rating at 90 to 100 PSI. If you have the storage space available, and you have access to electricity, you can find a reliable pancake-style portable air compressor that will do a decent job of inflating tires and performing other small household jobs when you’re not using the RV.
So there you have it. Understanding the basics about portable air compressors can assist you in selecting the right one for the job.