Debunking Portable Air Compressors
I think most RVers 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; you will get higher pressure readings and if you let air out the tires they are underinflated when they are cold. Another problem is when you are at the campground, and you want to check the tire pressure, there is no available air support to do it.
This is why you might want to consider purchasing a portable air compressor. But, air compressors can be confusing to understand, especially if you are trying to find one that is capable of increasing the inflation pressure in large motorhome tires. Let’s start with some air compressor basics.
Air compressors have 3 main ratings associated with them, horsepower (HP), Cubic Feet per Minute (CFM) and Pounds per Square Inch (PSI). Understanding these 3 functions of an air compressor can help us select the right portable air compressor for the job.
We’ll start with horsepower. Unfortunately when it comes to air compressors manufacturers don’t do a great job of explaining it to the consumer, almost to the point some would consider false advertising. Manufacturers tend to over-inflate HP ratings, assuming if the consumer see’s 6 HP advertised, rather than 2 HP, they are more apt to 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 by the name of James Watt. The same watt you think about when buying a light bulb. Watt is used as a measure of electrical and mechanical power. When we apply watts to HP, one 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. If you plug the compressor into a standard 15 amp, 120 volt circuit it would only produce about 2HP (voltage X amps = watts). The higher HP rating advertised is commonly referred to as peak HP, similar to a vehicles engine HP rating. The peak HP rating of a vehicle engine is only accurate at a higher RPM range (usually 5000 to 6,000 RPM’s), but rarely do you drive at 5,500 RPM’s. As for the air compressor it would require more amps, from a 240 volt circuit, to produce this peak HP rating.
The Cubic Feet per Minute (CFM) is an important rating when it comes to portable air compressors. CFM is basically a measurement of the rate a compressor can deliver a volume of air. CFM varies with atmospheric pressure. It also varies with the temperature and humidity of the air. 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. Let me explain
Let’s talk about our third rating, Pounds per Square Inch (PSI). 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 going from 0 to 90 PSI, but 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 too 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.
When shopping for a portable air compressor the key is to try and find one that can deliver about 4 REAL CFM based on the REAL HP rating at 90 to 100 PSI.
If you have the storage room available, and you will have access to electricity, you can find some reliable pancake style portable compressors that will do a good job inflating tires and performing other small household jobs. If you prefer a battery operated portable air compressor I came across a model that looks like it can handle the job, while I was researching the topic. It’s by VIAIR and it was designed specifically for RV and other large tire inflation jobs. I am in no way affiliated with this product, but if you would like more information click here.
Happy Camping,
Mark Polk
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Good article.. however, could you go into the starting currents for the compressors…
We found that a 1 hp compressor won’t start using a 4kw onan… and when we calc’ed it out we found for the elect motor it has.. it takes a whopping 4800 watts to get it to start … Might want to explore that issue also with the smaller ones and using the onboard gen set… seems 746 is not 746 watts when the thing wants to start…
thanks again
from 120 volts to 240 volts wouldn’t it take less amps… for the same unit?
Good article! Can’t you just imagine someone trying to pump up a tire on a bus-type chassis?? Using one of those little 2 HP 80-100 CFM pumps? WOW! What a headache. ALWAYS check tires cold—what I’ve learned over the years being a former auto mechanic. I do know that those battery operated pumps are good for riding lawn mower tires and such. Work fairly well on TT tires, to. I speak from the voice of experience. Please be careful using one of those units—most caution the user NOT to run over a certain time period. Burn that thing up and now, you’re in a worse mess that you were to begin with.
QUESTIONS MARK: Does more PSI necesarily mean more CFM? And vice versa?? Does more HP mean more of both?? A little clarification would help along those lines.
Why not just get a tire inflater instead of an air compressor? They are designed for the job and seem to work better at inflating tires.
I have the Viair 400P-RV compressor kit and really like it. It tops up my 80 psi tires quickly, stores in a bag and has close to 60 feet of hose that comes with it. All chucks and gauges are included. I clip the power cord to the truck’s battery and I can easily reach all of my tires. Shop around as prices do vary.
On a Class A motorhome, tap into the compressor system for the airlift bags and brakes. Use a quick disconnect and line to an easy access point on your vehicle. It is best to purchase a good quality commercial line and connector from a company that makes high pressure equipment lines and connections.
$250 – $300 is a bit rich for me. I bought a 115VAC 1.5 gallon Husky for around $100. If I need to use it off-grid I’ll have to start the generator, but that’s no big deal. This compressor can not only very quickly air up the tires but can also blow my water lines for winterizing the rig.
Thanks for the article…though a lot of it (and some of the other comments) kinda flew by me. After a couple of days of some rough driving in the SW this summer and over a month on the road, our Sears compressor (easy to hook up and use) was a real blessing to this RVer. Last summer (2008 was our rookie year pulling our Jay Feather) was a bear trying to find gas stations that catered to RVs (for gas and air) we bought an air compressor. I wish our RV dealer had told us how important it was to have this sort of equipment.
Is there a list somewhere of some must have equipment? If so I’d like to see it.
PS I love RV.NET…the most helpful place for us by far!
Okie,
I’ll try to answer your questions.
For the sake of an example let’s say you have tires that run at 85 PSI. You purchase a portable compressor rated at 3 CFM @ 90 PSI thinking it will be fine. The problem is the compressor might be able to supply the 3 CFM of air flow at 45 PSI, but as the pressure in the tire increases the volume of air flow decreases. It can supply a steady 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.
HP ratings and tank sizes apply more to larger compressors with more demand placed on them. If you have a garage and will be using multiple air tools for extended periods of time you need higher HP ratings to produce the higher CFM ratings required to efficiently run the tools.
What is important with smaller compressors, like we are discussing, is that the right amount of air pressure and air flow (CFM) can be met at the same time to more efficiently inflate the tire.
The literature for the VIAIR 400P-RV illustrates some additional ways that manufacturers can mislead us about the capability of an air compressor.
One item is the “duty cycle”, stated as 33% at 100 psi. The practical meaning is, if you run it for 2 minutes, you’d better let it cool for 4 minutes before going on, or you’ll burn it out, as Okie mentions.
Another “inflation factor” is in the performance data chart showing amperage draws up to 29 amps, at 13.8 volts. Since a fully charged battery only produces about 12.6 volts, the actual draw would be higher, or the output much lower. With the typical RV battery bank, that much load could be maintained for only a very few minutes. This compressor should be used only when a full sized converter or engine alternator is also running.
I guess you know your stuff? But after reading your artical.
I STILL DON’T KNOW NOTHING ABOUT WHICH COMPRESSOR TO BUY.
Carrying an air compressor is totally unnecessary and they are typically heavy. The first thing in the morning before hitting the road, check your tires with a good dial type analog gauge. If you need air in a tire, drive directly to the nearest gas station before hitting the road. If you have a flat, call AAA or Good Sam or change it yourself, Forget carrying an air compressor. I also carry a DeWalt electric impact wrench. This article makes no sense!
Charles,
When you buy an air compressor you need to size it according to your personal needs. If it is only intended for inflating tires and other small jobs the important thing is how much air pressure your tires run at.
A good starting place for a basic portable air compressor that can handle these types of jobs is to find one that specifies it is capable of delivering about 4 CFM @ 90 PSI
Phillip,
A portable air compressor may not make much sense to you, but we use ours to inflate air matteresses, a small 2 man inflatable boat, bike tires, and of course to inflate the RV ires when they need it. I can check and inflate the tires when they are cold, before traveling, for better accuracy and I don’t have to search out a gas station before heading out that day.
When we return home from a trip I use the same compressor for numerous jobs in the garage and around the house.
They are portable and most of the pancake models will weigh between 30 & 45 pounds.
So, for many of us it makes perfect sense!
I routinely boondock on my property in the wilds of the Sonoran Desert in rural SE Arizona. It is 15 miles over rough dirt roads to get to any place that even resembles a service station. Having a means to properly inflate my tires makes eminent good sense to me. Thanks for the article!
Mark,
For at home, I have a large industrial air compressor that handles heavy duty applications like painting, sandblasting, etc. If a pancake compressor is the only one you have, you need it!! I couldn’t make it on a pancake compressor.
Searching out a gas station is no problem. They are all over the place when you are on the highway. If not, you may want to get a GPS and load it with POI (Points of interest). It won’t affect tire pressure accuracy to drive a few miles, especially on cold mornings. If you are going to carry an air compressor, why not carry a oxy/acet. torch, a spare fuel pump, generator, water pump, spark plugs, heck carry a spare engine and transmission while you are at it!.
Phil
Phil,
As Pat stated above, sometimes finding a gas station that accomodates RVs can be a challenge, and some people dry camp, some people just like convenience.
I do not know why you have to be facetious.
We all after all here to learn. There are many different situations and concerns.
Thank-you Mark for your informative article that gives us food for thought. We do appreciate your perspective.
I agree with Francis: “Having a means to properly inflate my tires makes eminent good sense to me.”
I thought the article provide some good basic info. i.e. Do not always trust the advertisements.
General thoughts on the subject.
I am inclined to believe that if your tires require 80 psi or more you probably have a 120V generator in your RV so I would suggest you stay away from the 12v compressors unless you have absolutely no space. Their output is low and they tend to run HOT. I have managed to kill two while doing some tire inflation testing. If you have on-board air (air bags or air brakes) you should be able to have a connection added that will give you the couple of PSI needed to top off your tires.
Once you have confirmed that your tires are correctly inflated you should not need to add more than 3 or 5 psi over any given month period. If you need much more than that you probably have a problem (puncture or leaking valve) that needs attention at a service shop. I carry a small pancake compressor like this
http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=95275
that would provide enough air to get you to service even if you have been boondocking for 4 months. I got it on sale for less than $50.
While it is best to check and adjust “cold” tires i.e. not been operated for 4 hours and not in the sun. That doesn’t mean you can’t figure out how to add the correct amount of air after driving a few miles down the road. Simply record the cold inflation and calculate how many psi you need to add. You can then drive the 3 to 10 miles to the location with high pressure air. Measure the HOT inflation and simply add the number of PSI needed to correct the cold inflation. Since you have a cushion of at least 5 psi versus the minimum pressure to carry your load a +/- 1 psi error should not be a problem.
I would not worry about the time it would take to inflate your tire from zero to your 100 psi or so requirement. If you have a flat you have bigger problems that no portable compressor can fix.
Quick final thought.
At the Gypsy Journal Rally in Ohio last month I verified tire gauges for people attending my Tire Basics Seminar.
We discovered that 15% of the gauges were off by at least 5psi with a couple reading over 15psi HIGH!
Get a good digital gauge and compare it to another known good gauge.
Maybe *not *carrying an air compressor works for Phil, but my “air on the road” experiences over the past 20 years or so of RVing have made me a firm believer in the wisdom of carrying one, and I wouldn’t be without it!
*Very few* gas stations offer air hoses or even air stations any more, and even if there *is* an air hose, the access to it may be difficult or impossible, or the coin-operated “air machine” at a station requires a big supply of quarters (and maneuvering to postition the rig because the air hose is too short to reach!), or the air hose or connection has been run over too many times to provide a tight seal at the valve (or even to function!), or the kid in the station forgot to turn on the compressor when he opened in the morning… You see where I’m going with this?
And, finding a tire shop to air one’s tires is often impossible, or, at best , *very *inconvenient! By the time one finds a aplace with an air hose, the tires are well-warmed and the “fill” pressures are not accurate.
I did this “find a gas station” stuff, too, for a brief time when I started RVing quite a few years ago; I discovered very quickly that a small air compressor (I carry an old version of the currently-labeled Campbell-Hausfeld FP2040) made airing the tires a *lot* easier and *far* more convenient! The small , compact 2-gallon compressor works well (not fast, but it gets the job done), is simple to use on shore power or genset, and is reasonably lightweight.
Traveling with a small air compressor is a choice that I have never regretted; I simply wouldn’t go without one. YMMV.
Driving only a few miles with underinflated tires will damage the tires, and once damaged they don’t cure themselves. The damage accumulates and then you have killed another tire before its time.
I have a 38 ft DP with 22.5 inch tires It requires 110 psi in the tires. On my way to Alaska I had a dickens of a time finding air available at that pressure. Many stations offer it cutting off about 90. I think carrying it along makes sense to top off when needed. Plus using an air hose to blow out the dust after time on the ALCAN is a good use of the tool.
My wife and I are ROOKIE RV’ers and we both carried air compressors in our cars! Now I have to go double and triple check the 29 foot Puma’s tires. Its amazing to know how much we don’t know – and probably never will.
Thanks for the article. Even though you didn’t tell us what to buy, you told us some good stuff and are making us THINK (ouch).
M
Good article and comments (for the most part). We “fulltime” RV in a 40ft. diesel pusher, and the tires are rated for a minimum 110psi at full load capacity.
I have tried to use the coach air system exclusively, but it will only get the psi up to 107-108. I would like to carry about 115psi, but am just unable to get them that high with what I have. I have yet to find a service station that offers air up in that range, and even truck stops are generally not helpful.
I have looked at the VIAIR 400P-RV and see that it has a peak capacity of 150psi, but I am wondering if anyone has any info on its capability at those higher pressures. It may say 150psi, but once those big tires hit the 105-115psi range, they get very difficult to increase with any type small system.