Obviously, diesel air horns operate on air. If you want to blow them, you will need an air receiver that can supply air to your horn. Also, unless you plan on constantly pestering the folks at the local gas station, you will need a compressor to fill the tank. From there, you need some good plumbing and a valve to operate the horn. There are some other odds and ends that some people use, but which aren't necessary. These include regulators, air filters, etc. That's about it in a nutshell (of course assuming you already have a horn to blow).
First, I strongly recommend an ASME code air tank with safety valves. Other tanks, such as propane tanks, tanks that are not ASME rated, etc. are usually not designed for the stresses that using the tank as an air receiver put on the vessel. Considering the potential that exists for disaster or death, it's well worth it to be sure you have the correct type of tank for your setup. If you purchase the tank used, I would also urge you to have the tank hydro tested to ensure it is still capable of the pressure it was originally rated for. Because of the condensation that exists in compressed air, it's possible for air tanks to corrode from the inside. Basically, hydro testing involves pressurizing the vessel with water to some specified value beyond the rated pressure. Since water doesn't compress, if the tank fails, nothing will happen beyond a leak forming in the tank - much safer than the alternative, that's for sure!
Now, for the size of the tank, I would suggest a bare minimum of 20 gallons. This will allow you to honk most horns, at least for a short time. K and P horns will do OK on 20 gallons. However, if you can afford more space, I believe it's well worth it. I used a 50 gallon tank for a couple years, and was able to blow good, long blasts out of both M3s and S5Ts on it (two very hungry horns). I didn't have an M5 at the time, and so I don't know how well it would blow an M5, but you'd probably get at least one good blast out of it. For comparison, a good blast on the M3H, such as this one, brought my 50 gallon tank from 125psi down to about 70psi.
In addition to size, there are a couple other things to watch for on your tank. First, you will need at least a 1/2" ID port to connect to the horn's air line. Also, make sure you have a drain port on the bottom of the tank to get rid of any damaging condensation that will form. You will want to be sure you have safety valves, as well, rated for AT MOST the max working pressure of the tank. The maximum working pressure of the tank should be at least 125 pounds of pressure for horn blowing. (Diesel air horns are rated up to 140psi, though work nearly identically on 125psi.) A tank rated for higher pressures will give you longer blasts using a regulator, as well as slightly louder blasts. Lastly, be sure there is a gauge on the tank so you know how much air is inside, and NEVER fill a tank beyond its rated pressure! For instance, if you decide on a 125psi-rated tank, do NOT pressurize it to 140 pounds just to give your horn a little extra oomph. You may regret it.
There are several factors to consider when purchasing a compressor, as well as the tank. First, where will the compressor be used? If you intend to use the compressor while away from home, then a 12-volt DC compressor or gasoline compressor are your only real options. 12-volt compressors don't tend to be very large, and so will take a long time to recharge a tank. However, they can usually be fitted to run off a car's alternator if wired and spec-ed out correctly. On the other side, you can purchase a gasoline compressor that will have a very rapid recharge time, though it will take up much more space, and make a lot of racket. For years I used a 10HP two-stage gas compressor that could fill an 80-gallon tank from empty to 125psi in about five minutes, though there are gas compressors that will work much faster or slower than this as well. If you want to go all out, a gas or diesel contractor compressor will provide the ultimate in air recovery! Your choice all depends on how quickly you want to be able to recharge, what space you have, and how big your wallet is. Lastly, if you are satisfied charging your tank at home, an electric compressor will be cheaper than a gas compressor, and more efficient than a 12-volt compressor. When installation is complete, be sure that the compressor's shut off (if it has one) is set at most at the max working pressure of the tank, if not slightly lower.
I should note that others have had success blowing horns on gasses other than simple compressed air. Nitrogen is the most common, as it has roughly the same density as air and won't change the pitch of the horn like carbon dioxide would. There are many things to consider if planning to use Nitrogen, that stem primarily from the fact that nitrogen tanks are very high pressure - well over 2000psi in most cases. Regulators are critical, and safety of the pressure vessels is also of utmost importance. There are other important safety and non-safety considerations, as well, but I am not experienced to discuss them here. Please, please talk to someone who has experience with using high pressure vessels before seriously considering using one for horn blowing.
Plumbing and a Valve
Now that you have a tank and a way to put air into it, you need to get that air into your horn! In order to take full advantage of the horn (or even just to blow some of the larger horns like M5s), I recommend at least a 1/2" ID air line from the tank to the horn. Black pipe and hydraulic hose work well, though any hose rated for at least as high a pressure as the tank will do. Do not use PVC pipe, as it is not rated for gas pressure, only liquid pressure, and could potentially shatter! Also, you will want to keep the distance to the horn as short as possible, since this will give the most volume to the horn.
As an operating valve, there are many possibilities. I've seen ball valves, steam valves, solenoid valves, pedal valves, and regular horn valves, to name a few. By far, the quickest and cheapest solution is to run down to your local hardware store and get a 1/2" ID ball valve. While it doesn't automatically close, it is much cheaper than a horn valve, and will work for this purpose. You can always upgrade to a different valve in the future. You will want to mount the valve as close to the horn as possible, since this will give you the most control of the horn. When we initially set up our system, we had a 50 foot hose between the valve and the horn. On some of the smaller horns, they would continue sounding for several seconds after the valve was closed, just from the air left in the hose that needed to bleed off through the horn!
Odds and Ends
Now that you have your setup, you may want to consider a few additions for the future. A regulator can come in very handy. First, if you want to get more blow time out of your tank, regulate the pressure down between 60 and 90psi, and you will have more time with constant blowing. The regulator should be installed between the tank and the operating valve. Another use of the regulator is for tuning M horns. I prefer to tune my M horns at a low pressure, such as 20 pounds or so. If I can get them working well on this pressure, they will only sound better and have more of a dynamic range when given full pressure.
Another handy addition is the quick disconnect. Though not as easy to find in 1/2" ID connections, these fittings make adding and removing hoses, and horns, very convenient and quick. One other addition is an air line filter. There are all sorts junk and oil that can get into your air line, and eventually wear away at the horn nozzles, or do other damage. For this reason, it makes sense to put a filter on to keep the air line as clean as possible. Filters are not expensive, and should also be installed close to the tank so that anything using the air line will have a clean air stream to take advantage of.
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