Review By Dean Bielanowski  GMC Website - http://www.gmcompany.com


GMC DTC32L Dual Tank Compressor

Review
By Dean Bielanowski

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Please note: Since this review was published, Global Machinery Company (GMC) has gone into receivership and is no longer operating. As such, spare parts or technical support cannot be obtained directly through them. Their website at www.gmcompany.com appears to still be available online and offers some product information and manuals but contacting them will receive no reply. Note that OnlineToolReviews.com does not work for GMC, nor do we offer any support or spare parts for their products.

One essential piece of gear every woodworker or DIY enthusiast should have is a compressor. You could call it the swiss army knife of tools, because it provides the "power" to perform such a large variety of tasks. Whether it be frame or finish nailing, spray painting, drilling, chiseling, cleaning, inflating, cutting etc etc, the compressor is a must-have item in any woodworkers, tradespersons or renovators kit!

Ok, so the compressor itself wont be performing all these tasks straight out of the box. It is simply the supply of compressed air that drives all the different pneumatic tools available on the market for their respective tasks. Nonetheless, a good source of compressed air is a handy thing to have around. And what's more, if that air supply can convert from a "fixed" to a portable state, then all the better. This is the aim of the DTC32L Dual Tank Compressor.

The GMC DTC32L Compressor
This compressor is a twin tank model. The top tank can be readily removed from the unit and carried direct to the site of work as needed. And even when full of air, this tank is extremely light, so it is ultra portable and easy to manage. Having a total tank capacity of 32L, naturally, each tank holds 16L each of air. For some tasks, like larger spray painting tasks, or where a continuous air supply is required, the 16L tank capacity will likely not be enough for the task. However, the as both tanks can be connected to each other via a simple air hose, you can have the full 32L capacity at your disposal, but you then must cart the whole unit to the jobsite as the bottom tank is fixed and not removable.

The unit is powered by an oil-lubricated, two horsepower motor. This is a suitable motor size for a compressor of this capacity and fills the tank up to a working cutout pressure of around 120 PSI. Most pneumatic tools are not designed to run on air pressure higher than 120 PSI, so this is an adequate cutout limit. Once the tank pressure drops to around 80-85 PSI, the motor will kick back in and refill the tank back up to the 120 PSI cutout pressure. Now, the specs say this upper limit is 125 PSI, but I found the gauges to be reading closer to 120. This did not really have any effect on my use of the tool, or on the air tool performance. There are no sound level output specs listed but I'd say it would be easily over 80dB, which means it's ear protection time, especially if you are working near or next to the unit. Most direct drive compressors are quite noisy, and oil-less units tend to be even noisier still, but since this is an oil-lubed unit, it is a little quieter. Nonetheless, you don't want to be powering up this unit in the middle of the night. If you want a quieter unit, you need to look for a belt driven model. But noise and direct drive compressors are a given, so this one is no different in that regard, and certainly not a fault of the unit. Pump displacement is listed as 7.3 CFM.

The compressor provides up to 4 CFM free air delivery. This is four cubic feet of air per minute. As such, this is not the ideal compressor for continuous air delivery, like that required for continuous high volume spray painting, or for air sanders or grinding tools that are in constant operation. Having said this though, it doesn't mean you cannot use these tools. The thing is, you will have to use them in slower time cycles, meaning you have to stop operating them now and then so the compressor can "catch up" on filling the tank. In other words, the air used by some of these tools in constant operation will exceed the air volume the motor can replace back into the tank. I had no problem using a standard HVLP spray gun running at 50PSI for general spray painting tasks. But, I didn't have the trigger depressed constantly. It was shorter bursts of painting than continuous work. For the occasional spray paint tasks, the compressor will do fine. If you want to continuously spray paint, grind, or sand using pneumatic tools, you will need a compressor/tank size combo that can deliver at least 7-8 CFM free air delivery continuously. Now, airbrushing is a different matter altogether! Because these small spray painting tools use very little air, and at low pressure (often less than 30 PSI) you could airbrush all day with a 32L tank and the motor would not have to power up nearly as often as with higher volume air tools.

When it comes to nailing with pneumatic nailers, yes this compressor will drive all types and sizes of nailers, from the largest frame nailer, to the smallest brad or pin nailer and keep up. I had no problem here. Nail guns use very little air and it uses that air in very short bursts, so total air consumption is a lot lower than continuous air driven tools. The compressor will probably cycle once or twice for a full 100-nail framing nail clip, but that's not unreasonable given its size. You have to balance size vs use when considering purchasing a compressor. Sure, you could have a large compressor that will run your air tools all day no problem. But are you willing to lug this behemoth of a machine around all day too? Probably fine if you use it in a workshop in a fixed position, but then you have to have the room to place it and keep it too. It's a balancing act.

With the twin tank setup, as mentioned, the top tank can be removed from the unit and taken elsewhere. It comes fitted with a tire inflation hose and nozzle. I removed this and replaced it with a quick-connect air fitting so I could run my air hose and whatever tool I connected direct from the portable tank.

The portable tank is secured to the compressor via two stretchable rubber straps, which seem to do the job fine holding it in place for refilling or when using both tanks in a non-portable situation. These straps are easily removed to make the second tank portable. The portable tank is fitted with its own tank pressure gauge, one Nitto-style air outlet, and another Nitto port which connects to the hose on the compressor unit to refill the tank. These are all quick-release fittings for fast connection and removal. A drainage valve on the bottom of the tank allows water to drain after use to help prevent tank corrosion. The portable tank has its own feet to sit on when not strapped to the compressor, and as mentioned, is very light and very portable. I take it around the house often to use primarily with my brad and finish nailer for the ongoing renovations that seem to be occurring here all the time! Off memory, I think I can get roughly 25 brad nails driven before the portable tanks tarts to creep below the 80 PSI mark. I also use the portable tank extensively with my airbrush on my other hobby (Plastic scale model building) and for this task, the DTC32L is worth its weight in gold. I can spray for hours on the one 16L portable tank. Considering the cost of a specialized airbrush compressor, the DTC32L is cheaper, and more practical as a unit overall. And better yet, you can even purchase a second portable tank from GMC to have two portable tanks if need be. These spare tanks are offered with a standard tyre filling valve that suits petrol station's air hoses, so you could even fill it from there.

Given that I do most of my airbrushing late at night, I need an air supply that is quiet and has the capacity to last the night hours. Since I fill the portable tank before nightfall, so as to not wake the neighborhood up, the late night spray is essentially a silent affair using the 16L air capacity from the portable tank. When it comes time to refill the portable tank, simply take it to the compressor, attach the air line to the inlet port, switch on the compressor and refill. Note, that when the tank is refilling, both of the tanks on the unit will be receiving air. There is not a way to just fill the portable tank itself (unless you have one of the extra tanks with the tire valve as mentioned above). However, the upside of this is that if you bring the portable tank back empty to the compressor, and the other fixed tank is full, when you attach the air line to the portable tank and release the valve, the air from the fixed tank will begin to fill the portable tank on differential pressure. So you will then have both tanks equalize in pressure, essentially half-filling the portable tank without the compressor motor even engaging. But, of course, to fill the portable tank up completely, the compressor motor will be needed, and will again fill both connected tanks equally. The portable tank does not have a regulator fitted, so if you need regulated air, you will have to supply the regulator along the air supply line yourself. The compressor unit itself does have a regulator fitted with regulated air outlets for when using either the single fixed tank, or both tanks fitted in place.

Now, to the compressor unit itself. As mentioned, the unit offers a regulated air supply via two Nitto-style quick connect air outlets, so two air tools can be used at once direct from the unit at the desired air pressure. A regulated air gauge on the outlet panel will show regulated air pressure, while a second gauge shows tank pressure. A large yellow knob provides the means to adjust regulated air pressure. Note, however, that air pressure can only be adjusted properly when air is flowing through the system. Usually what I do is attach an air blower tool to the regulated air port, then start blowing air, and while this is happening, adjust the air pressure as required by turning the knob. Usually I turn it all the way to zero, so no air is flowing, then adjust upward to the desired pressure from there. Remember that the regulated air pressure reading showing on the gauge is the pressure "downstream" (i.e. toward the air tool) from the regulator.
The unit features a sturdy metal-encased air intake filter. I have a similar one fitted on my single tank compressor. They are much better than the cheap plastic capped filters that I always happened to break or snap off when I knocked them. It is good to see a sturdy air filter is used on this unit. Be sure to occasionally remove the filter and clean it out.

The lower tank is welded to the frame, and the motor is also fixed to the frame and remotely fitted from the fixed tank. I.e. it is not directly fitted to the tank like on many smaller portable  single tank compressors. A fold down/up handle allows to to move the unit around easily, and the 6" rubber wheels mean you can wheel it, rather than carry it. A small tool tray is fitted on the topside of the frame which is handy, and two carry handles are also located adjacent to this if you need to lift the unit up completely (to carry up stairs or over rough terrain). Be aware the the unit weighs around 37kg, and is a little awkward size and shape-wise for single person lifting. it can be done with correct lighting techniques just fine, but a two-person lift is so much easier. The only thing to check here is to tighten the nuts holding the handle to the frame. The ones on my unit where a little loose and cause quite a loud rattle of metal on metal when powered up. A quick turn of the wrench had these tightened up and virtually eliminated most of this noise. Do not overtighten them though as the friction created on the handle will make it harder to lift, or lower. Also, while you are checking the frame and motor, check all other fixing bolts are secure, and most importantly, don't forget to add the supplied compressor oil to the oil tank on the motor before you start it up for the first time! These motors are shipped "dry" to prevent oil leaks during shipping, so the oil must be added by the user or else you will quickly fry your motor. Fill the tank so the oil is showing level with the middle of the red dot on the glass oil level gauge on the end of the motor. Compressor oil should be changed often as well to ensure clean and fresh oil is being circulated to keep motor components well lubed with clean oil to extend their life time. I change my compressor oil about every 100 hours of use, or once every year, or even six months if used on a regular basis. If I was using it ever day for many hours a day, I might change it every 3 months even. Keeping good, fresh oil in the motor is perhaps the biggest factor to ensure good service life of your compressor. It is a small investment that pays off in the long run!

In Use Considerations
Compressors are pretty easy to use in general. Usually you just make sure all the drain valves are closed, check your oil level is ok, ensure the pressure relief valves are not stuck, and switch on. The compressor does pretty much everything else, including switching itself off when it reaches the prescribed cutout pressure. Some other things to consider though. It is recommended that air compressors not be run off long electrical extension cords. In fact, most manufacturers will not warrant the tool if it has failed after running off extension leads, and GMC is included here. These things need all the power they can get to operate correctly, and power loss from low quality or long extension cords can make a difference to the power available at the unit itself. Indeed, my other single tank compressor has sometimes not even started if I have tried to run it off too long of an extension cord, or from a power outlet where other devices are also consuming power. If you need to run it off an extension, and you do this at your own risk of voiding your warranty, ensure you have a good quality, large diameter electrical wired cord designed for industrial or trade use. Your best option, however,  is to just plug your compressor directly into a wall outlet, then running a longer air hose to where you need to go.

Another thing to consider is to drain the tanks after each use, or at least once a week. Usually when I am finished using the compressor, I release the drain plugs (by screwing them inward to the tank). Often I will take a blow gun and expel most of the high pressure air in the tank down to about 40 PSI or below, then release the air via the drain valves. The remaining air int he tank helps force moisture toward the drain hole where it can drip out overnight or while not in use. Removing as much water from the tank as possible will help prevent tank corrosion. The water gets there because air under high pressure separates it. In humid conditions, more water will be produced in the tank.

If you are spray painting a lot, consider adding a water trap along the supply line somewhere. Usually this is best placed near the end of the hose as it can also collect any moisture that is trapped within the supply hose itself. Often this is not practical for most larger painting applications, so consider attaching one closer to the tank. For airbrushing on models or fine detail work, a water separator will make a world of difference to the quality of the finish, especially if you are using oil-based paints.

Most air tools themselves require oil lubrication, although there are some air tools that do not, and should not be lubricated. For tools that do require it, simply buy some air tool oil (which is different to air compressor oil - which is added to the compressor oil tank) and place a couple drops into the air inlet of the tool before attaching it to the supply line. The high pressure air that runs through the tool will readily disperse this oil through the tool, lubricating seals and working components. This generally only needs to be done once per day, but if the air tool is being used heavily, a few extra drops should be added half way through the job.

Special considerations affecting the GMC DTC32L specifically and other compressors in general should include, checking all fittings are secure. If leaks are found, ensure the connections are removed and connection threads sealed with teflon tape. This tape can be found in most plumbing sections of hardware stores and costs about $1 for a small roll. Wrap the tape around the threads of the connecting pieces before re-joining them. This will ensure all connections are leak-free and you won't be losing air passively from the system. Occasionally check and clean the air filter on the compressor motor. If it is blocked or dirty, it is harder for air to enter and causes more load on the motor.

With the DTC32L's dual tank setup, there is a valve which controls whether air can enter the portable tank or not. This allows you to only fill the fixed tank if you require only a small amount of air. To fill both tanks, ensure the filling hose is connected to the portable tank, then open the valve so air can access both tanks. To double check this, look at both tank gauges to ensure they are filling equally.

Conclusion
In overall terms, use of the GMC DTC32L is not really much different from any other compressor, apart from the removable tank option. And this removable tank is what sets it apart from many others on the market. It is great to be able to take a light tank right to the site where it is needed without having to lug the whole unit with you. Naturally, the portable tank capacity of 16L will not be suitable for continuous use air tools, but it is great for the odd nailing, fastening, quick cutting or tire or football inflation task that doesn't require a lot of air. Or for the airbrushers out there, this is definitely one tool to consider! It will offer a continuous flow of air without the regular and annoying pulsing of small, tank-less diaphragm air compressors that make airbrushing such a pain in the backside! As a general air supply option, I found the DTC32L to be up to par in delivering a consistent supply of air. The regulated air supply works as you would expect and the mobility inclusions are adequate for the size and shape/weight of the machine. I can't comment on durability as I haven't really owned it long enough yet, but the tool is backed up by a 2 year warranty for peace of mind. I am quite happy with the compressor in general. There were no major flaws discovered and everything worked as you might expect, right out of the box.

The twin tank compressor is quite reasonably priced at AUD$199. Considering you get the portable tank option, plus twin regulated air ports onboard and most fittings supplied, it is not a bad price at all. It is well suited for any home DIYer, renovator or woodworker looking for an air supply to run their pneumatic tools in a non-continuous use environment.

 

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GMC DTC32L Photos
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The DTC32L with handle folded town and both tanks in place.


Here is the fittings on the portable tank. Left is the Nitto style connector which receives air from the fixed tank. The yellow valve with the ring is the pressure relief valve. The gold fitting is a Charge Air style quick connect fitting I replaced so I could use my air tools. And of course, the tank pressure gauge up top.


Part of the "Control Panel" with tank pressure gauge and large regulator adjustment knob.


The other side of the control panel features the regulated pressure gauge and the two regulated outlets. Note that the compressor comes with two Nitto style outlets here (right fitting) and I have swapped the left one for the Charge-Air style fitting I use in my workshop.


Main compressor ON/OFF lever (red tipped), and secondary tank flow valve (black lever).


Compressor motor is mounted remotely from tanks on the bottom of the frame. Yellow cap is oil reservoir fill hole. Note the oil level site glass below it.


Large 6" wheels offer good portability along with the large handle.


Note the stretchy black rubber straps that hold the portable tank in place when fixed to the compressor frame.


The portable tank is a great and useful feature of this package, although it has its limitations too.


 

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