Guide to Buying a Second Hand Bauer Compressor

 In Education

Would you like to have your own dive compressors and fill your own cylinders? A Bauer dive compressor does not come cheap, the best high pressure breathing air compressor manufacturer on the planet has a good reason to justify the price. High quality engineering, research and constant testing doesn’t come free. But, is the extra cost worth it? YES. How much is your safety or life worth? Please see the image of another manufacturers final stage head that separated during filling on a charter boat. That failure would have spelled disaster on the boat if it wasn’t for the pipe work that held it in place.

The cause: poor quality casting of the compressor block, cost cutting! A similar case in Asia caused the boat to sink.

But buying a brand new unit is relatively expensive, which often leads us to opt for a second-hand compressor. Then you search gumtree or eBay and find attractive marketing words such as: “in good running order“, “recently serviced“, “only operated for a couple of hours” and so on. Oh, and one more thing: “it’s cheap and the price is still negotiable“.

While these marketing descriptions may be true and applicable to the second hand unit being sold, buying a high pressure breathing air compressor is not like purchasing a home appliance where a simple visual inspection would suffice the quality. It requires some detailed information and a closer look at the unit may be similar to buying a secondhand car running on autogas, the motor may require a compression test or you end up blowing all your dough on a lemon.

Focus on the Filter Housing

Also known as the filter tower, the filter housing should be the first piece that you need to examine. As far as Bauer compressors are concerned in the lower cost range, the filter towers are the gold-colored canisters that houses the filter cartridge and usually comes in the P21 or P31 Triplex canister model. The filter tower is a high pressure vessel that performs the final filtering and supplies compressed air to the cylinder being filled.

Now, here’s the thing: overlooking, failing to examine, lack of knowledge or unable to physically see the secondhand compressor you wish to purchase as its on the other side the country can end up costing you an automatic $3,100 filter tower replacement after initial purchase of your new/old compressor. That is why the first thing we look at is the filter tower or yours and others safety maybe gravelly be at risk. A filter tower replacement could mean a financial loss for you or loss of life if you need to and don’t replace it. The $3,100 price for a new filter tower is only the trade price and does not include installation. Try taking your old compressor to one of the big names in the industry to trade it in and they may say no. Now you know why! And that’s only the financial side. Overlooking the filter tower may cause actual damage to property and even loss of life as a damaged filter tower can fail and explode generating tremendous energy equivalent to a hand grenade. A failed filter tower can produce a high pressure shock wave that has the potential to kill within a 7-meter radius. On top of the highly lethal pressure wave are the shrapnel that acts as a 360-degree shotgun.

In case you are not convinced as there are far less reported incidents with the filter housing exploding compared with SCUBA or BA cylinders, you can ponder on the following:

    • Layman’s Gas Theory (How not to get killed): A gas, in this case “Air”, can be compressed to extraordinary pressure, which once compressed, will contain potential <explosive energy>. With this, highly pressurized air must be treated with the same respect, as explosives, like a TNT.
  • Layman’s Metallurgy (Why stuff breaks): Have you tried cutting a wire without the use of a set of pliers? Simple logic will tell us that by wiggling the wire back and forth for awhile, it will eventually break in two. Now, using this logic with filter towers: even though they are made of thick metal, they bulge or flex out to a small degree when filled and contract back to their original shape when depressurized. Over time, this flexing of the metal by bulging weakens to a point that it will fail, possibly causing what is termed as a catastrophic failure. And just imagine, actually no, don’t imagine and see the following image and see our further articles.


Special Note: Bauer recommends replacing the filter tower after 15 years from date of manufacture (DOM). For ease of location, each filter tower has a date stamp that can be found on its base.

Pressure Cycle Explained

We guarantee you it’s not a matter of just assigning a random number. For decades, German Engineers at Bauer have done endless testing of the filter tower. They have come up with the pressure cycle lifespan based on metal stressing or fatigue (as described from the above layman’s metallurgy).

To give you a glimpse of what’s happening inside a filter tower, it is designed to remove the final parts of moisture (from the compressed air) and the fine oil aerosols (produced by the compressor’s lubrication cycle), and when mixed, forms what is known as condensates. The condensate are required to be drained every 15 minutes. NO MORE or NO LESS. You can do this manually by opening the 2 or 3 bleeding valves, or automatically when fitted with auto drains. You must continue the 15-minute interval while the compressor is running and filling cylinders. Every 15-minute drainage cycle is counted as “One Pressure Cycle”.

In every pressure cycle, the aluminum walls of the filter tower expand and contracts as pressure increases and decreases accordingly. This means that the vessel slightly bulges out when under pressure and contracts during depressurization as the drain valves are opened. In short, for every hour of compressor use, the filter tower vessel undergoes 4 pressure cycles. Over time this causes metal fatigue of the filter tower wall, weakens the metal and if not retired or replaced will eventually fail.

Maximum Pressure Cycle and Filling Hours

Now that you know what a pressure cycle is, let us focus on the recommended maximum number of pressure cycles and filling hours before a filter tower needs to be condemned and replaced.

Let us take the case of the Bauer P21 Triplex final filter tower (filter housing). Based on the assumption that there are four pressure cycles per hour and the condensate is drained every 15 minutes, Bauer Engineers have strictly calculated and determined the following:

  • For filling SCBA cylinders (with pressure relief configured/set to 330 bar)

Recommended Max. Number of Pressure Cycles: 4,000

Maximum Operating Hours: 1,000

  • Filling SCUBA cylinders (with pressure relief configured/set to 225 bar)

Recommended Max. Number of Pressure Cycle: 35,000

Maximum Operating Hours: 8,750

Note: The maximum number of operating hours is determined by dividing the recommended maximum number of pressure cycles to the constant 4 pressure cycles per hour of compressor use.


The Importance of the 15-minute Drain Interval

Also known as bleeding, the 15-minute condensate drain cycle is calculated by Bauer engineers ensuring maximum life of the filter cartridge and the optimum filter tower lifespan while ensuring safety.

Remember: NO MORE OR LESS THAN 15 MINUTES, otherwise you will either subject the filter tower to too many pressure cycles for what its calculated for or saturate the filter cartridge prematurely and compromise your breathing air quality.

To give you an example, here is a real life mistake: one compressor operator thought if they bled the compressor more frequently than the specified 15-minute interval, mistakenly they thought they would save on filter cartridge costs by extending the filter cartridge lifespan. What they did do is subjected the filter tower to far more pressure cycle than what it was designed for and the filter tower (a pressure vessel) eventually failed causing the filter tower cap to eject off with explosive force.

Ask for History: How and Where it has been Used

We can almost guarantee you most second-hand Bauer compressors sellers wont be able to offer you the vital history. But it doesn’t hurt to ask and some questions could at least give you an idea of the over-all condition of the unit. More than a few gumtree sold compressors were picked up by the seller at an auction. If your lucky and stumble across an owner operated compressor for sale, please do ask for the following:

  • Ask for photo’s including the Bauer manufacturers information plate to gauge the age and compressor output?
  • Ask for a photo of the date of DOM stamped on the lower base of the filter tower?
  • Ask for a photo of the pressure relief valve on top of the Filter Tower to establish if it’s 225 Bar set for Scuba or 330 Bar set for BA (firefighting).
  • What condition externally and internally is the filter tower, is there any corrosion or pitting?
  • Year bought?
  • How was it used? Personal, dive shop, liveaboard, commercial?
  • Frequency of operation?
  • Frequency of service and by whom?
  • Does it run on Mineral or Synthetic oil?
  • Does the fly wheel move freely?
  • How is it powered? Phase? Volts? Amps?
  • Does it have an hour meter and or log book of running hours?

Note: If the Filter Tower doesn’t have the screw down safety valve featured in the image of P21 Tower, as shown above, then its over 15 years old.

Compressor units that are used in charter boat operations are more prone to pit corrosion, especially filter tower models are made of aluminum.  If this is the case, then it needs to be condemned and replaced. You should also be aware that a compressor unit that is being used by a dive center or resort is most likely to have been running for long hours while filling multiple cylinders per day, or perhaps, weeks or months.

As far as the compressor block is concerned, Bauer compressors are designed to run trouble-free for 30,000 hours. As long the Bauer specified service schedule is followed or exceed which included primarily oil and valve changes. So, looking at the compressor log book or service receipts will come in handy for verification, if they exist.

If all of this so fare seems daunting or confusing, that’s where we come in. We buy and then resell fully rebuilt, refurbished to factory condition with warranty, documentation,  training and ongoing support with every compressor sold. Which in the end, will save you a lot of money and time down with an unusable compressor taking up space.

What to expect when buying online

As far as Australia is concerned, eBay and Gumtree are the two most popular sites to search for used Bauer compressors for sale. Not unless if you are planning to operate a busy dive center or have a unit operated in a Metro Fire Station that requires higher filling capacity, then most probably, the second hand units that you will be dealing with are Bauer Junior II, Oceanus, Utilus, Capitano and some older Mariner compressors. These Bauer compressor models generally use the Bauer P21 filter tower. For bigger units, the P31 filter tower is sometimes used.

Here’s an indicator for a good buy: let’s categorize the P21 or P31 filter towers as the norms for Bauer compressor. But if you see a unit that has a P41 filter tower, then put down that coffee mug you’re holding and press that inquiry button as this particular type of filter housing is more expensive and uses a higher strength metal. The P41 guarantees you a longer service life from new, but a secondhand one could have been flogged to death by a busy dive shop. Always ask the seller: Why are you selling?

Now, here’s the common scenario: Sellers are NOT aware of the filter tower vessel condition. More often than not, most seller’s don’t have a compressor log or record book showing the number of running hours that will become the basis of calculating pressure cycles, unless fitted with a B-Timer which is the subject of another article.

Go Air Safety at your Service

Bauer Company is a safety company and Bauer’s Engineers have invested a lot of time testing the compressor you’re looking at buying. However, you’re purchasing a secondhand compressor unit and need to consider the compressor’s history.

If in doubt of the second hand compressor you are about to buy, contact us and we will be very glad to share with you Go Diving’s Go Air Safety program. We can provide pre-purchase advice, inspection, testing, servicing, rebuilding and installation. We also offer fully refurbish pre-owned bauer compressors for sale with a guarantee.

On a Final Note, Bauer compressors, if operated, maintained and serviced according to specifications, will provide decades of safe and reliable service while producing safe and pure breathing air which everyone should expect and deserves.


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