When I was about sixteen I did what I would never recommend to anyone today: I built my own diving tanks. Three jetfighter crew oxygen tanks from the air force scrapyard and two ordinary industrial valves for oxygen tubes formed the core of the concoction. The tanks had a volume of about five litres each. Two of the tanks were the main depot of air and, once that was used up, I could easily turn on the third cylinder with ample reserve air. Testing the reserve valve prior to each dive was my routine.
The experiences gained as a DIY diver would help me later during my career as a surveyor. One experience, though, was quite special and I shall share it with you.
The tanks had apparently been tested before – a “150 kg” had been punched on each tank as the safe operation pressure. Still, after using them a few times, I decided to have them retested.
Firstly, I found out how to do that. It had to be done at a suitable facility. Initially, the tanks would be inspected internally for rust. Then, if they passed the visual inspection, they had to be tested with pressurized water of 50 % overpressure, thus with 225 bar in my case – or so I thought.
With the tanks on my back I took the suburb train to Maltepe, where a compressed gas facility was known to carry out such tests. This was very exciting stuff and I could hardly wait for the Maltepe station.
From the train I walked to the factory, and after placing some reasonable fee and receiving an elaborate receipt, I was allowed to proceed into the facility. There were all kinds of industrial gases, cylinders, compressors and you name it. Excited very much at what was around me, I took advantage of the opportunity and began strolling leisurely. I found my way through to the test facility. I remember distinctly that there was a dirt floor. A quite worn looking compressor and some coiled copper tubing caught my sight. A few adapters and rusty tools were lying on the floor as well.
While waiting to meet an engineer, I was also polishing my newly gained knowledge about compressed gases in order to “exchange knowledge” or, to be honest, show off a bit.
Eventually, a grimy guy who looked rather like a worker walked in. I was disappointed as I failed to recognize a partner in him with whom to exchange knowledge. On top of it, he was not really talkative. He removed roughly the valves and put through the neck of the tanks a small torch bulb attached to some stiff wires. “Are they sound?” I dared to ask. The reply was a short and meaningless grunt. But the guy must have been satisfied as he proceeded to hook up the tanks to the compressor. It was so exciting. I would now see my tanks pressed to 225 bars!
100, 150 bars read the huge gauge. 200 bars! I was very excited. But then the guy turned the compressor off, and after a few sharp blows with a hammer on the full tank, let the pressure off as well. Then he grabbed the next one.
Where were my 225 bars? Breathing through disappointingly, I aired a “Sir! Would you please test to the limit of the tanks, as required?” Not satisfied, I pointed at the punched “150 kg” legend and added, “150 bars plus 50% margin are 225 bars and not 200, if I may point out!”
An annoying silence followed.
The guy tested, in the same manner, the next tank and then the third – both only up to 200 bars. Then he started to put the valves back.
“Sir! Did you hear me? 225 bars, not 200!” I repeated, probably raising my voice in the process.
At that stage the guy looked straight into my face: “Look!” He was pointing at the huge inscription on one of the tanks that read “2000“. Painted over several times, the inscription was faint. Yet it was there. And I remembered noticing it while buying the tanks, but it had not meant anything to me at the time. “Can you read?” – “2000!” he retorted. Then he grabbed a piece of wire and, using it as a scribe, performed some calculus, right on the dirt floor. I was baffled.
Shouting in a voice that suppressed the noise in the factory, he pointed at what he had just scribbled: “2000 psi, boy, 2000 psi!” My eyes grew. “2000 divided by 15 makes 135 kg. Do you follow me, boy!” And he continued: “Some idiot in the past overlooked the manufacturer’s limit of 2000 psi and tested the tanks to 150 kg and punched that number in. You must never exceed the manufacturer’s pressure limits. Never.”
I was very quiet. That worker had done what was right and I was simply wrong. I apologized.
Then the guy thawed considerably. “The valves need servicing. Let me do that.” Remembering the limitations of my pocket money, I asked what they would charge for that. “Nothing“, he replied, “rarely seen a customer like you.” He dismantled both valves and remachined the seals before putting them together carefully.
On the way back, riding the suburban train, I recapped that day’s experience:
- Slap on the shoulder for having the tanks tested.
- The “2000” had been there from the beginning, I had seen it before, but I had never attempted to understand what it was.
- The “150 kg” had been punched in, at best, by mistake. I had not been sufficiently sceptical.
- Some people measure in psi, some in kg. Some in metres some in feet.
- A college student, how smart he may be, has much to learn from a professional, how grimy he may be.
- A candid apology is usually acknowledged candidly.
- Do not try to stretch limits with tanks from the scrapyard – not even for a few more breaths under water.
All the above notes are points a marine surveyor must keep in mind on a survey. Always.
Cert. Marine Investigator