1) The machine was made in Germany; the original thermocouples were German and given thet the machine was down and there were no remaining spares, I had to find a fast (and reasonably priced) solution. Which was to use a standard TC probe from Omega Engieering. The original probe was (apparently) a standard Off-The-Shelf probe, with a probe length of about a foot, coupled to a 5 foot cable with a DIN connector to mate with the control box. The entire assembly appeared to be a standard, stock item (these machines are not made in huge quantities, so custom TC probes were probably not on the designer's hot list.
2) The probe was mounted to the heater (at the cold end!) with a cable tie. The end of the probe - the last 2 inches or so was bent (by the machine operator) at right angles to fit into the hole in the side of the heater just below the heating element. The tip was inserted into the hole until it was in the heated air stream and locked (gently) in place by a setscrew. If the PID controller was set to (say) 450C and the temp. readout also read 450C, then you had it right, and got back into prodution - otherwise, you would adjust the probe in or out until things worked properly. Not exactly rocket science....
3) The operator, while certainly capable as a mechanic, was not familir with these sort of controls. The shop manager was also not really familiar with these details either. This was a small company without electronic personnel on board; I had done some work for them, so they called me in. Unfortunately, when I asked if they had any spare probes, they handed me a plastic bag with half a dozen of them - all defunct. They had just burned out the last one. The only good thing about this, is that all the dead probes had connectors, which can be used to make the new Omega OTS probes compatible with the control box.
4) The advantage of the design is that it uses OTS thermocouples and with the long probe, keeps the low-temp cable away from the hot zone, so ordinary cable can be used, thus keeping the cost down. Given that the TC probe is basically a consumable, that's not a bad idea, both from a cost and logistical point of view. The logistical issue is paticularly inportant, given the cost of downtime and the time involved (even using Fedex, etc.) of getting probes from Germany.
5) The PID Controller was a somewht older design, and while there was a safety relay to shut the machine don in the event of a TC failure, there were no diagnostics in the system.
The flaw was in not having a means for adjusting the thermocouple penetration. Designing in proprietary parts in order to assure that only the "right" part gets used should always cause a manufacturer grief! The solution that we used was to provide repair parts that were easier to install than the generic ones, or were already calibrated, or sometimes even less costly. Of course, we needed to be able to ship repair parts overnight, or faster, for some customers.
The fact that anybody cut off the end of the thermocouple shows that they had no idea what they were doing.
One more thing, which is that many thermocouple-based controllers have an option called "upscale-burnout", or "downscale burnout", and, using a combination of upscale burnout and an overtemperature alarm contact output, it would be simple to have an "open thermocouple" warning light.
Pretty hilarious that they were trimming that thermocouple and still having it work. I made a themocouple tip welder from three 10,000 microfarad capacitors in parallel, charged up from a 12 volt source through a 330 ohm 1-watt resistor. I connect one side of the charged capacitor bank to the twisted thermocouple wires (both together, with an alligator clip an inch from the end), and touch the tip to the other side of the capacitors. Typically 1mm or so of material disappears in the resulting flash (use eye protection!) and the themocouple is welded and ready to go. Then I discharge whatever's left in the capacitors through the resistor for safety.
I hear alarm bells when someone talks about "proprietary parts". We shy away from vendors who do too much of this. I had a vendor once admit to me that they used a custom made feed chain for carrying lumber through their machine because it made for good parts business. They did the same thing with their cutterhead spindle motors: completely custom motors. Needless to say, we ran away from them screaming. Being tied to the OEM too tightly is a sure fire way to spend way too much on replacement parts.
In the case where buying wrong sized replacements and people with no knowlege modify the spares, well, tough crap. Consider the servicemans visit a training excercise. Buy the spares (overpriced) from the OEM if you don't know what you are doing.
The other amazing thing on this fix is that the operator took the time to change the heater but stopped short of changing the thermocouple before calling in outside help. A quick resistance would have shown the failure.
The amazing thing is that they were able to form a working thermocouple junction - multiple times! - using nothing but a set of shears. I think these people are more skilled than we give them credit for.
The fix here, which has the negative downside of costing a lot more money and rendering the equipment useless once it gets too old (because parts would no longer be available) would be to have built the heater with a proprietary sized-and-threaded hole for the thermocouple, or some other way that only a vendor-approved part could be used for replacement. But where would the fun be in that?
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