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Diffusion Pump Story

Posted By: Dana
Date: Monday, 7 July 2014, at 3:28 p.m.

I know I am not the first to experience this, but it is related to safety;

Last week, my diffusion pump oil temp was over 200 degrees hotter than it should be, the silicone oil was bubbling up and splashing about.

Nothing (but discovery) was going to stop the diff pump from ruining itself. I found the ‘simmerstat’ in the control box to be heat-damaged and stuck ‘on’. The mantle was fine and heating well obviously.

Fortunately, the control part is standard for commercial mantle heaters like coffee makers and I found one on ebay quickly. All is back on and working, I am writing this to share what I learned.

I only knew about the overheating because I maintain a separate temperature monitor on the mantle (a 5” Omega sweep & sensor wire wedged between the glass & mantle) and have tick marks to see that is in range.

Simmerstats apparently operate by maintaining a separate internal bi-metal switch & heater system that has a hot surface inside the control unit, to maintain a relative (not measured) temp on the mantle. The simmerstat usage does not impress me as good design – in our application, it seems like feedback from the mantle would be better. This is a transient design brought to us from the laboratory, but their usage is not necessarily permanent like ours – my diff pump heater has been on continuously for ten years.

Bottom line is, once the simmerstat fails, there is no safeguard to keep the mantle from heating to the maximum temperature possible, which will result in boiling your diff pump/oil to damage. Quickly.

I have my diff on a separate switch and now I will be shutting it off between days of pumping sessions. Should have been doing that all along. This incident left me with fouled oil in both the diff and vac pump (from the boiling exhaust of diff oil) – I had just changed both recently.
What would have helped: Inspect the stat inside the control box for heat damage. My control itself was case-deformed right at the label, where the internal ‘control’ element is located inside. Apparently they can work a long time with visible damage. I am considering an over-limit arrangement to protect against this again. A fire hazard obviously if your mantle cracks up from the heat and lets the hot element shift or drop in your cabinet. Overlimit circuitry is required on element devices today, probably was when this was built too – at least for permanent installs.

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