Finding Leaks in Your Vacuum Furnace – Part XI

Over the course of nearly two years, we’ve covered quite the range of leak detection topics – from how to use and calibrate a helium mass spectrometer to locating inert gas leaks and understanding their causes and effects. In our most recent post for this blog series, we discussed how to determine whether the furnace or the pumping system is the root cause of any vacuum-related issues.

leak detection with a helium mass spectrometerToday, we want to leave you with a broader understanding of the different leak detection methodologies. To start, the two primary techniques used are the spray probe and the sniffer probe.

Spray and Sniffer Probe Techniques
The spray probe technique is the traditional form of leak checking; it involves pumping down the furnace, applying helium (e.g., with a helium mass spectrometer connected) and looking for a response. The sniffer probe technique, on the other hand, is for applications where it’s not feasible to evacuate what you’re testing for leaks.

This technique involves charging, or pressurizing, a test piece with trace gas (e.g., helium). Examples of such test pieces include the heat exchanger, water-cooled flange, power feedthrough, water jackets, water-cooled fans and shafts, etc. Once the test piece is charged with the trace gas, the probe sniffs around the part for any gas that may be escaping through a leak.

Pumpdown Difficulties
When the furnace won’t pump down, we recommend writing a manual cycle that creates positive pressure within the furnace chamber. Doing so inverts the leak paths that exist. You can then use the thin film (soap bubble) method, which involves spraying a leak-detector solution on high-suspect areas; this method is normally used for gross leaks. If it starts to bubble in any areas, you know you have found a leak.

Interesting Tip: One of our Ipsen experts recommends that, in the event you do not have a professional liquid soap, do not use a dish soap/water mixture as a replacement. Instead, he recommends stopping at a department or craft store and purchasing a plastic spray bottle and children’s bubbles to use (which he claims works phenomenally as an emergency replacement).

If on the other hand, the furnace pumps down to 2,000 microns and stalls – and you have no access to a leak detector at the time – we recommend taking a spray bottle and filling it with acetone. Then have one person watch the vacuum gauge on the control system as another person walks around and lightly sprays any suspect areas.

As the acetone vapor enters the vacuum environment through a leak, the expansion of the acetone within a vacuum causes a noticeable pressure rate of rise (i.e., a spike on the vacuum gauge), which is a tell-tale sign that something you just sprayed is leaking. It is important to note that this method is generally effective at pressures of 400 to 2,000 microns.

Leak Detection: ‘Farmer’s Trick’
If you are experiencing a large leak and are in a pinch with no leak detection equipment on hand, one older trick that can be used is to take a lighter or a handheld torch and run the open flame along certain suspect areas. Such areas include: fittings, ball valves, thermocouples, thermocouple penetrations, compression fittings, small flanges, etc. If you encounter a medium- or large-sized leak while doing this, you will see the open flame bend sideways as it’s pulled by the leak.

In the end, there a number of leak detection methodologies utilized based on the situation and resources available. Here at Ipsen, though, use of a helium mass spectrometer is always the preferred method of leak checking.

Have questions about how to leak check a certain piece of equipment, or need technical assistance? We’re here to provide support. Simply call our Aftermarket Support Helpline at 1-844-Go-Ipsen (Toll Free: 1-844-464-7736; International: +1-815-332-2530) or fill out our Ask an Expert form.


  1. I would have to agree that fixing the problem is easier than it sounds. You need to identify the problem first, and that can be really tough to do. If all else fails you can take it in to get the exhaust repaired by a professional.


  2. Hello
    This is Kaori company from Taiwan , which use Ipsen furnace for many years .
    I just found this web , and couldn’t stop read it .
    Finally , I finsish all of “Finding Leaks in Your Vacuum Furnace” . These are really interesting and helpful to me !
    However , there are still something I don’t understand . Would you mind to answer these question when you have free , please ?

    In this section , what’s mean with “professional liquid soap” ? Our company had used dish soap for many years …
    What’s is the different between them ?

    Another question is on section PART IV.
    As you said , it might be leak when furnace is hot .
    But , what kind of parts on furncae will leaking at high temperature ?

    Thank you very much !


    1. Hello Ivan,

      We’re glad to hear that our “Finding Leaks” blog series has been helpful and informative! I know an Ipsen expert already provided answers to your questions, but we also wanted to share his answers here with others.

      To start, the professional liquid soap is typically used to look for leaks on fittings and brazes. While it’s the most effective to use when checking for leaks, it also costs the most. On the other hand, we have found that children’s bubbles work very well as a replacement for the professional liquid soap; whereas dish soap just works okay.

      As for your second question, when a vacuum furnace heats, the metal expands and bolted flanges can move at different rates, causing rings to roll a bit. Power feedthrough seals can also get a bit softer when warm, and then the force of the vacuum pulling on them can compress them a bit and cause leaks. Finally, welds can have a small crack that passes a cold leak check, but once heated, the crack can open and cause a vacuum leak.

      Overall, while it’s always best to know the furnace’s cold linear leak rate, it is just as important to leak check the furnace at 1,000 °C (2,000 °F) at least twice a year.


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