Product Discussions: Sight Glasses with Quartz Bonded to Steel Housing

Heat resistance of anti-reflective coating for applications with laser beams.

The anti-reflective coating can be tuned to the specific wavelength, where the reflection will be less than 0.5 percent. In other words 99.5 percent of energy will pass through the window. The balance of 0.5 percent will be absorbed into the window in a form of heat. Uncoated quartz, per side will absorb about 5 percent of energy. It will make the window hot, and may destroy the bonding in bonded sight glasses.

The coating itself will not burn until lower levels of Gigawatts-range of power [1]. For applications in the MW range, a standard broad band anti-reflective coating (BBAR) will be sufficient to withstand the generated heat. Broad band antireflective coatings are applied using evaporative coating technique. Windows requiring to transmit gigawatts of energy are coated using ion beam sputtering. The cost goes exponentially up, from about $1,500 lot charge for evaporative coating to over $5,000 for sputtering for batches of windows in small lot quantities. For applications absolutely requiring ion beam sputtered coatings there is no way around the physics of wavelength transmission vs. heat absorption. The simple rule with anti-reflective coatings is the denser the coating, the more resistant to heat and scratch resistant it becomes.

Sapphire brazing to metal housing in sight glasses.

The sapphire window is brazed to the housing using metal alloys. This is the normal practice for high-vacuum, high temperature applications with medium pressures, where most high performing bonding materials disintegrate above 230 degrees C. Also due to a mismatch in coefficients of thermal expansion (CTE) between the glass and the metal, the housing will separate away from the window over a few temperature cycles, causing further break of the seal between the glass and the housing. This problem is solved by high temperature brazing of sapphire to metal, where the metal has a low CTE, closer matching the CTE of sapphire. Niobium would be a preferred choice of metal for the housing, but it can be price-prohibitive. Kovar and Invar metals are also used. An intermediary metal sleeve is also used to compensate the CTE mismatch between housing made from stainless steels and sapphire windows, sometime quartz windows too.

High temperature operations of bonded sight glasses.

Is the temperature going to be at 200C all the time, or is this a peak design temperature? If the expected operating temperature is indeed 200C, then we cannot use bonding, the adhesive should not exceed 230C. For high pressure of over 9,000 psi, for the sight glass housings we should use materials such as 17-4PH metal, or Titanium 10-2-3. If the customer can approve the use of these metals and the operating temperature is indeed below 200C, then the sight window design is our established process of bonding quartz GE-124 to a metal housing made from Ti or 17-4PH.

Sapphire windows in high temperature and high pressure sight glasses.

Sight windows meeting the criteria for:

  • Operating temperature over 300 degrees C,
  • Helium-leak tight, and
  • Withstanding high pressures, over 500 psi in larger aperture diameters, will have to use a mechanical C-seal between the sapphire and the housing.


This seal is internal to the sight glass assembly, where the housing, the seal and the window are all assembled into a single-piece part. Typically we use C-seals made from Alloy 718, which is Inconel. The C-seal is gold-plated to increased seal-ability as gold is soft and forms a He-leak tight barrier. Other methods of sealing the sapphire into the housing will not hold at 600C temperature. Bonding, fusing, brazing will not work. At temperatures below 500C  brazing will work very well but only where the operating pressures are low. Why? Because the brazing layer between sapphire (or quartz) and the metal is not as strong as a mechanically supported of window directly onto the housing.

Making bonded quartz sight glasses work at a higher temperature.

We looked at making a sight glass with the same threads as NPT 3/8" work at temperatures above 200 degrees C. For 200C up to 300C, we can use a Kalrez elastomer o-ring. For small sight glass in size NPT 3/8" the viewing diameter will be approximately 0.280 inches accomodating the O-ring. This is because the O-ring gland pushes all the adjacent dimensions farther away, specifically the internal diameter of the housing bore becomes greater and the aperture diameter smaller. The wall thickness where the threads are becomes thin, less than 1mm, which means the entire sight glass becomes weaker to handle installation torque and operating pressure.

So in short, in order to make NPT 3/8" work with bonded quartz at operating temperature above 200C, below 300C, the aperture dia. becomes 0.280 inches and the housing wall becomes thinner. If you absolutely have to have a larger aperture we have to step up to the next size NPT 1/2" and then it will work well.

Another alternative is to consider two entirely different approaches: First is to use a sapphire brazed to a sleeve and then weld the sleeve to the NPT 3/8" housing. This is risky as it will require some development. Here is an example of a sleeve brazed to the window: [2]. And the second approach is to use fused sight window with maximum operating temperate of 280C, but somewhat non-flat glass. Also the glass is likely to have strings, visible lines inside the body of the glass due to its fusing process. See product: Fused Sight Glass.


[1] John Ellis, Dominar Inc., Apr. 04, 2016

[2] MPF Products - Stock Products, Brazed Quartz and Sapphire Ports MPF Products, June 7, 2016