More than

24

years of use monolithic
Sapphire-alumina ceramic-shells for probes can't be a marketing gag:
 
 
 
Reliable process control over large time periods was successful demonstrated!

Why a probe made of ceramics?

No - not only ceramics!  
But Sapphire + Alumina ceramics!

The perfect optical window - Sapphire

Sapphire

is the most preferred material for optical windows in harsh application covering  the 200-5000 nm range, which is important for many UV-Vis and Vis-NearIR spectroscopic and photometric methods.

 
No other optical material has such a

unique combination

of:
 
* Spectral Transmission
* Scratch Resistance
* Strength especially at pressure
* Chemical resistance in a wide range of chemicals and
  wide range of pH including caustics
 

Compared to the competing quartz glass/fused silica, Sapphire has a better scratch resistance, chemical resistance and strength.

Very important for practical use of windows in chemicals is, that the Sapphire is in a wide range chemical inert while at quartz glass surfaces have always existing Silanole-groups which allow to grow up many chemistry, if the windows are wetted with chemicals, with water and containing microorganism.

Summarized:

Sapphire

has a much less "reactivity" than quartz glass at better mechanical properties, comparable optic properties and a competitive price!

It makes sense - a Probe made of Sapphire +Alumina ceramics!

Traditional probe designs cause lot of questions:

How to seal up windows?
What are the best sealing material and which design?
Why using O-rings and which material?
Does epoxy work?
Or brazing with special alloys to metal parts?  

Why all the efforts and trouble to get durable probes?

Why not direct fusing both types of Al2O3 ?

Answer: this would require a special technology and design!

Was this ever done?

Answer: 1991-1993 first time done.

Did this really work?

Yes  and the possible designs are

flawless inserted windows!

Both materials are basically  Al2O3 - the oxide of aluminum!