Taking Optical Probes into Harsh Environments

The need for optical probes to monitor industrial processes and machinery is

undeniable. The question is: How do you protect them from the harsh operating

conditions that have barred their use up until now?

Michael Pollack

Stephen C. Bates

Over the last 10 years, the growing use of optical probes and borescopes has

increased production yields and enabled the maintenance of processing equipment

during the production cycle (see Photo 1). Temperature, pressure, and harsh chemicals

commonly used in industrial processes, however, degrade conventional probes

quickly. To extend a probe’s life and expand its capabilities in harsh environments,

one option is to use a sapphire-window protective shroud.

To appreciate the benefits of these shrouds and the value of sapphire in this

application, it helps to understand how standard optical sensors are being used to

control and inspect processes involving harsh environments. For example, endoscopes

allow remote inspection; optical pyrometers measure temperature; and fiber-optic

spectrometers measure chemical composition. New developments in fiber optics are

being used to diagnose chemical and physical properties in various process systems.

Process instrumentation sensors must be simple and rugged, but standard probes

traditionally require relatively clean and mild environments, which greatly limits their

usefulness in industrial processes.

Challenges of Industrial Environments

Air cooled protective shroud with noble metal brazed sapphire window to 316 stainless

steel housing makes it possible for borescopes and optical sensors limited to 100°C to be used in environments with temperatures as high as 600°C.

Challenges of Industrial Environments Industrial processing involves tremendous variations

in the work environment, so diagnostic probes must be able to withstand a wide range of conditions. The basic parameters include:

• Pressure: vacuum of 10 -10  torr to pressures of 20,000 psi

• Temperature: 2000ºC to –270ºC

• Chemical: process chemicals that include corrosive acids and bases (e.g., HF to

NaOH)

Multiphase flow is also common in many forms, leading to a variety of additional

optical and mechanical constraints.

Industrial temperature monitoring, which is the most common diagnostic parameter of

interest, is performed by thermocouples and RTDs or by optical pyrometry in extreme

cases. Temperature-related degradation mechanisms include:

• Thermal shock

• Thermal cycling

• Thermal stress

• Thermal fatigue

• High heat fluxes

All these factors must be considered in elevated temperature design.