For over 25 years, our analyzers have measured H2O2 and H2O concentrations in various vapor mixtures using near-infrared (NIR), fiber optic-coupled analyzers. The HPV analyzer offers a simple, turnkey solution for measuring hydrogen peroxide and water (H2O2 and H2O) concentrations in the vapor phase. Since H2O2 and H2O concentrations are codependent, the analyzer measures both together in real time, eliminating guesswork during cycle development and throughout the actual sterilization cycle.

The complete system includes the HPV analyzer, one or two G-SST probes, and a pair of fiber optic cables for each probe.

The first-generation probe, designed to measure hydrogen peroxide in sterilizers, was a single-pass 25 cm probe. Later, we vented the probe for vacuum use, increasing the path length to 28.3 cm. This original design exposed the optics (lenses) to the gases being measured. Since lenses are sensitive to the index of refraction of the surrounding air, which varies based on the concentration of water and hydrogen peroxide vapor, this caused slight but detectable shifts in the effective focal lengths of the lenses. As a result, baseline shifts occurred, which decreased measurement accuracy and introduced a small bias in the reported hydrogen peroxide concentration during sterilization.

In 2003, we developed the first G-SST vapor probe, incorporating a double-pass design with a 50 cm path length. This longer path length improved measurement accuracy but still exposed the process side of the lenses to the sterilization chamber. As the air pressure and vapor concentrations changed, small baseline shifts were still detectable. Additionally, the glue holding the lenses in place degraded over time due to exposure to hydrogen peroxide, requiring service after several years of use.

In 2018, we launched a redesign project to reduce costs while improving the performance and service life of the G-SST probe. We retained the 50 cm folded path length, gold-coated second surface mirrors, and high optical efficiency. We also added an O-ring sealed window to isolate the optics from the process, effectively removing sensitivity to changes in the index of refraction.

Figure 1:  Original Double Pass 50 cm G-SST Probe with Sanitary Flange

Figure 2:  New 50 cm G-SST Probe

New 50 cm Pathlength G-SST Over the Previous 25 cm, 28.3 cm and the Original 50 cm G-SST probe

1. The double pass beam design (folded path) provides double the beam interaction with the gases in the sterilizer chamber. This increases the achievable signal-to-noise making for a more accurate and stable measurement. Since the absorbance of water and Hydrogen Peroxide vapor is very weak, this is a significant improvement in the measurement accuracy.
2. The optics are sealed behind a window which isolates the lenses from the sterilization chamber; thus the probe is no longer sensitive to index of refraction changes, this makes the measurements are more stable under widely varying conditions of deep vacuum to high concentrations of water and hydrogen peroxide vapor.
3. To allow the probe to be quickly serviced by a technician, the perforated cage around the beam path was changed to the open structure shown in Figure 2. With the lenses behind an O-ring sealed window, there is no adhesive degradation increasing the service life of the probe.
4. The newly designed G-SST probe is a form, fit, and function replacement for the older style G-SST probes. This allows for nearly effortless upgrading for existing customers who purchase the new and improved style of G-SST. The G-SST vapor probe is available with either a tri-clover sanitary flange for mounting on a chamber access port or without a flange for placement within the chamber. Inserting the probe into the sterilizer through a 2” [50 mm] flanged port allows the fiber optic cables remain outside of the chamber and reduces measurement noise. Also, both the flanged and flangeless versions of the G-SST vapor probe can be 100% immersed in the sterilizer chamber with the addition of 2 small o-rings and a dual fiber feedthrough.

Improved Signal to Noise Through Folded Path Optics

The original 25 and 28.3 cm path length probes utilized single pass optics. As part of the redesign, a folded mirror configuration was developed, so that the light passes through the probe twice without significantly increasing the footprint of the probe. This design also keeps the optical fiber connections on one end and, when installed through a flanged port, outside of the process. To intentionally avoid measuring scattered light, the two paths are separated. By increasing the effective path length to 50 cm, the absorbance and therefore the signal-to-noise is doubled.

Original G-SST Suffered From Variable Index of Refraction

The original G-SST probe and the even older 25 cm gas probe had the lenses exposed to the vapors and air.  The lenses collimate the light in the probe and refocus the light onto the end of the small return fiber.  The focal length of the lens is dependent on the index of refraction of the air surrounding the lens.  For most practical applications, the index of air is taken as unity and not of any concern.  However, in this application, the chamber medium can change from vacuum to pure N₂ to very high humidity air with Hydrogen Peroxide vapor. Absolute Vacuum is defined to have an index of refraction of 1.0 exactly.  Air has an Index of Refraction of 1.0003.  Water vapor and Hydrogen Peroxide vapor will change the index of the air depending on their concentrations.  As the index of refraction changes, so does the degree of focus of the probe.  This changes the baseline offset of the absorbance measurement.  The baseline offset is also wavelength dependent. 

In other words, we found that the original probes suffered from a baseline sensitivity under different operating conditions.  This was most notable when going from vacuum to high relative humidity conditions, such as during sterilization.  The wavelength sensitivity did cause a slight change in the water and Hydrogen Peroxide Vapor measurements produced by the Hydrogen Peroxide Monitor.

Controlling the Index of Refraction in the Redesigned G-SST Probe

The new G-SST probe has a window which separates the lenses from the vapors.  This window is in a collimated portion of the beam, so the focus is not sensitive to the index of refraction of the sample.  The result is a more stable baseline under varying process conditions, hence the removal of one small source of measurement error.