- The double pass beam design (folded path) provides double of the beam interaction with the gases in the sterilizer chamber. This increases the achievable signal-to-noise making for a more accurate and stabile measurement. Since the absorbance of water and Hydrogen Peroxide vapor is very weak, this is a significant improvement in the measurement accuracy.
- 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.
- To allow the probe to be quickly serviced by a technician, the perforated cage around the beam path was 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.
In addition to these engineering improvements, 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. By 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.
The 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, Guided Wave 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.
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