Advantages of Redesigned G-SST Probe for use with Hydrogen Peroxide (HPV) and Water Analyzer or other low-Pressure Gas Applications

Guided Wave has been measuring H2O2 and H2O concentrations in various vapor mixtures for over 25 years using near-infrared (NIR), fiber optic-coupled analyzers. Guided Wave’s HPV analyzer is a simple turnkey solution for the measurement of hydrogen peroxide and water (H2O2 and H2O) concentrations in vapor phase. These are both measured together because they are codependent. The analyzer operates in real time, which takes the guesswork out of determining the H2O2 and H2O concentrations during cycle development and throughout the actual sterilization cycle. 

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

The first generation probe developed for measuring hydrogen peroxide in sterilizers was a single pass 25 cm probe.  Later this probe was vented for vacuum use yielding a path length of 28..3 cm.  This original design, exposed optics the optics (lenses) to the gases being measured.  Since lenses are sensitive to the index of refraction of the surrounding air which is a function of its constituency, in this case varying water and hydrogen peroxide vapor, this resulted in a small but detectable change in the effective focal lengths of the lenses.  As a result, baseline shifts could occur, which would decrease the accuracy of the absorption measurements and ultimately causes a small bias in the reported concentration of vaporized hydrogen peroxide during sterilization.

In 2003, GWI developed the first G-SST vapor probe, a double pass design with a pathlength of 50 cm.  The longer pathlength increased the accuracy of the measurement.  This probe still exposed the process side of the lenses to the sterilization chamber.  Once again, small changes in baseline could be seen as the air pressure and vapor concentrations changed.  Furthermore, the lenses were glued into place and after a couple of years of heavy service, the glue degraded due to exposure to hydrogen peroxide, necessitating service.  Below is a photograph of the original G-SST probe with its perforated tube design.

original Gas sst probe with 28.3cm pathlength
Figure 1 Original Double Pass 50 cm G-SST Probe with Sanitary Flange

In 2018, the engineering team at Guided Wave began a redesign project with the intention of reducing cost and improving the performance and service life of the G-SST probe.  GWI retained the 50 cm folded pathlength, the gold-coated second surface mirror and the high optical efficiency.  An o-ring sealed window was added to isolate the optics from the process. 

Primary Advantages 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 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.
  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 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 probe is available with a KF flange, sanitary flange, or flangeless. Obviously, the flanged probes are intended to insert into the sterilizer through a 2” [50 mm] flanged port thus the fiber optic cables remain outside of the chamber. However, all versions can be 100% immersed in the sterilizer chamber with the addition of 2 small o-rings.

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.

G-SST Vapor Probe, 50 cm Pathlength
Figure 2 New 50 cm G-SST Probe

Improving the Service Life of the G-SST

In the old G-SST probe design, the lenses were glued in using TorrSeal.  This adhesive, while being low outgassing for vacuum use, was attacked by the vaporized Hydrogen Peroxide. As a result, the service life was reduced and many probes had to be rebuilt or replaced.  The window in the new G-SST probe is o-ring sealed, hence there is no adhesive exposed around the lenses.  The mirror in the far end of the probe remains the same second surface gold-coated mirror.  Being the second surface, the coating is not directly exposed to the gases.  We use gold rather than aluminum because the aluminum would be attacked chemically by the peroxide.  In addition, we pot the backside of the mirror in with RTV to prevent any vapor for getting to the mirror coating.

Existing Installations Can Be Upgraded to the Redesigned Probe Today!

The new style of G-SST is compatible with all generations of Hydrogen Peroxide Vapor Analyzers. However, older analyzers may require a firmware upgrade before they can accept the new double pass 50 cm probe. This is due to a path length normalization feature which must be set to account for what generation of gas probe is connected to the analyzer.

The HPV analyzer along with the G-SST probe delivers accurate, real-time measurement results. The long term stability and no maintenance requirements of this system make it a cost-effective smart choice to help optimize productions, ensure product quality, and ultimately enhancing profitability.

Customers interested in upgrading to the redesigned G-SST probe should contact Guided Wave for price, lead time, and upgrade procedure specific to the serial number of the HPV analyzer.

Posted in Guided Wave News.