* * * INSIGHT * * *

Use of the Pirani Gauge in the Lyophilization Process

By

Thomas A. Jennings, Ph.D.

ABSTRACT: In July, 1998 a question was submitted to the PharmSciTech group regarding the calibration of a Pirani gauge used on a freeze-dryer to produce pharmaceutical product(s). However, what prompted this writer to consider the above topic for an issue of INSIGHT was not the question - this writer did respond  directly in writing to the individual who posed the question - but the general nature of the responses that were submitted to the discussion group. All of those who responded to the question offered suggestions on how the calibration should be performed. One of the respondents did caution that one should take into account, presumably during the calibration procedure,  that the "behavior" of the Pirani was dependent on the nature of the gases but then went on to encourage gauge calibration using a McLeod gauge or a calibrated Pirani gauge. What surprised this writer was the general acceptance of those working in the pharmaceutical industry, by silence, that the Pirani gauge is a suitable pressure gauge for the lyophilization process.

In the Vol. 1 No. 2 issue of INSIGHT,  it was shown that during the primary drying process there was a relationship between the shelf-surface temperature, the product temperature and the chamber pressure. If one were to fix any two of these parameters, then the third would also be fixed. It was assumed that the lyophilization was conducted in vials that were in direct contact with the shelf-surface. The main concern expressed in this earlier issue of INSIGHT was that, by not measuring the product temperature, one would not have a means for verifying that the chamber pressure and shelf-surface temperature were within their prescribed limits.  Without knowledge of the product temperature, one must then rely heavily on the accuracy of the pressure measurements. If there is now an uncertainty regarding accuracy of the measurement  of the chamber pressure, then the lyophilization process will no longer be based on science or technology but on art or chance. 

The remaining portion of this INSIGHT will describe the construction,  operation of a Pirani gauge and the effect that the composition of the gases in the chamber can have on the pressure readings.

Volume 1 Number 6                                                                                           August 1998

     3 Pages          3 References          No Figures

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