A Real-world Application Of The ICH Standard Stability Testing Points

A while back our application engineers received a question on uncertainty in probes used in stability chambers. Here we offer two good references and help with calculating uncertainty  in RH instruments. This customer wrote:

Dear Vaisala,

We are hoping for some specific advice regarding the concerns we have been having with the humidity sensors monitoring our stability cabinets.

The issue revolves around the uncertainty of measurement on the calibration of the probes we use to profile and monitor the cabinets. The ICH guidelines state that the conditions must be maintained within ±5%RH from the setpoint.

If we take the example of a cabinet at 25°C/60%RH this provides us with an allowed range of 55-65%RH. Our procedures require us to map the unit, place the monitoring probe at the mid-range location and set its alarm limits to take into account the variance from the high and low extreme points.

For example, if we assume a scenario where we map a cabinet and the probe reading the highest gives a mean value of 61.0%RH (after adjusting for its offset), while the probe reading the lowest gives a value of 59.0%RH (after adjusting for its offset) we would place the monitoring probe in a location that gave a mean value of 60%RH and initially determine the OOS alarm limits for the monitoring probe at 56.0 and 64.0%RH.

Our probes are calibrated on site with a calculated uncertainty of ±2.9 (±1LSD) %RH. This would then be adjusted for any offset/error on the monitoring probe based on its most recent calibration. For this exercise I am going to assume there is no offset. If we then take into account the uncertainty of the monitoring probe ( ±2.9%RH) we are left with alarm limits of:

58.9 + 2.9 = 61.8%RH (low limit)
61.1 - 2.9 = 58.2%RH (high limit)

Clearly these are not feasible limits to set and I can find no way to make this work without ignoring the uncertainties on the probe calibrations. Even if we could get the probes calibrated with uncertainties below ±2%RH we would still end up with very narrow alarm limits. Any help you could provide would be greatly appreciated!