Mapping Sensor Placement in Stability Chamber Manufacturing
In this week's blog Paul Daniel answers a question on mapping sensor placement from a stability chamber manufacturer:
We design and manufacture stability chambers for the pharmaceutical industry and naturally, we map them as part of our product testing. In some models, the control sensors are placed away from the sample area and there can be a significant offset between the sample area conditions and the conditions seen by the control sensors. If we include the control sensor's location in the mapping we could get a failure at that position so we exclude it. What are your thoughts on this? Thanks in advance, D.
As the designer and manufacturer of the chambers, I think you have some more latitude. Creating a stability chamber that can operate within tight limits -- ±3C and ±5%RH (for ICH) -- is not easy. I can understand that this could require placing the control sensor in a location that experiences greater extremes than the required chamber parameters. You captured the essence of this point when you said: "The control sensors are placed away from the sample area in some models."
The primary goal of mapping is to map the storage areas. Other locations don't matter. In most cases, the control probes are placed in, or very close to the storage area along a wall of the chamber. In your case, you have an obvious exception as it is not in the sample area.
In my experience, there are up to three sensors in a stability chamber – the controlling RTD, a display RTD, and a building monitor RTD. These are often, but not always, placed in the same location. In truth, we really don't care what is going on at the control RTD, especially if it isn't in the storage area. But we do care about the display and monitor RTD locations. We want to include these in our mapping because we want to create some correlation between our display/monitor data and the mapping data.
We generally recommend that an additional mapping sensor be placed at the control sensor. But, in some cases what we need to recommend that an additional mapping sensor be placed at the display and/or monitor. Thank you for emailing! I appreciate being made aware of the exception to the rule, so I can clarify the rule better.
More on Stability Chambers
For further reading, we have an application note on Mapping Stability Chambers; plus, check out our other blog on stability chambers: Humidity Sensing in Stability Chambers: Two Important Guidances, Redundant Humidity Sensors, and Intervals for Requalification