Your Idea of Normal RH vs. an Auditor's idea of Normal RH: How to Reconcile the Results of Mapping Studies

Almost as follow up to our blogs on remapping and buffering sensors in mapping studies, we had a question from a blog reader about results from a temperature and humidity mapping study with some high humidity.

Hi Paul,

After reading your blog on remapping, we have some questions that arose when we performed remapping on a chamber adjusted for 25 ºC / 40 %RH. Our requirements are ±2 ºC and ±5 %RH. No value outside limit more than 30 min. Temperature is OK and the humidity varies from 36 to 48 %RH at different positions, as you can see… See entire study. Position 5 and 6 are both high (40-48 %RH). We think the average humidity is OK but the peaks made us wonder. Our question is: Are those values within normal variations? Thanks for any advice you can offer!

Paul's reply:

Dear L,

As you are obviously aware, humidity is hard to control and hard to measure.  The instruments we use aren't usually as accurate as we need, and for ICH stability guidelines, the tolerances are usually quite small.  Thank you for being thorough in your question. I think that your chamber is probably operating fine, and your products probably won't experience any different chemical activity at 48 %RH instead of the prescribed 45 %RH. 

But here's the crux. We have to be able to convince an auditor of this fact.

Sometimes RH is all about Temperature (they're like, related)
Usually, if I saw widely varying humidity like yours, I would try to explain this with temperature swings, as temperature changes have a large impact on RH%.  You can see this in the relationship between your average temperature and your average humidity.  Points 5 and 6 have the lowest average temperature and the highest average humidity.  However, your temperatures are INCREDIBLY stable – less than 0.2 °C variation in some cases – I call that plenty stable.  Understanding the relationship between temperature and RH is fairly critical, and I would review that fundamental knowledge before an audit. This PDF on Humidity Theory may help, and here is a blog.

Buffered Conditions for Mapping?

I suspect that you are buffering the temperature signal in some way, by monitoring a sensor in a vial rather than temperature in air.  If you buffered the temperatures it hides the temperature swings you would need to explain the RH% spikes in this strategy.  If this is the case, I would consider remapping without the temperature buffering.  This will give you more transparency into what is actually happening inside the chamber so you can better diagnose the fault. Or remap and buffer the RH% probes as well using the same rationale – put them in a small box or something. 

What does the Map Say? (Now that we've stopped to ask directions…)
So, now I am back to looking at the map.  I note that both points 5 and 6 are in the lower left edge of the unit.  Is there something about the humidity/temperature control system or venting that would cause this spot to be colder or more humid? 

It is not possible to say what is within normal variations in your chamber because we don't know what normal is yet.  But there are a couple other questions that come up for me. One: Does you facility allow for acceptance of average values?  If so, then just follow that.  And two: Or is it the case that by your study/facility guidelines that this unit will fail if you can't explain the high humidity?

Thank you so much for sharing your study data to review. Please reach out to me if you have further questions!