Mean Kinetic Humidity? Answers & Advice from our MKT Webinar
Industrial Manufacturing and Processes
Life Science
These are a selection of questions that were answered in our recent webinar on Mean Kinetic Temperature. These questions are a sample, to hear all the questions and answers, view the recorded webinar and get the slides here.
But since Vaisala is the world-leader in humidity measurement, folks often ask us questions based on our favorite parameter, so we include it here prior to the webinar Q&A.
M wrote:
Moisture extremes impact in-process product quality. Quality sometimes defines parameters that are not well defined. Similarly, from alarming strategy, what impact does duration and/or the magnitude of an excursion have on product quality?
Also, requirements are often stated at less than 50%RH at 20°C; since relative humidity is "relative" to temperature, can dew point or vapor pressure control be used as an equivalent parameter? Is there a difference in product impact moisture requirements that use RH versus dew point temperature?
Paul Daniel, Vaisala Senior Regulatory expert answers:
1) Moisture extremes impact on in-process product quality.
When in-process, the product is not protected by final packaging. However, processing usually takes place in well-controlled manufacturing suites, so we often have an opportunity to control moisture and humidity in ways that are not possible downstream, for example in storage and distribution.
2) Quality sometimes defines parameters that are not well defined.
Agreed: In my experience, the only times I have seen this lack of definition overcome is when there is good communication between quality and manufacturing. Ideally, this is a by-product of a holistic QMS, but it doesn’t always happen. Quality may seem more concerned with passing audits and can miss the practical concerns we see on the manufacturing floor.
3) Regarding the impact that duration and/or magnitude of excursion has on product quality…
This depends entirely on the product, the process, and the manufacturing environment. For temperature, because of the phenomenon described by Arrhenius in his equation, we can generally state with confidence that the higher the temperature and the longer the duration, the larger impact on product quality.
With humidity, it is harder to say. Because higher humidity increases the concentration of one of the reactants (gaseous H2O), the reaction kinetics can be quite different. And often, high humidity effects can be reversible, say for example, in product clumping in powdered product, which can often be fixed by some extra agitation or mixing. But low humidity effects, as you know, can be a lot harder to reverse, especially if something has completely dried out. In short, the humidity effects must be decided on a case-by-case basis. Humidity lacks the huge body of empirical data that we have for temperature and its effect on reaction rates.
4) Regarding requirements often stated at less than 50%RH at 20°C and your question: “Can we use dew point or vapor pressure control as an equivalent parameter?”
You are absolutely correct that relative humidity is exactly that… relative to the temperature. But it would be a mistake to think that the temperature affects the amount of water vapor (gaseous H2O) in the air sample – it does not. Increasing the temperature, or lowering it (as long as we don’t lower it past the dew point) will not impact the amount of gaseous water in the gas sample, it only affects the reported RH% value.
For example…. 20°C and 50%RH is a vapor pressure of 11.7 mbar and a dew point of 9.2°C. While 22°C and 44%RH is a vapor pressure of 11.7 mbar and a dew point of 9.2°C.
As you can see, while temperature affected the reported %RH value, it didn’t have any effect on the actual amount of moisture in the sample.
5) Finally the difference in product impact moisture requirements that use RH versus dew point temperature…
There are two general reasons why we might choose to report our measurements in one parameter over another.
The first reason is linked to what we are trying to achieve. If we are interested in promoting or preventing drying processes, then we want %RH, because drying processes are entirely dependent on the ability of a gas sample to accept more gaseous H2O. But if our process goal is to prevent condensation in a process, we might be better served with a dew point measurement.
The second reason is accuracy. Percentage of relative humidity measurements are not as reliable when things get really dry (<10%RH), really humid (>90%RH), or in extreme temperatures and extreme pressures. In these instances, we are better served with dew point or mixing ratio as our reported parameters because we’ll get better resolution and more accuracy.
With ambient temperatures and humidity between 10% and 90%RH, relative humidity is probably your best measurement when dealing with pharmaceutical products. The relative nature of relative humidity is not a variability in the amount of moisture in the air, but a temperature-dependent variability in the maximum amount of gaseous H2O that can be present in a gas sample.
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