- Most people are familiar with %RH, which measures how close the water vapor content of the air is to saturation (raining) at a given temperature
- RH is temperature dependent!
- Typical temp/RH probes output %RH based upon the temperature they measure, because of this set point must be defined in %RH
- Absolute humidity measures the amount of water water in the air per volume of air and is typically measured in g/m^3 or PPMv (parts per million volume).
- Absolute humidity is NOT temperature dependent
Why is humidity important for accelerated aging if it is not part of the Arrhenius equation?
- Incorrectly chosen humidity can cause faster aging than calculated by the standard Arrhenius equation
- Too high or too low can result in faster aging depending on the materials involved
- Poorly controlled humidity can also result in deviations to the Arrhenius calculated aging
- ASTM F1980 recommends ± 5% RH (relative humidity) for accelerating aging limits
How can high and low humidity levels affect product during accelerated aging?
High humidity can cause the following:
- Hydrolysis, which is material degradation in the presence of water
- Think about how some materials (salt for instance) dissolve when placed in water
- Also think about how elevated temperature results in quicker or more complete dissolving
- Degradation of any water soluble or water activated components
- Oxidation
- Ozone degradation (Ozonolysis)
- Drying/cracking of materials with variable moisture content
- Nylon and natural rubber are examples of materials with significantly variable moisture content
So at both ends of the humidity spectrum there can be unintended effects on the Arrhenius aging calculation. The key point is to discuss with your materials engineer or expert which type of failures your materials are most likely to have.
If you have a device/package with both, I may consider performing half your aging at high humidity and half at low humidity to verify the design at the extremes it will see during its shelf life.
How are absolute humidity and relative humidity different?
As stated earlier the key difference between relative humidity and absolute humidity is that relatively humidity is temperature dependent while absolute humidity is not.
Below is a graph of how absolute humidity increases with temperature (25-60°C) at 50% RH.
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| Plot of absolute humidity (g/m^3) with increasing temperature at 50% RH |
What's important to notice about the above graph?
- The absolute humidity at 60°C is than the standard conditions (25°C / 50% RH)
- It's actually 5.6 times higher, which a huge difference in the intended amount of available moisture in the air
- If we calculate the %RH at 25°C based upon that at 60°C / 50% RH we obtain 282%, which means there is almost 3 times as much moisture in the aging chamber air than would be required to generate rain at room temperature
- I don't think the intent is to have devices/packages submerged in hot water during storage (water vapor passes through packaging for EO sterilization)
So given that keeping %RH constant at elevated temperatures increases the absolute humidity we will need to reduce the %RH set point as the aging temperature is increased as shown in the figure below.
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| Plot of constant absolute humidity for standard conditions of 25°C @ 15% (green), 50% (blue) and 85% (red) RH |
What can be done to properly account for humidity when selecting accelerated aging parameters?
1 | Determine the device/package storage conditions
- Controlled Storage
- Use temperature/RH monitoring records from the facility over 1 year to calculate the min, max, mean and standard deviation in temperature and absolute humidity measured
- If data is not available, then a reasonable assumption for room temperature controlled storage is 50% RH at 25°C (standard temperature)
- Uncontrolled Storage
- Use temperature/RH monitoring records from the facility over 1 year to calculate the min, max, mean and standard deviation in temperature and absolute humidity measured
- If data is not available
- Obtain data from a local environmental monitoring station (if calibrated)
- A reasonable assumption for the low/high RH bounds could be 15/85% RH at 25°C
2 | Calculate the absolute humidity based upon the determined %RH value at 25°C (or other standard temperature)
- Create a table using a calculator from a calibrated probe vendor to determine the absolute humidity for the standard conditions AND the selected aging temperature (see figure below)
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| Table of relative humidity set points for a given absolute humidity at 25°C and 15/50/85% RH |
- Create a spreadsheet based upon accepted formulas AND validate it
- Vaisala has a great white paper on the relevant formulas available here
- Going into the details of how to create such a spreadsheet is extensive enough for another post, which I will complete and link if enough interest is generated
3 | Perform your aging study at the calculated set points and appropriate control limits
Closing Remarks
I hope this has been informative and has help you gain some insight into the importance of humidity when aging devices/packages. Please feel free to leave comments, questions or suggestions below.
