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How to Read a Humidity Indicator Card

May 6, 2021

You’ve developed a moisture management and monitoring system for your project but after a while, your humidity indicator card has turned from blue to pink. Or worse, it’s kind of blue and kind of pink… what do you do? Do you need to change desiccant? Replace the indicator card? Is everything ruined? To answer these questions, you need to know how to properly read your humidity indicator card. You could read through the FAQ page and find the answers, but we’ll just tell you below:



How to Tell if Your Card is Nonreversible or Reversible

You first need to know if your card is nonreversible or reversible to properly read it. Here’s how to tell non-reversible from reversible HIC’s:

Both nonreversible and reversible humidity indicator cards may be either rectangular or circular in shape. Also known as Maximum Humidity Indicator (MHI) cards, nonreversible cards are frequently rectangular and often printed with “Maximum Humidity Indicator” directly on their face. This marking easily identifies them as nonreversible indicators. However, some nonreversible MHI's are round and may not feature the printed designation. Such cards require activation via humidity to properly differentiate them from reversible cards.

Reversible humidity indicator cards will not feature the "Maximum Humidity Indicator" designation. However, reversible indicators may share the same shape as MHIs and feature no other identifying markings.



Reading Nonreversible Humidity Indicators

Nonreversible humidity indicator cards commonly feature a color-stain element that changes to indicate specific met or exceeded humidity levels. Once exposed to 55% RH or greater for a continuous eight-hour period, the stain element will change from white to orange (fades to a dark brown over time). This change occurs after two hours if the card is exposed to a continuous 85% RH. The rectangular MHI cards, when activated by humidity, will display a non-uniform, blob shape. The blob will turn an orange or black color in each segment indicating the relative humidity percentage that occurred.

Humitector Nonreversible Maximum Humidity Indicator Card
New Rectangular Nonreversible Maximum Humidity Indicator



Reading Reversible Humidity Indicators

Reversible humidity indicators change color from blue to lavender to pink once their monitored environment has achieved the given humidity level threshold. The color change occurs rapidly, normally within minutes. However, the transition time is dependent upon the humidity level and temperature unto which the card is exposed. The card does not need replacing once it turns pink; rather, exposing it to a dryer atmosphere than the card’s rating will reverse the color change. There is one caveat, however. Exposure of 36-hours or more to humidity levels in excess of 90% may wash out the coloring element and potentially ruin the card.

Rectangular, reversible humidity indicator cards. Blue and pink indication. No indicators triggered.
Rectangular Reversible Humidity Indicators. Pink Indicates that the Indicated RH Has Been Met

A note on behavior. Reversible humidity indicator cards may change color during cold nights to indicate the rise in humidity level that occurs naturally. The card then reverses color during the following day as RH falls. As humidity indicator cards are most often checked during the day, in this particular situation, a reversible indicator card would provide no indication as to whether or not excessive humidity levels had been reached during the night.

For More Information

For information about humidity indicator cards and plugs or designing moisture mitigation systems in your project, email AGM Engineering or call us at (520) 881-2130.

AGM Container Controls, Inc.
3526 E. Fort Lowell Rd.
Tucson, AZ 85716
520.881.2130
agmwebsales@agmcontainer.com

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