Humidity Indicator Theory and Characteristics

Reversible Color Change

A color-change element that turns from blue to lavender to pink as the humidity increases, and turns back to blue when the humidity decreases. This characteristic enables these elements to be reused as long as they have not been exposed to humidity in excess of 90% for periods of 36 hours or longer, which tends to wash out the element. Such conditions are not uncommon in non-desiccated empty containers during periods of storage. The element has long-term calibration and accuracy to within ±5% when read at 75°F (24°C). At other temperatures a small correction factor of 2-1/2% for each 10°F (5.5°C) higher or lower than 75° must be taken into consideration. For example, at high temperatures the element will indicate a lower humidity than is actually the case, and conversely, a higher humidity will be indicated at low temperatures. As indicated above, these elements are water soluble and should not be placed in direct contact with water or steam or exposed to extremely high humidity for protracted periods of time. Also, ammonia fumes will “set” the elements, causing them to lose color reversibility.

Humidity Indicator Cards Theory and Characteristics

The elements are available in 10% increments from 10% Relative Humidity (RH) to 90% RH, as well as various combinations of these humidity levels. All elements are available in plug-type housings, which are leak-proof and will withstand shock and vibration. The plugs can be installed quickly and easily with common hand tools, and are suitable for almost all applications for Method II packaging of MIL-P-116. RFI/EMI shielding and CARC finish are optional features.

Humidity Indicators should remain in their original packaging until ready for installation. Recommended torque for installation of all Humidity Indicators is 30 in. – lbs. (3.4 N•m).

Maximum Humidity Indicator

Humidity Indicator Plugs Theory and Characteristics

A color-stain element warns that certain humidity levels have been exceeded, indicating that the desiccant may need to be recharged or replaced. The element warns of dangerous humidity levels by staining a white disc a brilliant orange (turning later to a dark brown) when it has been exposed to humidity ranging from 55% RH for an eight hour continuous period to 85% RH for two hours. Humidity levels below 55% will not cause the indicator to stain, regardless of duration.

A non-reversible humidity indicator disc is often superior to a reversible humidity indicator disc because it maintains a record of past conditions. For example, a reversible indicator could trip during a cold night when the relative humidity level naturally rises, but then reverse itself during the warmer daytime hours when the indicator is most likely to be checked. In this case, the reversible indicator would give no sign that a dangerous humidity level had occurred overnight. By contrast, after a non-reversible indicator trips, it will always show that its humidity threshold has been exceeded.

Because these elements are irreversible, they should only be installed in a desiccated atmosphere. While it has been common practice to install a humidity indicator plug complete with the irreversible indicating element into a container at the time of container fabrication, this procedure can cause the indicator to trip before the container is put into service. Therefore, it is best to install an empty plug housing into a container during fabrication or refurbishment, and then install the indicating element into the housing at the time that the container is loaded and desiccated. For this reason, you will find these irreversible indicating elements TA378-HC-MHI listed individually. The empty housings are sold as Observation Windows TA456.

The indicator plug is leak-proof and will withstand shock and vibration. It installs quickly and easily with common hand tools, and is suitable for almost all applications for Method II packaging of MIL-P-116. RFI/EMI shielding and CARC finish are optional features.

Recommended torque for installation of all Humidity Indicators is 30 in. – lbs. (3.4 N•m).

Related Products

Humidity Indicators are often incorporated into plug-type Desiccators for small enclosures. AGM also has custom Humidity Indicators.

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