top of page


The actual moisture content of air within a package, expressed as a percentage of it's saturation moisture content,
is termed the Relative Humidity (RH).
The crucial factor in preventing corrosion and mould growth to stored or in transit items,
is to keep the RH below 40 to 50%.

Carefully calculated, the use of silica gel or molecular sieve sachets can keep the RH below these levels. 
This can be confirmed by the use of humidity indicator cards to give a visual reading of the RH level
and to provide an indication as to when the desiccant needs changing.


Humidity indicator cards are made of high quality blotting papers, with the spots impregnated with
indicator solutions. They are 'read' at the point on the card, where the spots are changing from blue to pink.
They will continue to reflect the changing surrounding RH or can be fully reversed, to all blue,
by placing them in a sealed container of desiccant (or a similar very low RH environment).

We currently offer the following humidity indicators:

6-spot Humidity Indicator Cards (10 - 60%)










Each spot represents a RH level from 10 - 60%.
The higher up the card the spots change from blue to pink, the higher the RH.
This allows an accurate reading of the humidity level and again, aids in indicating when silica gel sachets require replacing.


3 or 4-spot Humidity Indicator Cards

Similar to 6-spot cards, but with each spot representing a different RH set:
10 - 20 - 30 - 40%
30 - 40 - 50%
5 - 10 - 15%
5 - 10 - 60%


Cards with low RH spots, 5% and 10%, will usually be used in conjunction with molecular sieve desiccants

as molecular sieves (but not silica gels) can reduce the RH down to these very low levels.

The cards typically meet the international standards and industrial requirements
of MIL-I-8835A, IPC/JEDEC J-STD-033B-1 and GJB-2494-95.

Safe/Unsafe (one spot) Humidity Indicator Cards or Paper

The one-spot card and paper will reflect an 8% RH level and are simply a safe/unsafe indicator.
A blue card indicates that the RH is at safe levels, a pink card indicates unsafe.

Humidity indicator cards and papers are sold in small or modest quantities - please ask.

bottom of page