Why you should worry about desiccant and humidity indicator cards!

Let’s talk “desiccant” and “humidity indicator cards” this week! “Desi-what” I hear you say???
Seems like we chose the right subject…!

But let’s rewind a little at this point: A few months ago we published a post on Moisture Barrier Bags (MBB) and ESD Control. The article described that Moisture Barrier Bags shield (no pun intended here!) ESD sensitive devices from 2 potential risks:

  1. The faraday cage created when using these bags correctly protects contents from ESD Damage.
  2. Specialised layers of film controlling the Moisture Vapour Transfer Rate (MVTR) also protect contents from moisture.

13950Moisture Barrier Bags

The blog post went on to explain that “desiccant” and “humidity indicator cards” must be used for proper moisture protection.
So what are “desiccant” and “humidity indicator cards” and how are they used? These are the questions we’d like to clarify with today’s blog post.

What is desiccant?

Desiccant is a drying agent that absorbs moisture from its surrounding area. Desiccant will stay dry to the touch even when it is fully saturated with moisture.
In a Moisture Barrier Bag it is used to ‘soak up’ moisture from the air inside the bag AFTER it has been sealed. Any moisture that gets through the bag from the outside will also be absorbed.

How is desiccant purchased?

Desiccant is available as a “unit” or fractional “unit”. A unit of desiccant absorbs a specific amount of moisture. One unit of desiccant weighs about 28g.

How is desiccant packaged?

Desiccant is packed in small sealed pouches made from a white plastic called “Tyvek” or brown “Kraft” paper. Tyvek pouches are very clean and sulphur free. Kraft pouches are economical.

204519A desiccant pouch

Pouches of desiccant are placed into metal pails – this ensures the desiccant is kept dry during transport and storage.

How much desiccant do you need?

A word of warning before we move forward – the following calculations may look a bit daunting at first sight. But honestly, once you’ve read through it all and calculated a couple of examples, you’ll be fine! Promise!!!
There are 2 different methods you can use – so let’s dive right in:

1. Method 1 per MIL-P-116

  • Formula: Unit = 0.011 x bag area in square inches
  • What you need: Bag area (2 times the surface area of your bag as there are 2 sides to a bag)
  • Example: 10″ x 20″ MBB bag
  • Apply formula: 0.011 x (10″ x 20″ x 2) = 4.4 rounded up to 4.5 units of desiccant

2. Method 2 per EIA 583 (allows you to tailor desiccant to your specific needs)

  • Formula: Unit = 0.231 x Bag Area x Bag MVTR x Months divided by Moisture Capacity
  • What you need: Bag area, Bag MVTR, Months of Storage, Maximum Interior Humidity (MIH), Moisture capacity table below:

    10% MIH 3.0 g/unit
    20% MIH 4.8 g/unit
    30% MIH 5.8 g/unit
    40% MIH 6.2 g/unit
  • Example: 10” x 20” bag with a 0.02 MVTR, a 12 month storage time and a MIH of 20%
  • Apply formula: 0.231 x (10″ x 20″ x 2) x (0.02) x (12/4.8) = 4.62 rounded down to 4.5 units of desiccant

Phew – told ya’ it wasn’t that bad! Now that we’ve covered the desiccant, we’ll take a quick look at humidity indicator cards. And I really mean “quick” because they’re much easier to explain/use….

What is a humidity indicator card?

A humidity indicator card allows for quick visual inspection of the relative humidity levels within its surrounding area. They are printed with moisture sensitive spots which respond to various levels of humidity with a visible colour change from blue to pink.
In a Moisture Barrier Bag they provide a low-cost method of verifying the effectiveness of the moisture barrier packaging. If you are using Moisture Barrier Bags, moisture will be an issue in your application so you’re obviously aiming for as little moisture as possible. However, if you happen to open your MBB and the humidity indicator card shows a relative humidity of 60%, you’ll know that the contents of your bag have been exposed to moisture and may not be safe for use anymore.

How are humidity indicator cards purchased?

Humidity indicator cards come in many shapes and forms. Some will show relative humidity from 10% – 60%; others from 5% to 15%. Depending on the sensitivity of your application to moisture, the correct type of card should be chosen.

204502Example of a humidity indicator card

Bear in mind that not all humidity indicator cards are reversible. Some cards will measure the relative humidity only once and then halt at that reading. These types of humidity indicator cards are NOT re-usable. This is important to know so make sure you check before purchasing!

How are humidity indicator cards packaged?

Humidity indicator cards are sold in containers. It is recommended that cards are stored in their original un-opened canister in a dry, well ventilated room with a reasonably consistent temperature of 20°C. Humidity indicator cards should not be stored in ultraviolet sunlight, moisture or heat.

How many humidity indicator cards do you need?

One humidity indicator card per MBB is needed for proper verification of relative humidity.


Moisture Barrier Bags, desiccant and humidity indicator cards all play a very unique and important role when protecting ESD sensitive devices from moisture. They should always be used together to ensure maximum protection. However, remember that all three tools need to be used correctly as otherwise all your efforts have been in vain. And don’t forget: your Moisture Barrier Bag must be heat sealed with a vacuum sealer to eliminate the amount of “moisture laden air” within the package.


Well, that’s it – that’s all we have for today! Did we answer all your questions? If not, let us know your questions in the comments below!

About Vermason

Vermason is a manufacturer of ESD protection products and was founded in Letchworth in 1979. Our mission is to maintain our position as the primary ESD solutions provider for the electronics industry in Europe. Vermason strive to manufacture quality ESD Control products with innovation, leadership and partnerships which deliver long term commercial benefits. We understand the many challenges our customers face regarding quality and reliability. We support these requirements with products and services of exceptional value which help them gain competitive advantages in their markets. We also appreciate that the control of ESD is one of many necessary links in a long chain required to bring a customer’s project to a successful completion. We endeavour to make that happen. We sustain our vision and mission by constantly seeking renewal via continuous education, application of new technologies and good business practices. Employees are expected to serve the customer with the highest level of technical knowledge in the industry.

Posted on 2016-03-31, in Articles, Bags and Labels, ESD Information, How-To, Packaging, Handling and Storage, Resources. Bookmark the permalink. 3 Comments.

  1. Hello, can you tell me if desiccant bags and humidity cards are safe to put into ESD bags with complete assembled boards? They are made of paper and I thought that paper was an ESD generator?

    Thank you for your time!

    • We do not claim that either desiccant or humidity indicator cards are ESD protective products. However, for many years they have been successfully used in the packaging of ESD sensitive items by high-tech companies. It is the dry pack required and as described in JEDEC J-STD-033.
      The best explanation is that both desiccant and humidity indicator cards are hygroscopic and readily absorb mositure. We would expect that in application they would reliably measure in the disspative range less than 1 x 10^11 ohms.

  1. Pingback: 6 Steps to defining ESD Protective Packaging Requirements for ESD Sensitive Items | Vermason Blog

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