Electrostatic Charges and how they affect weighing

Understanding what electrostatic charges are and their effect on different samples and weighing scales is critical. If there is an electrostatic force in an analytic scale, it will cause offsets that will affect the weight measurement directly. It is critical to take preventable action whenever possible to reduce or eliminate the build-up of static charges. This helps in avoiding error, slow weighing results and instability. Read on to know more.

How Does Static Charge Originate?

Static is the accumulation of electrical charges on the surface of a non-conductive material.

Whenever two different materials come in contact, there will be an exchange of some amount of charge between them. The amount of charge depends upon the electron affinity of the materials, details of the contact and the surface conditions. When they separate, they will have a potential difference (voltage) between them.

If the two materials are grounded conductors, the charge will rapidly shunt to the ground and no net charge will remain. If one or both of them are ungrounded conductors or insulators, the charge will remain.

Most powders being weighed are insulators and so are the windows in the analytical balance. When you remove the reagent from the glass container, it will charge it through separation and contact from the glass. Even brushing against the window will leave it charged.

The static charge level that is normally experienced in the weighing process can affect the balance readings by 10 mg.

Effects of Electrostatic Charges on the Accuracy of Weighing
Two main effects of electrostatic charges on weighing are:

* The reported value being more or less than the real weight

* There may be errors in the range of 1-100 mg

You may often notice the numbers changing on the scale display when you place a weight boat on the weighing pan. If you pull the pan while trying to tare the balance, then it would be difficult to get back to zero to offset the effects of the static charge on the pan.

You can observe a drift in the 'zero' based on the amount of humidity in the air. This can lead to inaccurate measurements in critical applications.

How to Manage Static Samples
New advances in weighing technology have enabled the latest generation of analytical balances to automatically detect the presence of electrostatic charges during the weighing operation.
You can measure and record the magnitude of the electronic charge. It is possible to even eliminate the electronic charges using an integrated ionizer module to avoid influence on the weighing result.

The static detection can be performed while the balance settles and the detection barely takes a few seconds. Therefore, there will be no delay in getting the weighing result.

3 Ways to Control Electrostatic Charges

1). * Prevent the Static Charges from Building-Up
Some ways to prevent a build-up of static charges include:

1. Use electrically conductive or anti-static treated materials. Avoid using plastic and glass vessels as they charge quickly.

2. Don't handle the container unless absolutely necessary. Avoid handling it especially when you are wearing protective gloves.

3. If you are handling dissimilar materials, avoid contact between them.

4. Increase the humidity of the air. The optimum range should be 45% to 60%.

5. Avoid wearing footwear that is electrically insulated.

6. Make sure that the weighing pan and balance are electrically grounded.

2). * Reduce Forces that Static Charges Produce

1. Use a smaller tare vessel.

2. Use a conductive underlay.

3. Use an ErgoClip to hold the tare vessel. It will act as a Faraday cage to electrically screen the sample material during weighing.

4. Make sure you place the sample in the center of the weighing pan.

3). * Neutralize the Balance

You can neutralize the entire balance through air ionization. The air ionizer produces a balance of positive and negative air ions which flood the environment of the analytical balance.

The charged objects in the work area will attract the oppositely charged air molecules, thus neutralizing the entire balance. Once the balance is neutralized, there will no electrical charges to affect the weighing process. As a result, you will get accurate weighing results.

Sensitive weighing applications may suffer from offset due to static charges within the balance and the objects in their environment. By implementing
good weighing practices in the lab, you can get accurate results.

Author - Kevin Hill

Kevin Hill heads the marketing efforts at
Quality Scales Unlimited in Byron, CA. Besides his day job, he loves to write about the different types of scales and their importance in various industries. He also writes about how to care for and get optimized performance from different scales in different situations. He enjoys spending time with family and going on camping trips.

October 2019


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