Electrostatic Charges and their Effects on Weighing

Understanding electrostatic charges and the effects they have on your samples and weighing vessels is critical to achieving good quality weighing results. Wherever possible, preventative action should be taken to reduce or eliminate the build-up of static charges on samples or weighing containers, in order to avoid any errors, instability or frustratingly slow weighing results.

What is static?

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

Managing static samples

Exciting new advances in weighing technology enable the latest generation of analytical balances to automatically detect the presence of electrostatic charges during a weighing operation. The magnitude of this force can be measured and recorded. Using an integrated ionizer module, these electrostatic charges can be eliminated to avoid any influence on the weighing result. As the static detection cycle is performed while the balance settles, and takes only a few seconds, it causes no delay in obtaining the weighing result.

Benefits for the balance user

StaticDetect™ simplifies the handling of statically charged samples or containers, making the process more efficient for the balance user, to ensure the highest accuracy and most reliable weighing results.

White Paper: Electrostatics and Weighing

Samples or containers which are electrostatically charged can be difficult to weigh, often causing problems with balance stability or measurement drift. This white paper explains how to:

Prevent static charges from building up

Dissipate static charges once they are generated

Recognize the disruptive effect static charges have on weighing

Choose the appropriate method for deionization of your samples/containers

XPR analytical balances simplify the handling of static samples, by using automatic detection and elimination of electrostatic influences, to ensure the highest accuracy and most reliable weighing results.

Download the "Electrostatics and Weighing" white paper

Understanding Electrostatic Charges

Friction is the most common way that electrostatic charges are generated. Examples include:

Drying a glass beaker with a cloth
Handling a volumetric flask with disposable gloves
Unpacking a laboratory vessel from a plastic bag
Filling a vessel with loose materials (bulk)

The presence of static is usually recognizable by:

Balance struggling to settle
Drifting measurement readings
Non-repeatability of the measurement results

However, there are cases where a stable reading can be obtained with a net electrostatic force present, which makes it difficult or impossible to identify the presence of electrostatic effects.

► Download the "Electrostatic Test" guide to determine the presence of static

The reported value may be more or less than the real weight (because the charges generated can be negative or positive, and either attract or repel each other)
Errors of 1-100 mg have been observed (which is highly significant when weighing small samples)
Dry powders are very susceptible to static and therefore difficult to weigh

Electrostatic charges dissipate over time. The dissipation rate is affected by charged body surface conductivity, which is influenced by air humidity and surface contamination.
Charge dissipation can take a few seconds or minutes, or several hours in a dry atmosphere (RH < 40%).
Good insulators, such as borosilicate glass or laboratory grade plastics, can make charge dissipation slower.
A glass flask cannot become charged at 80% relative air humidity.

However, for PTFE containers, even when the air humidity is high, significant weighing errors are observed over several minutes.

Here are some recommendations for practical solutions that can be put into place:

1. To prevent build-up of static charges

Use electrically conductive or anti-static treated materials – plastic and glass vessels charge quickly so are non-ideal materials
Avoid contact between dissimilar materials when handling
Avoid unnecessary handling of the container – especially while wearing protective gloves
Increase air humidity (optimum 45-60%)
Ensure balance and weighing pan are electrically grounded
Avoid wearing electrically insulated footwear

2. To reduce forces produced by static charges

Use an "ErgoClip" to hold tare vessels, as it acts as a Faraday cage to electrically screen the sample during weighing
Use a smaller tare vessel
Ensure the sample is placed centrally on the weighing pan
Use a conductive underlay

3. To dissipate static charges

Use a high-voltage ionizer (internal or external) to discharge samples
Use a radioactive source to ionize the air

But the best way to control electrostatic charges is to avoid generating them in the first place!

► Download the white paper "Electrostatics and Weighing" to learn more

StaticDetect™ is a patented innovation to help balance users who have issues with electrostatic charges, and is available on METTLER TOLEDO XPR balances. It involves the automatic detection of the presence of an electrostatic charge on the tare container and/or sample using a sensor. If a static charge is detected, the magnitude of the effect of this charge on the weighing result is measured. A warning is given if the user-defined limit of acceptability is exceeded. An icon on the balance terminal screen indicates the presence of static charges.

This technique is immune to electromagnetic interference and works over a large range.
It operates successfully in any laboratory environment, even in challenging conditions such as in fume hoods or containment systems.
Alternative methods exist for detecting whether a body is electrostatically charged, but none of them are able to estimate the EXTENT by which the charge influences the reported weight.

The electrostatic detection cycle begins automatically as soon as the sample is placed on the balance and the door is closed. As the static detection cycle is performed concurrently with the weighing step and takes only a few seconds, typically during balance settling time, it causes no delay in obtaining the weighing result.

A concentric electrode is integrated below the grounded weighing pan, and an AC voltage signal is applied. The positive half-wave of the square wave signal generates positive charges at the electrode (a). The negative half-cycle of the square wave generates negative charge carriers at the negative electrode (b). The difference in the measurement result between the two situations (a and b) can be attributed to the electrostatic force (F_E). This will be zero if the sample or container is not electrostatically charged. Using this method, the sensor can determine the true weight of the sample.

Attractive force - electrostatic charges

(a): An attractive force is acting between the measured object and the electrode. The vertical component of this force is added to the weight and influences the measured result by making the reported result heavier that the true result.

(b): A repulsive force is acting between the measured object and the electrode. The vertical component of this force is subtracted from the weight and influences the measured result by making the reported result lighter than the true result.