Understanding electrostatic charges and the effects they have on your samples and weighing vessels are 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 whilst 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.
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:
Understanding Electrostatic Charges
What causes static?
Friction is the most common way that electrostatic charges are generated, such as:
- Drying a glass beaker with a cloth
- Handling a volumetric flask with disposable gloves
- Unpacking a laboratory vessel from a plastic bag
- Filling vessel with loose materials (bulk)
How to recognize static?
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.
What effect can static have?
- 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
How quickly do static charges dissipate?
- 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 dry atmosphere (RH < 40%).
- Good insulators, such as borosilicate glass or laboratory grade plastics, can make charge dissipation slower.
- A glass flask is unable to 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.
Electrostatic charge dissipation
How to control electrostatic charges?
Here are some recommendations for practical solutions that can be put into place:
What is Automatic Static Detection (StaticDetect™)?
How long does the Static Detect™ process take?
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.
How does StaticDetect™ work?
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 (FE ). 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.