analytical weighing

Electrostatic charging as an interference factor during analytical weighing processes

 

Electrostatic charging as an interference factor during analytical weighing processes

Dieter Feller and Karl-Armin Opfer

Electrostatic charges

Every one of us will have had an experience of electrostatic charging. Either in the form of a mild electric shock when getting out of a car or touching a hand rail, or trying to control unruly “flying hair”. These are the usually unwanted effects of electrostatic charging.

From the point of view of a lab operator involved in weighing processes, this phenomenon could be dismissed as an annoying but trivial problem affecting our private lives, if not for the fact that it causes continual problems when it comes to the process of analytical weighing.

Before we turn our attention to solution possibilities, first of all let us take a brief look at the “phenomenon of electrostatic charging”. The first findings in this field stretch back to the Greek scientist Thales (640 to 547 BC.), who also made a name for himself as a mathematician.

As long ago as 90 years after the founding of Rome, he discovered that a piece of amber rubbed against his clothing attracted lightweight papyrus particles in the same was as a magnet. The Greek word for amber is “electron”; the word static is derived from Latin and describes a standstill or idle status.

So our knowledge about static electricity originates from a long way further back than our understanding of “electrodynamics”, which is more familiar to us as the electrical circuit.

Charging phenomena can be separated into three categories:

  • charging involving relative mechanical motion of objects and surfaces
  • charging as a pure contact phenomenon (no relavtive macroscopic motion occurs)
  • charging as a primarily chemical (or electrochemical) procces

 

The first category is one of the oldest known forms of charging. As a result of friction, electrons are “released” from the body with the lower discharge energy (donator). They jump over to the body with the higher discharge energy (aceptor). As a result, ions are created which charge the body either negatively (in case of an excess of electrons) or positively (in case of a deficit of electrons). These charges are temporary, i.e. they are discharged again as soon as they have the opportunity, for example as a result of earthing. As it is regrettably not possible to create a galvanic earth connection everywhere on the surface of an insulating material, a different solution has to be found.

At least in theory, the following conveivable possibilities exist for this.

  • To reduce surface resistance levels
  • To reduce contact surfaces
  • To apply separating of distance agents
  • To increase relative humidity
  • To apply antistatic agents
  • To provide reliable earthing
  • Passive or active ionization

 

From the practical point of view, most of these options are not feasible when applicable to analytical weighing processes.

So what can be done?

There are two possibilities here:

  1. The passive option, i.e. to take all the necessary precautions for “proper discharge” in the scale itself. This involves for instance the comprehensive use of high-grade steel from the scale pane through to the power supply unit
  2. Active high-voltage ionization.

 

An active high-voltage ionizer is made up fundamentally of a high-voltage power pack and the actual ionizer.
The power pack generates an alternating voltage of around 7 kV and is connected to the ionizer by means of a flexible high-voltage cable. The positive and negative ions created at the tips of the ionizer neutralize the charge on the surface of the charged material.

A major advantage of high-voltage ionization is that, in accordance with the sinus curve of the applied alternating voltage, both positive and negative ions are created. Therefore it makes no difference whether there is positive or negative charge to compensate.

If a U-shaped electrode is positioned immediately in front of the weighing area, the discharge process takes place at the nearest possible position to the scale pan. This saves awkward handling and takes place practically automatically when filling the analytical scale.

Is there no element of danger due to high-voltage at the ionizer?

This is something you are certain to have asked yourself. Unfortunately it is the case that certain ionizing equipment manufactures still work with so-calles “hot” electrodes, i.e. electrodes in which the high voltage is actually connected galvanically to the ionizing pin. Although the flowing current is generally only a few milliamperes, contact os not totally without danger.

Modern ionizers, in contrast, are based on the principle of capacitive current decoupling and are accordingly completely shockproof. In conjunction with a modern, self-calibrating analytical scale, it is possible to guarantee both, the reliability of measured results and also electrical safety for the operator.

Surface resistance of different materials

Material Ohm cm
Wood (dry) 109 (1 Gigaohm)
Paper 1010 (10 Gigaohm)
Glass 1011 (100 Gigaohm)
PVC (polyvinyl chloride) 1014 (100 Teraohm)
PS (polystyrene) 1015 (10 Petaohm)
PP (polypropylene) 1016 (10 Petaohm)
PE (Polyethylene) 1017 (100 Petaohm)



Surface resistance of different materials