When carrying out precision weighing applications, it can be difficult to achieve accurate results, even when using some of the best equipment available. This is because external factors such as draughts or humidity can affect accurate weighing.
One external factor that may not be top of mind is static charge. Static charge is usually caused by contact between objects, and even a small amount can impact your weighing result.
In this post, we explore how static charge may be created in the lab and how it impacts precision weighing. We also look at steps you can take to minimize static charge so you can enjoy greater accuracy in your weighing applications.
How Static Charge Is Created
Generally, objects have a balance of positive and negative charges. Static electricity is created when there is an imbalance of charges on an object. When objects come in contact with each other and then are separated, electrons (which are negatively charged) can be transferred from one object to the other. This is known as triboelectric charging.
The amount of charge depends on multiple factors, including how rough or smooth the materials are, the contact surface area, the speed of separation of the materials, and the amount of humidity present (we’ll discuss the latter below).
If the material is an insulator, it will remain charged. If it is a conductor, it can drain the charge to another object or the ground. The human body is a conductor, but shoes or flooring materials often insulate us from the ground. When a static charge is built up on our hands, it will remain there until we transfer it to another object, for example, another person or a metal door handle. This is why we experience “static shocks.” The electrons built up on our skin or clothes are being discharged to whomever or whatever we have come in contact with.
The Impact of Static Charge on Precision Weighing
In a laboratory setting, there are many ways in which everyday items and actions could be causing static charge. Here are a few examples:
- Wiping or drying surfaces or equipment with a cloth
- Removing or putting on PPE such as a lab coat or hair net
- Picking up or putting down glassware while wearing gloves
- Separating weigh boats
- Putting powder in a weigh boat
It’s very feasible that components of a balance as well as samples and weigh boats could hold a static charge.
Objects with the same charge repel each other and those with opposite charges are attracted to each other. This electrostatic force can result in very slight movement of objects. It can also attract particles from the air to various objects, including your weighing sample and scale components. The attraction of particles and any movement caused by static charge could cause errors in weight measurements.
How to Control Static Charge to Ensure Accurate Weighing
While static charge is a recurring issue when it comes to precision weighing, there are actions you can take to control it, including:
- Utilize an ionizer
- Use antistatic equipment
- Increase the humidity
- Ground lab personnel
1. Utilize an Ionizer
A handy piece of equipment to have at hand when precision weighing is an ionizer. These continuously generate negative and positive ions and send ionized air to the charged body (samples and weighing containers). As a result, the static electricity will be eliminated.
An Ohaus Static Ionizer ION-100A and an Accuris compact anti static ionizer.
The Accuris ionizer has a blower which can help rapidly dissipate static charge, but can be switched off when working with lightweight material such as powders.
The Sartorius Stat-Pen works in a similar manner to the ionizers above to remove the static charge from samples and filters.
Some balances, for example, Radwag’s XA 4Y M A PLUS Microbalances, have a built-in ionizer. Others, like Ohaus Pioneer models, are equipped with a static removal bar (shown below) which you simply have to touch with the charged object to eliminate static charge.
2. Use Antistatic Equipment
It’s also a good idea to look out for equipment that is made from antistatic materials, such as:
- Weigh pans, for example, those offered by Sartorius
- Balance components such as the draft shield
- Storage containers (some integrate conductive compounds for this purpose)
- Dispensing tools (many of these are designed for use in the manufacturing of sensitive electronics)
3. Increase the Humidity
You’ve probably noticed that you experience more static shocks during colder weather. This is because the air is drier (has less humidity). Air is naturally an insulator so it’s difficult for electrons to travel through. However, water is a conductor and electrons can be transferred to water droplets present in air. Drier air has fewer water molecules to transfer charged particles to a ground. Working in a more humid environment lowers the risk of static charge affecting your results.
It is worth noting that too much humidity can negatively impact the precision of weighing too. Water droplets can bind to sample particles, adding mass. Optimal humidity for a lab environment is usually around 50 percent.
4. Ground Lab Personnel
As mentioned, people are conductive, but the charge won’t be released through their shoes, so it will be released upon contact with another conductor.
There are many options available to help ground lab personnel, including antistatic wrist straps, antistatic mats, electrostatic discharge (ESD) flooring and ESD footwear. That said, these are more commonly found in environments where very small electric charges are a concern such as places where microcircuits and other sensitive electronic components are manufactured.
In most laboratories, trying to prevent the buildup of static charge and removing it with an ionizer should suffice. Antistatic PPE can help reduce static buildup as can wearing natural materials (synthetic materials can act as insulators).