Who hasn't experienced results floating around on an analytical weighing display?
But have we thought about the potential reasons?
Most weighing operators who have to perform analytical weighing of bulk powdered substances have experienced electrostatic charging, where in the most extreme cases the sample literally "dances" as soon as it is picked up on the sample paddle.
In order to still be able to perform accurate weighing in such cases, it is important to understand the nature of the electrostatic charge, its effect on the weighing process, and possible solutions to the problem.
Whenever possible, preventive action should be taken to reduce or eliminate the build-up of charge on the sample or weighing container, thus avoiding erroneous results, instability and an annoyingly slow weighing process.
Static is the accumulation of electrical charge on the surface of non-conductive materials
Friction is the most common way in which electrostatic charges are generated. Typical examples would be:
- Drying/wiping a glass beaker with a cloth
- Using a volumetric container with disposable gloves
- Unpacking a laboratory container from a plastic bag
- Filling the container with loose material
- Transport of bulk material
The presence of statics is usually indicated by:
- the balance is unable to stabilise
- Display reading "floating"
- Very poor repeatability of results
- The mass value shown may be greater or less than the true value of the sample. Because the charges generated may have a '+' or '-' sign, thus attracting or repelling each other.
- The error in absolute terms can range from 1 to 100 mg, which is significant when weighing tiny samples.
- Dry powders are very susceptible to static and are therefore very difficult to weigh
Electrostatic charge dissipates in time. The rate of dissipation depends on the surface conductivity of the charged body, which in turn is affected by the relative humidity and surface contamination
Charge discharge can take from a few seconds to several hours in a dry atmosphere (RH ≤ 40%)
Good insulators, such as borosilicate glass, or laboratory grade plastics can give slower discharge
Glass containers cannot be charged at 80% RH, while PTFE containers, even at high RH, will be charged and produce a significant weighing error.
Here are some suggestions and practical tips to consider:
1. To avoid electrostatic charges
- Use electrically conductive or anti-static materials - plastic and glassware charge quickly, so are not ideal materials. Try special weighing boats with an anti-static coating!
- Avoid contact between dissimilar materials when working
- Avoid unnecessary handling of the weighing container, especially when using gloves
- Increase the humidity in the room (optimally between 40 - 60%)
- Ensure that the weighing pan and the weighing pan are electrically earthed
- Avoid wearing electrically insulating shoes
2. To reduce the effects of electrostatic charge
- use the special holder from the ErgoClip series. This acts as a Faraday cage and electrically isolates the sample during weighing
- Use smaller containers
- Make sure the sample is placed in the centre of the weighing pan
- Use a conductive insert
3. To dissipate pre-existing static
- use high voltage ionizers (built-in or external) to dissipate the charge
What is automatic static detection (StaticDetectTM)?
It is a patented innovation available on the most advanced Mettler Toledo scales from the XPR series. It involves the automatic detection of a charge in a sample or tare container using a sensor. If the sensor detects a static charge, that's not all - the magnitude of the charge effect is also measured. This is followed by a warning if the user-defined warning limit is exceeded, or even a prohibition to continue weighing.
StaticDetect starts working fully automatically as soon as a sample is inserted. And since the process takes only a few seconds (usually during the stabilisation of the scales), it does not increase the time of the weighing process in the slightest
Derox team will be gappy to advize possible solutions if you are facing described problem with electrostatics.
We also make an educational seminars by explaining theoretical and practical background of electrostatics - please ask if you are interested to have one!
But have we thought about the potential reasons?
Most weighing operators who have to perform analytical weighing of bulk powdered substances have experienced electrostatic charging, where in the most extreme cases the sample literally "dances" as soon as it is picked up on the sample paddle.
In order to still be able to perform accurate weighing in such cases, it is important to understand the nature of the electrostatic charge, its effect on the weighing process, and possible solutions to the problem.
Whenever possible, preventive action should be taken to reduce or eliminate the build-up of charge on the sample or weighing container, thus avoiding erroneous results, instability and an annoyingly slow weighing process.
What is statics?
Static is the accumulation of electrical charge on the surface of non-conductive materials
What causes static charge build-up?
Friction is the most common way in which electrostatic charges are generated. Typical examples would be:
- Drying/wiping a glass beaker with a cloth
- Using a volumetric container with disposable gloves
- Unpacking a laboratory container from a plastic bag
- Filling the container with loose material
- Transport of bulk material
How to recognise static?
The presence of statics is usually indicated by:
- the balance is unable to stabilise
- Display reading "floating"
- Very poor repeatability of results
How much impact can statics have?
- The mass value shown may be greater or less than the true value of the sample. Because the charges generated may have a '+' or '-' sign, thus attracting or repelling each other.
- The error in absolute terms can range from 1 to 100 mg, which is significant when weighing tiny samples.
- Dry powders are very susceptible to static and are therefore very difficult to weigh
How fast do projectiles dissipate?
Electrostatic charge dissipates in time. The rate of dissipation depends on the surface conductivity of the charged body, which in turn is affected by the relative humidity and surface contamination
Charge discharge can take from a few seconds to several hours in a dry atmosphere (RH ≤ 40%)
Good insulators, such as borosilicate glass, or laboratory grade plastics can give slower discharge
Glass containers cannot be charged at 80% RH, while PTFE containers, even at high RH, will be charged and produce a significant weighing error.
How can you control electrostatics?
Here are some suggestions and practical tips to consider:
1. To avoid electrostatic charges
- Use electrically conductive or anti-static materials - plastic and glassware charge quickly, so are not ideal materials. Try special weighing boats with an anti-static coating!
- Avoid contact between dissimilar materials when working
- Avoid unnecessary handling of the weighing container, especially when using gloves
- Increase the humidity in the room (optimally between 40 - 60%)
- Ensure that the weighing pan and the weighing pan are electrically earthed
- Avoid wearing electrically insulating shoes
2. To reduce the effects of electrostatic charge
- use the special holder from the ErgoClip series. This acts as a Faraday cage and electrically isolates the sample during weighing
- Use smaller containers
- Make sure the sample is placed in the centre of the weighing pan
- Use a conductive insert
3. To dissipate pre-existing static
- use high voltage ionizers (built-in or external) to dissipate the charge
What is automatic static detection (StaticDetectTM)?
It is a patented innovation available on the most advanced Mettler Toledo scales from the XPR series. It involves the automatic detection of a charge in a sample or tare container using a sensor. If the sensor detects a static charge, that's not all - the magnitude of the charge effect is also measured. This is followed by a warning if the user-defined warning limit is exceeded, or even a prohibition to continue weighing.
StaticDetect starts working fully automatically as soon as a sample is inserted. And since the process takes only a few seconds (usually during the stabilisation of the scales), it does not increase the time of the weighing process in the slightest
Derox team will be gappy to advize possible solutions if you are facing described problem with electrostatics.
We also make an educational seminars by explaining theoretical and practical background of electrostatics - please ask if you are interested to have one!