EXTERNAL FACTORS IN WEIGHING PROCESS
All balance parameters presented in here are related to typical laboratory conditions, that is the ones which take interfering factors into account. The term includes all factors originating from the ambient area and people who influence measurement result.
It is not possible to define all of them, but the main factors include:
• Oscillations, vibrations
• Breeze of air
• Temperature drifts
• Evaporation and absorption phenomena (hygroscopicity)
It should be remembered, that the factors influence both balance and the weighed object. When estimating weighing process as a whole, factors should be distinguished and diagnosed, which of them is of decisive significance for analysis correction.
Oscillations, vibrations: – They are transmitted by the ground and walls, and the source that generates them are the devices and objects moving in communication course and the staff. Visible effect is longer measurement time and higher indication dispersion. Vibrations are transmitted to balance mechanical system, so the most successful method of preventing them, is to remove them from weighing process.
Another commonly used way to prevent vibrations is application of anti-vibration table. It is a construction based on a double rubber console, which task is to suppress vibrations.
Breeze of air:- Visible result of their influence is indication instability and long weighing time. Balance workstation should not be located close to doors or windows.
Closeness to devices such as air-conditioning, fans, as well as places like communication courses should be avoided.
Temperature in analysis process:- It is one of the most important factors of weighing process. Weighing room temperature should be maintained at constant level. Before analysis start, while performing it and after its finishing, temperature should be stable. The of “Constant level” temperature is a disputable matter. It is defined differently in relation to balances with 1mg elementary unit and still differently for balances with d=0,01mg. It is accepted that temperature is treated as stable if its changes are not higher than 0.50C/hour.
For balances equipped with automatic adjustment system, balance precision restoring process is carried out automatically, with consideration of temperature and time changes.
The statements above result in practical aspects for weighing process:
• acclimatize the samples before analyzing their mass
• operator should not put their hands into weighing chamber (causes change of weighing chamber temperature)
• pick up the samples with use of tweezers or other holders (if touched with hands, samples change their temperature)
Balance temperature changes:- The effect of dynamic temperature changes in a weighing room is dynamic temperature error. In practice, if dynamic temperature error occurs, measuring parameters of a balance, like repeatability and linearity, may get worse. The problem of dynamic temperature error is particularly dangerous for balances equipped with system of external calibration. Such balances are not equipped with system notifying on temperature change.
In case of slow temperature changes, a balance will warm together with a weighing room. Such process is not source of errors due to its slow dynamics, and static errors compensating factors. Considering temperature influence on weighing process, balance and ambient area warming up process caused by operator influence should be taken into account. It is assumed, that a man is a source of about 70W power. Bearing in mind this factor, quantity of operators working in the weighing room at the same time cannot be large. Practically, it depends on the room size and air conditioning system.
Electrostatics in weighing process:- Evaluation of electrostatics in weighing process is complicated, as an operator has to determine a factor that is not visible. The operator can only observe the effect of electrostatic presence.
Electrostatic discharges may occur on:
• compensated ions (positive or negative) are taken over from the air,
• by rubbing two non-conducive substances,
• touching a sample with hand
• low humidity in weighing room
Visible effect of electrostatics presence is:
• slow drift of weighing result,
• large dispersion of weighing results in a series of measurements, and
• no return to zero if a load is taken off the weighing pan.
As it is not possible to remove the source of electrostatics, it is common to use factors that eliminate or compensate the influence of undesired electrostatic charges.
One of the methods that partially eliminates the problem is providing proper air humidity in a weighing room. It is recommended to set relative humidity of a weighing room in between 40 % and 60 %. There are, however, cases where setting such humidity is not desired or impossible. If such is the case, then operator should install an ionizer – ionizing frame. Ionizing frame is a device that generates ions, so called aero-ions, which charge is opposite to charges that operator wants to eliminate. The means of operation of an ionizing frame is eliminating charges that are on operator’s hand and inside weighing chamber on its opening. If balance conditions, ionizing rate is equal to recombination rate, which maintains constant ionization rate.
Additionally, an operator can apply specially designed mats, antistatic foils and selected uniforms.
Sample structure, hygroscopicity:- Weighing result, as obtained during an analysis period is influenced by a series of factors relating to balance and ambient conditions. However, one should also consider sample features during weighing process. Samples that are liquids, can undergo process of evaporation.
In such case, balance indication will be influenced by a drift. Weighing result will continuously decrease. Additionally, it can be concealed by balance filters. In order to prevent such situations, liquid samples should be weighed in weighing vessels, like bulbs with narrow necks or vessels with top cover. If operator is determining evaporation level, not the mass, balance settings should be modified, so that it is possible to perform ordered analysis. A factor reverse to evaporation is absorption of moisture from ambient air by a sample. it is very important in case of hygroscopic samples. The effect of moisture absorption is differences in mass determination, each measurement will have higher mass readout than the previous one. For the purpose of proper weighing of such substances, weighing vessel should be clean and dry. The easiest way to eliminate moisture absorption factor is application of hermetic vessels.
Magnetism as weighing process interfering factor: – Most of weighing mechanisms of high resolution balances are constructed on basis of electromagnetic sets which include a force-motor and magnet. In case magnetic loads are measured, there is a risk, that electromagnetic field of a balance is disturbed or weighed sample is influenced by magnet installed in a balance. The effect is incorrect mass reading of a weighed sample.
A solution for this problem is removal of a weighed sample from electromagnetic field of a balance, i.e. increasing a distance between a sample and balance mechanism. It is possible through so called under-hook weighing with application of special racks or hooks made of aluminum.
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