A measuring instrument's lifecycle begins from evaluation of ab operator’s requirements. The operator should be aware of the production and control requirements of the balance. A balance is expected to provide more than just an accurate measuring device but must be compatible with a company’s network to send data to other devices.
When selecting a balance, it is important to consider all the requirements. The operator should compare the balance features with economic considerations, the balance supplier’s responsiveness to defects and any further configuration possibilities.
The company control department is responsible for the installation of balances with low readabilities. In the case of high-accuracy balances, such as ultra-micro balances or microbalances, the balance equipment must be installed by an authorized manufacturer. This ensures the configuration and set-up of the balance is to company standards and ensures the required performance. For example, this may include stability and filter settings. RADWAG offers these activities as a supervision packet over a balance (technical inspection book).
Figure 1. Life cycle of a measuring instrument. Image Credit: Radwag Balances & Scales
Calibration of the balance equipment is a fundamental part of the life cycle of a measuring instrument and enables the actual balance errors to be determined. Calibration should be connected with the determination of minimal mass if such data is necessary.
An operator performs routine activities that aim to decrease the level of risk that occurs in the weighing process. This includes calibration and adjustment processes.
Balance Routine Examination
Instrument accuracy is guaranteed by routine tests, which are used for analysis and to minimize the frequency with which errors occur. The basic aspect of routine examination refers to its intervals:
How often should routine texts be performed?
Routine tests must be performed in accordance with the specified company intervals. A balance is assumed to operate properly between these routine test intervals. i.e. its measurements are within the set acceptance tolerances. It is helpful to introduce two limits for controlled parameters. The first is a cautionary limit, which, if exceeded, does not affect balance operation.
The second one is a critical limit, which, if exceeded, causes a balance-recognition defect. For sensitivity control, it is necessary to consider actual balance errors for loads below the max capacity, for example, 50% of the max allowable load. Balance errors in this instance may be acceptable. Therefore, exceeding the critical limit does not always result in separating a balance and recognizing it as out of order.
Figure 2. Sensitivity drift – cautionary limits. Image Credit: Radwag Balances & Scales
This raises the question of what specification a balance should be checked against. Generally, it can be assumed that this depends on :
- The expected weighing accuracy
- The parameters which are being tested
- The weighing process and its acceptance within specified tolerances
The alternative approach is testing a balance before each measurement process. This is unpractical if a balance is not highly utilized (relatively low frequency of measurements, and a couple of operators). It would be necessary to create a standard operating procedure, according to which a balance will be tested. Test results should be recorded with key information such as the date, time and operator code. However, implementation of such a procedure may be a challenge, as it requires very high exploitation of mass standards and supervision over this mass standard.
Is it possible to decrease test frequency?
It is recommended to decrease the number of tests, as too many tests may disrupt the balance’s working cycle. To decrease the frequency of testing, it is recommended to observe the stability of balance measurement and, depending on this stability, adjust the frequency of testing. Analysis accuracy is the main consideration here and the frequency of testing needs to be adjusted to consider this. Generally, the rule is:
high risk = frequent tests
Low Mass Measurement
When a balance is only used for low mass measurements (up to 5% balance maximum allowance capacity) the only justified test is repeatability control. In such a case, it is assumed that even if prominent tare is used, e.g. 30% of Max capacity, it is still located at the central pan point. With low mass measurement, other balance errors that influence deriving from linearity, sensitivity change or eccentricity, are negligible.
Figure 3. Low mass measurement – parameter checking. Image Credit: Radwag Balances & Scales
The ideal situation for balances is for their utilization to be at 100%, which, in practice, means good measurement ability for both light and heavy loads. Under these conditions, sensitivity drift should be checked more often.
Figure 4. Sensitivity change test – balance adjustment. Image Credit: Radwag Balances & Scales
Most balances are equipped with self-adjustment systems. However, sensitivity drift can still occur. Device drift can occur due to a change in ambient conditions. If it does occur, then the situation from drawing no.4 takes place.
As standard practice shows, most companies only test adjustment effectiveness, while balance accuracy is controlled by adjusting the balance using a precisely specified weight. Balance indication is not checked considering the elapsing time from the previous calibration. In such a case, the result would show the stability of balance indications in time, which is stability of sensitivity.
Good measuring practice states that, before a measurement is taken, an operator must perform an adjusting procedure. All sensitivity errors will then be zeroed. After the operation, an error-free value should not be expected, as the adjustment process relates to mass weighing and these are not ideal measurements.
- For heavy load weighing, sensitivity change tests should be performed.
- For low mass measurements, repeatability tests should be performed, as this is the main source of errors. However, repeatability tests should not be neglected for heavy load measurements. There is also an equipment risk during heavy load measuring if a load hits the weighing pan and damages the precise balance mechanism.
- Eccentricity should be checked if the loads being weighed have large dimensions or untypical shapes. In which case, the load being measured may not have a central position on the weighing pan. Eccentricity errors may be avoided if special holders for weighed objects are applied.
- Non-linearity – does not refer to low mass measurements. This parameter is stable in the entire measuring range of a balance, and therefore its contribution to overall errors is relatively low. In most cases, non-linearity error results from a non-repeatable operation of a balance.
- The specified frequency of balance testing should take into consideration the range of jobs performed on a balance, their intensity, balance stability in time and the required weighing measurement precision. The following balance parameter control periods can be fixed, assuming the external conditions are stable:
- Calibration - annually
- Repeatability - monthly
- Centricity - monthly
- Sensitivity change - weekly
- Adjustment - daily
The above-specified parameters for balance control are not mandatory. Each metrological unit should establish the requirements and specifications for its balance operation.
About Radwag Balances & Scales
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The ideas are realized by supplying the customers with more and more innovative products, which are employed by most demanding end users with great satisfaction. Measurable effects of our activity are positive opinions of numerous users, thanks to which RADWAG is placed among leading manufacturers of balances and scales.
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