Non-Automatic Scale Calibration
Having explained the basic functions and metrology requirements of non-automatic weighing instruments, we can now give some information about their calibration.
Calibration consists of:
- the measurements performed
- calculation of the measuring results
- determination of the uncertainty of measurement
- issue of a calibration certificate
When calibrating a scale, the object of the calibration is the indication provided by the instrument when applying a load. The value of the load indicated by the scale is affected by local gravity, the load temperature and density and the temperature and density of the surrounding air.
When we calibrate a weighing scale we proceed as follows:
- Apply test loads to the scale under specified conditions
- Determine the error or variation of the indication, and
- Evaluate the uncertainty of measurement corresponding to the results
Range of Calibration
Normally the calibration must cover the full weighing range. (from minimum (Min) to maximum (Max) capacity). Of course the customer may define the desired calibration range or individual calibration values. On multiple range instruments, the customer should specify the ranges that shall be calibrated.
Place of Calibration
The best method would be to perform the calibration on the site where the instrument is used. There are many effects that can alter the performance of an instrument in the case when it is moved to another location and might invalidate the calibration. Some of these effects are:
- variation of the environmental conditions
- difference in local gravity acceleration
- mechanical and thermal conditions during transportation
Before proceeding with the calibration, the weighing scale must be checked to ensure that it works correctly, without any malfunction, and the indications are easily readable. The scale must be energized before calibration for an appropriate period, according to manufacturer’s instructions. If applicable, the instrument must be levelled. Any special manufacturer instructions describing the operation of the scale, must be taken into account.
The measurements taken during calibration determine the following:
- Repeatability of indications
- Errors of indications
- The effect of eccentric application of a load on the indication
The test is performed by depositing repeatedly the same load on the weighing scale, under identical conditions (of both the load and the instrument). There is no need for the test load to be calibrated or verified.
For the repeatability test, a test load LT must be selected which, for weighing instruments with a constant scale, usually has the following relationship with the Max:
0,5Max £ LT £ Max
Before testing repeatability, the indication of the weighing scale must be set to zero. Then the load is applied at least 5 times (or at least 3 times when LT ³ 100 kg).
Every indication corresponding to each deposition of the load is recorded. It is important to check, after each removal of the load, if the indication returns to zero. If this does not happen it must be reset to zero.
Example: On a scale with 200g capacity and scale interval of 0,1mg, we perform repeatability test with a 100g load and record the following indications:
The repeatability error is determined by the standard deviation s according to the following formula:
where n = 6 (the number of repeats)
By applying the formulas to our results, we have a repeatability error: s = 0.13 mg.
Test for errors of indications
The purpose of this test is the evaluation of the scale’s performance over the complete weighing range. The standard loads used for this test must be properly calibrated with an established traceability and measurement uncertainty.
This test is performed with at least 5 different test loads, fairly distributed over the instruments weighing scale. For example, 5 test loads can be zero or Min, 0,25Max, 0,5Max, 0,75Max and Max. Other test points can be used upon customer’s special requests.
Prior to the test, the indication must be set to zero. The method of calibration can be one of the following:
- Increasing load by steps with unloading between separate steps (this is the procedure corresponding to the majority of cases
- Continuously increasing loads by steps, without unloading between separate steps, but adding a new load
- Continuously increasing and decreasing by steps
- Continuously decreasing by steps, starting from Max
The indication for each load is recorded. After each removal of the load, the indication must be checked for returning to zero and if not, to reset it to zero.
Example: On the same scale as above, with 200g capacity and scale interval of 0,1mg, we perform calibration by applying the following loads and having the following results:
Standard Load (g)
This test is performed by placing a test load in different positions of the weighing scale in such a manner that the centre of gravity of the load takes the positions indicated in the following figures or equivalent positions, as accurately as possible.
- Front Left
- Back Left
- Back Right
- Front Right
The test load used for the eccentricity test does not need to be calibrated. The value of the test load should be at least Max/3 unless otherwise specified by the manufacturer.
Before starting the test, the indication must be set to zero. The load is first put on position 1 and then moved to the other four positions in arbitrary order. At last, it is put again on position 1.
Each time, indications must be recorded and after each removal of the load, we must make sure that the indication returns to zero.
Example: To our weighing scale, with 200g capacity and scale interval of 0,1mg, we perform eccentricity test by applying to the five positions a 100g load. The results of the measurements are as follows:
In order to calculate the eccentricity error, we calculate the difference between each position reading with the position 1 reading:
The eccentricity error presented in the calibration certificate is the absolute value of the higher difference. In our case:
Scales must be regularly calibrated in order to provide accurate results.
Written by Sofia