Weighing balance calibration is important to get accurate results of weighing; Most of the electronic weighing balance comes with a feature of Internal and external calibration, for batter accuracy it is recommended to calibrate the weighing balance with an external standard weight of known mass value.
For routine calibration of weighing balance using standard weight, operating manual of the weighing balance can be followed, normally all weighing balances comes with a inbuilt calibration function that can be access by pressing ‘Cal’ button for rest of the process manual can be followed.
For detailed calibration of weighing balance following of the test are performed.
“Repeatability is the ability of a weighing balance to repeat the results when same load is applied again and again”
Repeatability test is performed on full and half capacity of the weighing balance, to perform repeatability test two weight are required one of full capacity of weighing balance and one of half of the capacity , for example, for a weighing balance of 200g capacity weight required for repeatability test will be 200g and 100g.
After warm-up, set the balance display to zero.
For repeatability at half load repeat same process with half load weight and calculate the standard deviation
“Eccentricity is error due to off centre loading, that may occurs if measurand is not placed in the centre of the weighing balance pan”
Eccentricity test is carried out at a load recommended by the manufacturer of the balance or if it is not known, a load between one-third and half of the maximum capacity of the balance may be used, basic of selecting weight for eccentricity test is that surface area of the weight should be between one third and half of the pan area.
Always use single weight for eccentricity test.
Mark five locations on the pan including one centre and four corners for both square and round type pans as shown in figure.
The difference between the actual mass of a calibrated weight and its display on the weighing balance is known as the linearity error of that weighing balance
The linearity of the weighing balance determined by successively placing calibrated weights of the known mass value on the balance pan, and observing the difference between actual mass value and the display of weighing balance.
The departure of indication from nominal value or the linearity of the weighing balance is measured at sufficiently and equally spaced points over the full ranges of the weighing balance. Usually minimum ten such points are taken.
Read zero reading of the balance with no load on the pan.
Repeat the above steps with all other weights. Until the maximum capacity of the balance is reached.
Linearity correction for a weight is calculated as:
Standard mass value - (average observed value - average zero value)
Note-: This correction is the amount that has to be added to an indicated mass to give the correct mass of the weight on the balance pan. The negative of this correction is known as the departure from the nominal value
Hysteresis means lagging behind, weighing balance must not show any error when it is loaded to full range and between the full range multiple times at the same time.
Hysteresis may be caused due to overloading of weighing balance or any accidental dropping of object over the balance pan, the balance should be tested for its hysteresis when it is calibrated for the first time or after a major repair. This test is carried out near the mid-range of the balance, two weights of approx. half of the balance capacity used, to make the capacity of full range of the weighing balance, Hysteresis test is carried out in the following steps:
Repeat the above process for a second set of observations R5, R6, R7 and R8.
Hysteresis Error is the calculated as:
Hysteresis error = (R1+R3+R5+R7) - (R2+R4+R6+R8) / 4