To prepare a procedure so to perform the verification / calibration of weighing balance.
MASTER USED: Standard Weights (E2 Class / F1 Class / M1 Class)
Check the condition of the UUC for any sort of damage that might have occurred to the instrument. Then we also have to check the display to ensure that the reading is displayed correctly. Note down all the details of the instrument with respect to range, make, ID / Sr. No, Customer details and environmental conditions.
REFERENCE STANDARD METHOD: EURAMET CG 18
a. Make sure the weights and weighing scale is clean. Calibration of Weighing Balance
b. Place the standard weights that have to be calibrated near the instrument.
c. Note the room temperature using a thermometer and the temperature of weights by using the same instrument.
d. Leave the instrument and weights undisturbed for 30 minutes along with the balance so as to stabilize them to the temperature conditions. Calibration of Weighing Balance
e. During the calibration of weighing balance check for any specific instructions given by the manufacturer or if there are any specific requirements of the customer.
3. PROCEDURE FOR CALIBRATION OF WEIGHING BALANCE
1. Warm up test:
a) Set the scale at zero value and record the room temperature.
b) After 1 minute record the indication with an empty pan. Calibration of Weighing Balance
c) At the center of the pan place a standard weight that is equal to 98% of the capacity of the scale. If by any chance the scale does not operate at the stated capacity then this test should be performed at 90% of capacity. When the indication is steady, only then the reading should be recorded and then the weight removed from the pan. Calibration of Weighing Balance
a. Set the balance to zero as in warming up instructions.
b. Place weights on the pan in increments of 10% up to 100% of its capacity, observe the readings and record them. The deviation from standard indicates inaccuracy from the true value. Calibration of Weighing Balance
It is assumed to be constant for the entire range of the balance. A set of five readings are taken to calculate the error due to repeatability at the same load. Repeatability is calculated at 50% of capacity of the balance. Calibration of Weighing Balance
At each interval of the scale the repeatability test needs to be conducted. E.g. Capacity of scale 200g, interval 0.01 mg up to 100g, 0.1mg up to 200g. Then, the repeatability will be calculated for both ranges. Prior to the test, we need to tare the indicator of the instrument to zero.
4. Off-center Errors / Eccentricity Test:
Place the weights at 30% of the capacity in 5 positions on the pan, as shown in the below figure and then note the reading for each position. Error is calculated as the difference between the 1st reading and each of 0 then 2, 3, 4, 5 readings. Prior to the test the reading needs to be set at zero. Calibration of Weighing Balance
Normally hysteresis is performed on new balances or if there is any specific instruction from the customer.
The test can be performed as explained below:
a. Firstly set the balance to zero reading.
b. Place a weight or weights on the pan equal to about 50% of the balance capacity and once it is stable then record the reading.
c. Add more weights to the balance, which is 90% to 100% of the total capacity. Wait for a stable reading. The actual value need not be recorded.
d. Remove the weights added and record the balance reading after it has stabilized.
e. Remove rest of the weights from the balance and record the readings as soon as it is stable. Calibration of Weighing Balance
f. Find bellow the sequence of operations.
|Operation||Weight On Pan||Balance Reading|
Calculate (W1–W1’) + Z/2 and any deviation due to hysteresis.
6. Temperature Effects Calibration of Weighing Balance:
The ambient temperature could affect the zero reading or even during full scale calibration. Precautions need to be taken to avoid moving the instrument from one location to the other, as it could have a detrimental effect due to change in temperature. Calibration of Weighing Balance
4. Instructions for operating the weighing scale. Calibration of Weighing Balance
1. Check the following before using the weighing scale.
· The balance is stable at the work station.
· Work station is clean and dry.
· Balance should be placed far away from objects and equipment which possess magnetism or generate a magnetic field, sound, vibrations or air circulation.
· Operating temperature should ideally be in the range of 23 +/- 5 C and humidity 50 +/- 15%.
· Note that the calibration of the balance needs to be performed at the location where the instrument is used.
· Calibration points to be taken at approximately 10%, 20%, 40%, 60%, 80% & 100% so as to cover the entire range. Care should also be taken to keep in mind if there were any other instructions from the client.
· E.g. 0.1%, 0.2%, 0.5%, 1.0%, 2.0%, 5.0%.
E.g. 220 gm balance
20 (10%), 40 (20%), 80 (40%), 120 (60%), 150 (80%) & 220 (100%).
5 (10%), 10 (20%), 20 (40%), 35 (60%), 50 (80%), 60 (100%)
· The F1 class weights needs to be handled using the tweezers only.
Level the balance if the leveling is not satisfactory. To check the leveling of the balance, take the help of the inbuilt spirit level indicator of the
balance. If it is not centered, center it by adjusting the leveling screws placed at the bottom toward the back of the balance.
Once the balance is leveled, close all the chamber doors if available and press the control bar on the front of the balance. After a few seconds, a row of zeros will appear. This indicates that the balance is zeroed and ready for use.
5. Instructions for use:
· Connect to power supply as per the rating of the balance.
· Switch on the balance.
· If the display load is not zero, tare the load.
· Carry out the calibration as per the procedure.
· Switch off the balance.
6. RECORD OF FINDINGS:
Record the readings in observation sheet.
7. ACCEPTANCE CRITERIA:
Calibrated instrument must be accepted as per Manufacturer specification or as per the customer criteria if it has been provided by the customer.
Procedure for Calculation of Uncertainty of Weighing Balance
The following method of calculating the uncertainty is based on UKAS Document No. M3003. The Expression of Uncertainty and Confidence in Measurement.
The parameters used for calculating the uncertainty are:
UB – Type B uncertainty
UA – Uncertainty due to repeatability
Ustd – Uncertainty due to the standard mass used for calibration
Uoff – Uncertainty due to the offset error (eccentric loading) seen on the balance
Uer – Uncertainty due to drift in readings
Uhys – Uncertainty due to the hysteresis of the balance
Udrift – Uncertainty due to the drift value in between calibration
3. TYPES OF ERRORS
The laboratory should consider various uncertainties that could affect calibration and group them as per relevant category of errors.
Type A (Systematic Error)
These are errors that depend on the biases of measurement which lead to measured values being repeatedly too low or too high. These happen typically due to the repeatability of measurement. Uncertainty due to this error is Urep
Type B (Random Error)
These are errors in measurement that lead to measured values being inconsistent. The various uncertainties due to this are: Ustd, Udriff, Uoff, Uhyst, Ures, Usens.
Type A Error
Series of Measurement (10)
Average = 1+2+3+4+5+6+7+8+9+10
where 1,2,3…..10 are set observations with applied load in kg or g.
Standard (Sd ) = (A-1) 2 + (A-2)2 ….. (A-10)2
where n = no. of observations which is 10.
UA Uncertainty due = Standard Deviation
to rep (Urep) √n
Type B Error
Ustd = Uncertainty of master weight %
Udriff is the change in the weight after re-calibration.
Udriff = Actual drift
Ures = Actual resolution of machine √2
Uoff = Actual Offset error x 2
Offset Error = Max Reading Offset – Min Reading Offset
(2/3) factor is considered so that the weights are kept at a distance of (2/3) from center.
Uhys = Actual Hysteresis Error
|Observation||Weight On Pan||Reading|
Hysteresis Error = W1 – W1 + Z/2
Uerr = Error from the master
Uncertainty due to Air Buoyancy U buoy
The air buoyancy is assumed to be equal to 1ppm of the nominal value as per UKAS guideline M3003.
U buoy = 10,000,000 x nominal weight value
Total Type B Uncertainty
UB = Ustd2+ Udriff2+ Ures2+ Uoff2+ Uhys2 + Ubuov2
UC = UA2 + UB2
Expanded Uncertainty (Uexp)
Uexp = K x UC
where coverage factor k = 2 for 95% confidence level