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Calibration of Balance

Bangladesh BEST Programme. Calibration of Balance. Sri Lanka. Nihal Gunasekara. Types of Balances. Bangladesh BEST Programme. Electromagnetic force balance (Top loading) Single-pan, two-knife-edge balance Two-pan, three-knife-edge balance

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Calibration of Balance

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  1. Bangladesh BEST Programme Calibration of Balance Sri Lanka NihalGunasekara

  2. Types of Balances Bangladesh BEST Programme • Electromagnetic force balance (Top loading) • Single-pan, two-knife-edge balance • Two-pan, three-knife-edge balance • Triple beam balance • Weighing scale

  3. Analytical Balance Bangladesh BEST Programme Balance with an enclosed weighing chamber and a resolution 0.01 or 0.1 mg Micro Balance • Balance with a resolution of 1 microgram

  4. Micro Balance Bangladesh BEST Programme

  5. Bangladesh BEST Programme Semi Micro Balance Balance with a resolution of 10 microgram Ultra (Super) Micro Balance Balance with a resolution of 0.1 microgram

  6. Bangladesh BEST Programme Resolution or Discrimination The smallest change in mass which can be detect by the balance Sensitivity This is a measure of the ability of the balance to detect changes in the load applied to it

  7. Response time Bangladesh BEST Programme Time taken for the weighing instrument to indicate the value of a change in load Stability A measure of the time for which the reading on the balance remains unchanged

  8. Correction Bangladesh BEST Programme Correction = True Value - Reading Error Error = Reading - True Value

  9. Departure From Nominal Value Bangladesh BEST Programme The amount by which the reading on an instrument depart from its nominal value Hysteresis The indications of a measuring instrument when the same value of the quantity measured is reached by increasing or decreasing that quantity

  10. Standby Bangladesh BEST Programme Operating state of an electronic instrument where one part is switched off for energy saving reasons Accuracy class. (mass) Legal metrology term. Classification of weights for calibration purposes by OIML R111-1 weights of Accuracy class E1, E2, F1, F2, M1, M2, M3 from 1 mg to 50 kg

  11. Bangladesh BEST Programme Different accuracy classes of mass

  12. Bangladesh BEST Programme Repeatability • Closeness of the agreement between the results of successive measurements of the same measurement carried out under the same conditions of measurement. • Repeatability condition include : • the same observer • the same measuring instrument, used under the same conditions • the same location • repetition over a short period of time

  13. Bangladesh BEST Programme Reproducibility • Closeness of the agreement between the results of measurements of the same measurement carried out under changed conditions of measurement. • A valid statement of reproducibility requires specification of the conditions change • The changed conditions may include • principle of measurement • method of measurement • observer • measuring instrument • reference standard • location • conditions of use • Time

  14. Influence Factors Bangladesh BEST Programme • Air movements and heat sources • Support surfaces and vibration • Electrical and electromagnetic interference • Magnetic effects • Buoyancy effects

  15. Bangladesh BEST Programme Influence factor Influence factor Parameter Source Effect Test for effect Precautions Environmental Vibration Various, Eg. Machinery, roads, railways, aircraft operator Vibration causes damages Changes forces on the weighing machine due to acceleration Primary Stability of reading under constant load Secondary Accelerometer measurements Use of a high-mass support, set on a solid surface, for the weighing machine, independent of the adjacent flooring, Eg fixed to bedrock Note : low frequency, vibration is likely to be more serious in its effects than high (acoustic) frequencies Rubber pads used to damp high frequencies may pass low frequency vibration, and may cause tilting of the weighing machine

  16. Bangladesh BEST Programme Influence factor Influence factor Parameter Source Effect Test for effect Precautions Environmental Temperature: air, weight, weighing machine Sunlight, heating systems, operator’s body, greenhouse heating in the weighing chamber Temperature changes and differentials can change the weighing machine response, cause convection effects and change buoyancy Temperature monitoring instruments Avoid direct sunlight and other connections to the external environment; avoid changes in heat and lighting input. Leave balance cases ajar when not in use. Avoid excessive handling of equipment during weighing. Allow weighted object to acclimatize

  17. Bangladesh BEST Programme Influence factor Influence factor Parameter Source Effect Test for effect Precautions Environmental Humidity Air supply ,air conditioning, changes in activity levels Buoyancy, absorbed moisture, condensation Rh monitoring instruments Avoid liquid water in the working area unless rh is too low . Air conditioning, humidity controller

  18. Bangladesh BEST Programme Influence factor Influence factor Parameter Source Effect Test for effect Precautions Environmental Airflow Air conditioning, wind, convection, operator movement Varies force on weighing machine Primary Stability of reading under constant load Secondary Enclose weighing machine and observe change of reading stability Avoid temperature gradients. draught free working space, enclose weighing machines

  19. Bangladesh BEST Programme Influence factor Influence factor Parameter Source Effect Test for effect Precautions Environmental Magnetic Constructional steelwork, ferromagnetic weight (s) electrical induction earth’s magnetic field, moving-coil meters Change of current in force compensation coil, direct attraction to/from surrounding structure Motion-dependent restoring forces Primary Use a magnet to check for induced reading changes Secondary Monitoring readings when weights are rotated and at varying heights above the weight receptor (place on/under non-magnetic mount) Avoid weights with residual magnetism (de-gauss) Test with compass or gaussmeter Limit magnetic permeability of weights weighed check for magnetic fields with gaussmeter Screen with mumetal

  20. Source of error Type of error Load cell hysteresis Repeatability Electrical Interference Repeatability Mains fluctuation Zero Incorrect horizontal support Sensitivity, Zero, Repeatability Mechanicalerrors Repeatability Bangladesh BEST Programme

  21. Bangladesh BEST Programme Nominal value Tolerance in mg Class E1 Class E2 Class F1 Class F2 Class M1 Class M2 Class M3 10 kg 5.0 50 160 500 1600 5000 16 5 kg 2.5 8.0 25 80 250 800 2500 2 kg 1.0 3.0 10 30 100 300 1000 1 kg 0.5 1.6 5.0 16 50 160 500 200 g 0.10 0.3 1. 0 3 10 30 100 1.6 100 g 0.05 0.16 0.5 5 16 50 50 g 0.030 0.10 0.3 1.0 3 10 30 10 g 0.020 0.06 0.20 0.6 2.0 6.0 20 5 g 0.016 0.05 0.16 0.5 1.6 5.0 16 2 g 0.012 0.04 0.12 0.4 1.2 4.0 12 1 g 0.010 0.03 0.10 0.3 1.0 3.0 10 100 mg 0.005 0.016 0.05 0.16 0.5 1.6 50mg 0.004 0.012 0.04 0.12 0.4 10 mg 0.003 0.008 0.025 0.08 0.25 5 mg 0.003 0.006 0.020 0.06 0.20 1 mg 0.020 0.06 0.20 0.006 ACCURACY CLASS (OIML R111-1 :2004) 0.003

  22. Bangladesh BEST Programme Lowest class of weights required for different types of weighing machine Capacity Resolution 100g 10g 1g 10mg 1mg 0.1mg 0.01mg or less 100mg Up to 200g M1 M1 F2 F1 E2 200g to 1 kg M1 M1 F2 F1/E2 E2 E2 1 kg to 30kg M2 M2 M1 F2 E2 E2 E2 30kg to 100kg M2 M1 F2 F1 E2 Above 100 kg M2 M1/F2 F1 E2

  23. Calibration of Top Loading and Analytical Balances Bangladesh BEST Programme

  24. Before use a Balance………… Bangladesh BEST Pregramme • Keep the Balance power on appropriate time to warm up. • Clean the balance as best as no contamination on the balance pan. • If it is applicable, level the balance before use. • Don’t move the balance after the calibration. • Lint or any other soft gloves should be worn by the user. • A pair of forceps or a pair tweezers with thermal insulating ends should be used to lift and place weights.

  25. Bangladesh BEST Programme Before use a Balance • Make sure with the soft placement of weights on the balance pan as level best of possible. • Readings should be taken after the balance is stabilized well. • The place should be free from vibration. • Any other relevant precaution practiced by the user.

  26. Balance Calibration Bangladesh BEST Programme The calibration of the electromagnetic force compensation balance consists of the following tests: • Test 1 - Scale Value • Test 2 - Repeatability of Reading • Test 3 - Departure from Nominal Value • Test 4 - Off centre Loading • Test 5 – Hysteresis

  27. Scale Value Bangladesh BEST Programme Internal Scale Adjustment –Internal Mass External Scale Adjustment – External Mass Procedure – Balance Operating Manual

  28. Repeatability of Reading Bangladesh BEST Programme Objective of Measurement : The repeatability test is carried out to determine the ability of the balance to give under defined conditions of use closely similar responses for repeated measurements The balance Standard Deviation is the “repeatability of measurement”

  29. Repeatability of Reading Bangladesh BEST Programme : • M 1 -Set the reading to zero without any mass on the pan • M 2 - Record the zero reading of the balance (z) • M 3 - Place the mass on the pan and record the reading (r) • M 4 - Remove the mass from the pan • M 5 – Repeat M2 to M4 ten times • during the process the balance should not be zeroed

  30. Bangladesh BEST Programme Repeatability of Reading: • Performed at three points : • Near zero • Near half load • Near full load

  31. Bangladesh BEST Programme DATA RECORDING Mass 1: Mass 2: Mass 3:

  32. Repeatability of Reading Bangladesh BEST Programme Mass 1: Mass 2: Mass 3 : Mean value : Std. Dev. ( σ ) : Max. Diff. : Uncer. Type A :

  33. Bangladesh BEST Programme Repeatability of Reading Repeat three different values of M ; near zero, half maximum and maximum load. Load on Pan (g) Standard Deviation of Reading (mg) Maximum Difference Between Successive Readings (mg) 0.05 100 200 0.067 0.052 0.052 0.2 0.1 0.1

  34. Bangladesh BEST Programme Departure from Nominal Value: Objective of Measurement : “The departure from nominal value measurements are carried out to determine the deviation of the balance reading from the expected value” This is the Correction to Balance Reading

  35. Bangladesh BEST Programme Departure from Nominal Value: 1 Divide the range into at least 10 equally spaced steps 2 The masses corresponding to M, 2M, 3M, 4M, 5M, 6M etc. until the capacity of balance is reached 3 In addition choose four more test points corresponding to 20 %, 40 %,60%and 80 % of the first step for single range.

  36. Departure from Nominal Value: Bangladesh BEST Programme 1 - Read the zero z1 2 - Place the weight on the pan &record reading r1 3 – Remove the weight momentarily and place it back on the pan and note the new reading r2 4 - Remove the weight and read the zero z2 5 – Repeat 1 to 4 for weights of masses 2M,3M,4M,5M etc until the capacity of balance is reached

  37. Bangladesh BEST Programme Departure from Nominal Value: • Average zero reading zi = (z1 + z2 )/ 2 • Average mass reading ri = (r1 + r2 )/ 2 • Difference di = ri – zi • Correction ci = Mi – di • Mi is the conventional value of the mass

  38. Bangladesh BEST Programme Correction to Balance Reading

  39. Bangladesh BEST Programme Departure from Nominal Value: Reading (g) Correction(mg) Expanded Uncertainty k=2 +(mg) Before the Calibration After the Calibration 4 8 12 16 20 40 60 80 100 120 140 160 180 200 220 -- -- -- -- 0.2 -- -- 0.4 -- 0.6 -- 0.4 -- 0.5 -- 0.0 0.0 -0.1 0.1 -0.0 0.0 0.0 -0.1 -0.1 -0.1 -0.1 -0.2 -0.3 -0.3 -0.3 0.096 0.097 0.097 0.098 0.099 0.098 0.098 0.099 0.099 0.099 0.10 0.10 0.10 0.11 0.11

  40. Bangladesh Best Programme Off Centre Loading: Objective of Measurement : “The off center loading is carried out to find the variation in the displayed weighted as the object is moved to various positions on the weighing pan”

  41. 3 4 1 5 2 3 4 1 5 2 Bangladesh BEST Programme Off Centre Loading This test to find the variation of the reading for several place of the balance pan. Test should be done for rectangular type balance pans and as well as for circular type balance pans. A single weight should be used

  42. Bangladesh BEST Programme Off Centre Loading: Place a mass on the centre of the pan and move it successively to the front rear, left and right positions ( position being 2/3 of the distance between the centre and the lip of the pan) on the pan recording the reading each time Back (g) 100.0000-100.0000=0.0000 Center (g) 100.0000-100.0000=0.0000 Right (g) 99.9999-100.0000= -0.0001 Left (g) 100.0001-100.0000=0.0001 Max Difference =0.0002 g Front (g) 100.0001-100.0000=0.0001

  43. Centre(g) Front(g) Back(g) Left(g) Right(g) Maximum Difference (g) 0.0000 0.0001 0.0000 0.0001 - 0.0001 0.0002 Bangladesh BEST Programme Off Centre Loading: Mass of 100g was moved to various positions on the pan. The balance readings obtained relative to the centre reading Place a mass on the centre of the pan and move it successively to the front rear, left and right positions ( position being 2/3 of the distance between the centre and the lip of the pan) on the pan recording the reading each time

  44. Bangladesh BEST Programme Hysteresis: Objective of Measurement : “The hysteresis measurements are performed to determine the difference in the indication at the same load , depending upon the direction of approach to that load ie increasing or decreasing the load”

  45. Hysteresis Bangladesh BEST Programme Perform the test at one point about mid range • Zero the balance and record the reading z1 • Place a mass M1 on the plan and record the reading r1 • Add extra mass M2 to bring the balance close to full range • Remove the extra mass M2 and record the reading with the mass M1 still on the pan • Remove the mass M1 and record the reading r2 • Repeat above steps three times • Take the average of three readings

  46. Bangladesh BEST Programme HysteresisMass M = g

  47. Pan Load (g) Hysteresis Error(mg) 0 0.0 100 0.1 Bangladesh BEST Programme Hysteresis:

  48. Bangladesh BEST Programme Hysteresis: Hysteresis error

  49. Calibration of Single Pan two Knife Edge Balances Bangladesh BEST Programme

  50. Balance Calibration Bangladesh BEST Programme The calibration of the single pan two knife edge balance consists of the following tests: • Test 1 - Repeatability of Reading • Test 2 - Departure from Nominal Value • Test 3 – Built-in-Mass • Test 4 - Off centre Loading • Test 5 – Hysterisis

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