mass measurement n.
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  1. MASS MEASUREMENT by Mr.Vikram Joshi

  2. MASS MEASUREMENT INTRODUCTION: - • Measurement of mass one of the mist frequently carried out operation • Accurate mass measurement required for such purpose as • Obtain a known quantity of Sample for analysis • Preparation of analytical reagents • Preparation of calibration standards

  3. MASS MEASUREMENT • Units of mass measurement • in the international system (SI) of measurement unit base unit of mass is the kilogram (Kg) • Kilogram is one of the seven base measurement units • In routine analytical work, it is more convenient to work in terms of grams (g) where 1000 g = 1 Kg or 1 g = 10-3 Kg

  4. MASS MEASUREMENT • Other sub-division and multiples of the gram are

  5. MASS MEASUREMENT TYPES OF BALANCE USED IN ANALYTICAL LABORATORIES • Balances are distinguished by specific features

  6. MASS MEASUREMENT TYPES OF BALANCE USED IN ANALYTICAL LABORATORIES • Balance capable of weighing to 0.001 mg are commonly referred to as micro balances • Normally used for weighing quantities of < 0.1 g. • Balance weighing to 0.01 mg, 0.1 mg or 1 mg are commonly referred to as analytical balance • They are normally used weighing quantities of about 0.1 to 100 g

  7. MASS MEASUREMENT MASS: - Mass is the amount of material in an object and does not change with environment in which the object is located WEIGHT : - Weight is a force arising from the interaction of mass with earth’s gravitational field which varies with the location. • Sophisticated balance provide “error-free” measurements • All the measurements have error that trouble the accuracy and create uncertainty about the quality of the measured mass.

  8. MASS MEASUREMENT • Weighing process is distorted by several influences that introduce both systematic and random error • Bias is due to buoyancy effort or to the deviation of reading from the reference mass • Common random contribution involves readability, repeatability • The measure of the weighing process is the mass “m” weighing object which is evaluated from the reading R of a calibrated and verified electronic balance

  9. MASS MEASUREMENT LOCATION OF THE BALANCE • DRAUGHTS: - Chosen to minimize draughts from doors, windows, passer-by • VIBRATION: - Vibration had affect on balance for measurement 0.01 g or less, locate the balance on a vibration free surface. • LEVEL SURFACE: - The surface on which the balance is mounted should be level and balance feet should be adjusted, using the spirit-level device to show when the balance is level

  10. MASS MEASUREMENT • TEMPERATURE: -Ideally the ambient temperature should be stable to within ± 3°C. temperature fluctuations can cause gradients in the balance mechanism • HUMIDITY: - Humidity is relatively un-important provided condensation does not occur • MAGNETIC FIELDS: - There to be avoided as they may cause permanent changes in the response of the balance • ELECTRICAL INTERFERENCE: - Subject to electrical interference and left on all the times

  11. MASS MEASUREMENT OPERATIONAL PRINCIPLES • Typical balance consists of a single pan (Usually SS) • For analytical balance is enclosed by a movable transparent shield to protect pan from draughts • EMF COMPENSATION BALANCE: - • A coil is rigidly attached to the balance • Pan assemble is placed in the field of magnet • Object placed on pan lowers it, causing and increase in the current in the coil. • Magnetic counter-force generated which returns the pan to its original position

  12. MASS MEASUREMENT • The increase in current in the coil is measured as a voltage on a digital voltmeter • Mass of the object is directly proportional to the measured voltage. • A digital read-out of the mass of the object readily obtained • The balances are called electronic balance

  13. MASS MEASUREMENT PERFORMANCE VERIFICATION: - • The need for regular and appropriate assessment of balances vital both in providing traceable and accurate results. • Achieved by means of calibration and verification • CALIBRATION: - set of operation that establish under specified conditions, the relationship between values of quantities indicated by a measuring instrument or measuring system or values represented by a material measure or a reference material and the corresponding values realized by the standards

  14. MASS MEASUREMENT • Calibration shows how the nominal values of a material or the indication of an instrument relates to the convention values of the measured. • The conventional value is realized by a traceable reference standard • Calibration not sufficient to ensure the suitable performance of the balance and comparability of its measurements • Number of specifications to accomplish • Examination of conformity of results with the specifications (manufacturers) generally expressed as tolerances

  15. MASS MEASUREMENT • Daily or before-use checks should be made on balances results recorded. • If the balance has the adjustment facility that allows the output to be adjusted between zero and an internally or externally applied weight, this facility to be operated regularly before the balance is used. • The checks to include adjusting the zero of the balance (performed with adjusting facility) followed by the placement of a single weight (Calibrated or control weight) on the pan. • This procedure for daily check should define on action limit or maximum permissible error, and if it is exceeded, a full calibration should be carried out. • Other regular checks called intermediate checks required between full calibrations

  16. MASS MEASUREMENT E CENTERING LOADING: - • This testing verifies the instrument delivers the same weight reading regardless of where on the weighing pan the object is placed. • Procedures • Select a test weight close to the weighing capacity of the instrument • Place the weight in the center of the pan and re-zero the display • Move the weight on half way from the center to the front edge of the pan • Repeat step (c) at the half way locations for right, rear and left edges, recording the reading as appropriate • Compare there readings with the corresponding corner load tolerances


  18. MASS MEASUREMENT REPRODUCIBILITY TESTING • Repeatability/Reproducibility refers to instrument’s ability to repeatedly deliver the same weight reading for a given object. • Expressed as standard deviation • Select a test mass equal to, or nearly equal to the weighing capacity of the instrument • Procedure • Tare the balance to read all zero • Place the test mass on the pan. Record the reading under heading “full scale” • Remove the weight (do not re-zero) and record the reading under zero • Repeat the above two steps 10 to 11 times

  19. MASS MEASUREMENT • Calculated the standard deviation for the measurements set of “Zero” and “full scale” • Calculated standard deviation larger than allowed in the instrument specification indicated instrument is either operating in an unstable environment (air draft, static, warm-up, vibration, etc.) or the instrument is need of repair.

  20. MASS MEASUREMENT LINEARITY TESTING • Linearity: - A balance to exhibit perfect linearity if two masses that differed by a particular factor give read-out that differed by the same factor • Perfect linearity is not capable of being achieved in practice or of being demonstrated experimentally • Linearity checks verifies the accuracy of the instrument at intermediate values of weight • Specified by the manufacturer as a tolerance which represents the maximum deviation of the balance indication from the value that would be obtained by linear

  21. MASS MEASUREMENT • Interpolation between the adjusting points (zero and internal/ external weight loading) • The linearity error typically takes the form of an S-shaped curve • Routine QC ensures that linearity remains within acceptable limits • A convential procedure • Use two weights, each of approximately one-half the weighing capacity of the instrument • Imperative that these two weights not be interchanged within this procedure

  22. MASS MEASUREMENT • Refer to the individual weights as “A” and “B” • Re-zero the display place “A” on the pan (at the center) record the reading (x) • Remove “A” and place “B”. Re-zero the display • Again place “A” on the pan. Record the reading (y) • Calculate the difference between the two readings. • The difference should be less than the advertised tolerance and for linearity and accuracy.

  23. MASS MEASUREMENT SPECIAL NOTE: - • A common error in linearity (accuracy) testing is simply place test weights on the pan and observes the difference between the indicated weight and nominal value of the test weight. • This process fails to account for the fact that test weight are imperfect and that difference between nominal value and weight might be significant • True with analytical balances, where the balance may be more accurate then any standard test weight. • Nullifies the problem by comparing the weight readings of the same object, both with and without a preload. • The accuracy of the test weight is thus immaterial

  24. MASS MEASUREMENT THE CALIBRATION PROCESS • It refers to the difference between the weight reading of a given mass standard and the actual value of that standard • Calibrated weight: - A weight with a mass value and an associated uncertainty that has been formally established by a calibration laboratory that ensures that traceability of the mass value the national standard of mass. • Calibrated weights have and uncertainty that is 1/3 to 1/5 of the OMIL tolerances

  25. MASS MEASUREMENT • TRACEABILITY: - A measurement is said to exhibit traceability if it can be related to a recognized reference of appropriate quality through an unbroken chain of comparisons, each comparison having a known uncertainty. Ideal, all measurements should have such traceability. • CLASS OF WEIGHT: - Classified by International Organization of Legal Metrology (OIML) according to their material of construction and maximum permissible error (tolerance)


  27. MASS MEASUREMENT • To perform calibration suitably, the balance should be clean and located in a position free from vibration and thermal sources. • Standard weights used for calibration should be cleaned • Calibration to cover at least ten points evenly spread over the loading range. • A series of repeated measurements are carried out (six to ten) for each standard weight • From there data, at each calibration point, the average of readings of standard weight is evaluated. • All measurements are subject to unavoidable measurement uncertainty

  28. MASS MEASUREMENT • Balances are highly engineered, sophisticated and reliable items of equipment • Source of uncertainty • The reproducibility of balance readings • The linearity of balance readings • The uncertainty of calibrated weights used to calibrate the balance response. • The uncertainty of calibration procedure • The repeatability provides decisive contribution to the uncertainty of measurement

  29. MASS MEASUREMENT DETERMINATION OF THE MINIMUM SAMPLE WEIGHT: - • On site determination the minimum sample weight • To ensure that influence of the person performing the weighing and environments are included. • The lowest permissible sample weight is defined by • Repeatability of a balance at its location • Readability • Require relative tolerance limit of the maximum error of measurement • A probability of occurrence

  30. MASS MEASUREMENT • USP-30 section 41, weighing is to be performed with a weighing device whose measurement uncertainty (random plus systematic error) does not exceed 0.1 % of the reading • The minimum weight for a balance is that which will have loss than 0.1 % of that weight • Associated with expanded uncertainty with a coverage factor of 3 which takes into account the defined probability of occurrence of 99.73 % • The minimum weight that can be weighed on an analytical balance is 3000 SRP or 1200d which is ever is larger SRP -> Standard deviation for n replicate weighing d-> Readability of the balance

  31. MASS MEASUREMENT “Reference standards are weighed on and analytical balance and weights are recorded to 4 decimal places (e.g. 0.0100). Typically 10 mg quantities are weighed out, however in some instances smaller quantities are weighed.” A FDA 483 OBSERVATION • Ensure to use and appropriate analytical balance and sufficient material is weighed to reduce the error • The greater the number of decimal point could reduce the amount we weigh, whether it is fit for its purpose? • Large number of analytical results that are out of its expected range, one of the factor may be weighing

  32. MASS MEASUREMENT CHOICE OF BALANCE • The choice of balance depends on the quantity to be weighed and nominal accuracy (number of decimals) required in the weighing • Balance selection for a particular weighing operation

  33. MASS MEASUREMENT CHOICE OF BALANCE • Mass values with a nominal relative accuracy of 0.01 % or better which is more than accurate for all routine analytical work CONTAINERS AND TYPE OF SUBSTANCE TO BE WEIGHED • Substances are nearly always weighed in containers of same sort • Empty container should be clean, dry, free from dust and be of a design that facilitates easy transfer of the weighed substance in to the vessel or apparatus subsequently required for analysis • Mass of the empty container should not be larger than necessary compared to the mass of the substance being weighed

  34. MASS MEASUREMENT CONTAINERS AND TYPE OF SUBSTANCE TO BE WEIGHED • For volatile liquids being weighed, a container with a well-fitting stopper to minimize losses through evaporation • For hygroscopic substances, a stopper container helps to minimize pick-up of the moisture from the ambient air. SEQUENCE OF OPERATION INVOLVED IN WEIGHING A SUBSTANCE • Setting up the balance • Switch on the power supply and at least 20 minutes allowed for balance to “warm up” • Recommended that balance are left permanently switched on and left in the stand-by mode

  35. MASS MEASUREMENT SEQUENCE OF OPERATION INVOLVED IN WEIGHING A SUBSTANCE • Check that balance is level and adjust if necessary, using the leveling feet to centre the bubble in the spirit level device • Gently clean the balance pan with a brush to remove any dust or loose particulate matter

  36. MASS MEASUREMENT CHECKING THE ACCURACY OF THE BALANCE • Set the balance to read zero and check that zero is displayed Note: - Depending on the balance, its location and the substance being weighed, it may be un-realistic to expect the stability in the last decimal place of the balance reading • Some judgment will have to be exercised as to what digit is taken for the final decimal place of the reading • An additional uncertainty in measured mass value is introduced by such instability • Analyst should satisfy them selves that uncertainty is acceptable in terms of fitness-for-purpose • The calibrated weights should be handled with forceps and not with hands

  37. MASS MEASUREMENT CHECKING THE ACCURACY OF THE BALANCE • The forceps should be made of plastic or non-metallic material • If made of metal they should be tipped with material such chamois lather • Place the calibrated weight on the center of the pan close the balance door and weight for mass reading to stabilize, record the reading • Record the result of the accuracy check • The difference between the measured mass and the expected value should be within the stated limits • This check only needs to be carried out at the start if series of weighing, not before every individual weighing

  38. MASS MEASUREMENT CHECKING THE ACCURACY OF THE BALANCE • Remove the calibrated weight and check that zero is displayed on the balance • If zero is not displayed, check the balance pan is clean, the doors have been closed • For small departure from zero (i.e., in the final decimal place of the reading, the balance may be zeroed • Larger departure from zero should be investigated and if necessary the balance should be serviced

  39. MASS MEASUREMENT WEIGHING BY DIFFERENCE • Place the empty container on the centre of the pan, close the balance doors, wait until the reading is stable (W0 grams) • Remove the empty container from the pan, transfer the substance to be weighed to the container and place the container on the center of the pan. Close the balance doors, wait for the reading to stabilize and record the reading (W1 grams) • Remove the container from the balance pan • If the substance is transferred quantitatively (by washing with water or solvent) to the required vessel, the mass of the substance is obtained by subtraction of the mass of the empty container from the mass of container plus substance (W1-W0)

  40. MASS MEASUREMENT WEIGHING BY DIFFERENCE • If the substance is transferred ‘dry’ to another vessel, once the transfer has been carried out replace the container on the center of the balance pan, close the doors, wait for the reading to stabilize and record the reading (W2 grams). The mass of the substance transferred is then obtained by subtraction (W1-W2)

  41. MASS MEASUREMENT USE OF TARE FACILITY • If a substance is to be weighed directly on to the vessel required in the subsequent analysis, the tare facility is used • Place the empty container on the center of the balance pan, close the doors and press the tare button. Wait for reading to stabilize at zero • Remove the container from the balance, add the substance to the container, replace the container on the balance close the doors, wait for the reading to stabilize and record the reading. The reading gives the mass of the substance in the container

  42. MASS MEASUREMENT COMPLETION AND TIDY UP • On completion of the weighing activities, gently clean the balance pan with a brush, collect any debris and discard it appropriately • Close the balance door and leave the balance in the stand-by mode • Ensure any calibrated weights used are returned to their storage box • Ensure that an appropriate and permanent record of the weighing is made

  43. MASS MEASUREMENT EFFECT OF BUOYANCY ON WEIGHING • Buoyancy is uplifting force on a weighed object due to the fluid in which it immersed. • Magnitude of the uplifting force depends upon the density of the air • True mass = Ma {1+ PA (1/Ps – 1/Pcw)} Ma = Mass value obtained by weighing in air PA = density of air (kg per meter cube) Ps = density of object weighed) Pcw = density of the calibrated weight used to calibrated the balance

  44. MASS MEASUREMENT OBSERVATION SIGN THAT INDICATE ALL IS NOT WELL • Messy area where the analyst has been working • Spilt solid on or under the balance (and the pan) • Chemical left out on the open bench rather than being returned to the place where it is stored • The door(s) on the balance being left open when weighing • Moving a balance but not checking to see if it is level, before use • Weighing results not recorded, written down on a scrap of paper, or inappropriately recorded, e.g., rounding a weighing such as 20.3974 g and recording as 20.4000 g

  45. MASS MEASUREMENT • Incomplete recording of information, e.g., not including what has been weighed, when and by whom • Re-calibrating the balance for no apparent reason • Modern balances are reliable instruments and some automatically re-calibrate whilst others rarely need to be re-calibrated • If there is a problem with a balance then the problem should be identified and rectified before re-calibration of the balance • Using the tare facility in a haphazard way - this is particularly important to get right in a busy laboratory where several analysts may be using the same balance • Balance not recording zero when there is nothing on the balance pan

  46. MASS MEASUREMENT • Quality control checks not performed at the correct time to determine that the balance is in calibration • Quality control check performed but the analyst does not know the defined limits for acceptable performance, e.g., analyst records the measurement of a check weight but does not check to see if it passes or fails • The manufacturers' calibration certificate • When to calibrate a balance and the appropriate calibration interval(s) • As part of a routine calibration procedure • After a balance has been moved • The calibration expiry date which should be clearly visible to analysts, e.g., displayed on the balance

  47. MASS MEASUREMENT • The use of standard certified weights for balance calibrations • The use of check weights as a quick way to determine whether or the not there is a problem • The need to monitor run to run variations by calculating the standard deviation of successive weighing • How to monitor daily drift by, e.g., constructing and using a control chart? • How to determine linearity using standard certified weights?

  48. MASS MEASUREMENT Thank you