mass measurement by mr.vikram joshi [email protected]
TRANSCRIPT
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
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
MASS MEASUREMENT
Other sub-division and multiples of the gram are
Unit Symbol Relationship to the gram
Fematogram fg 1fg = 10-15 g
Picogram pg 1pg = 10-12 g
Nanogram ng 1ng = 10-9 g
Microgram Mg 1Mg = 10-6 g
Milligram mg 1mg = 10-3 g
Gram g 1g = 1 g
Kilogram kg 1Kg = 103 g
(metric) tonne t 1t = 106 g
MASS MEASUREMENT
TYPES OF BALANCE USED IN ANALYTICAL LABORATORIES
Balances are distinguished by specific features
Typical minimum
capacity Typical
readability Type of balance
10 g 0.001 mg micro-balance 6-figure balance
150g 0.01 mg or 0.1 mg
Analytical balance 3,4 or 5-figure balance
1000g 0.01g general purpose or “top-
pass” balance 5000g 0.1g
3000g 0.1g or 1g
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
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.
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
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
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
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
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- 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
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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
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
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
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.
Proceduresa. Select a test weight close to the weighing capacity of the
instrument b. Place the weight in the center of the pan and re-zero the
displayc. Move the weight on half way from the center to the front
edge of the pand. Repeat step (c) at the half way locations for right, rear and
left edges, recording the reading as appropriatee. Compare there readings with the corresponding corner load
tolerances
MASS MEASUREMENT
Corner load resolution
Tolerance (digits) for
Lab environments
Capacity 1mg 0.1mg 0.01 mg
30 g - 2 5
100g 2 4 10
300g 4 10 -
1000g 10 - -
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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
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- 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.
Line Zero Full scale
Standard deviation (SD)
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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
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
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- 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.
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
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
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)
MASS MEASUREMENT
CLASSIFICATION OF WEIGHTS ACCORDING TO OIML
OIML classification
Material of construction Nominal mass value
Permitted tolerance
E1 Integral stainless steel without marking or adjustment chamber
50g ± 0.00003 g
E2 Integral stainless steel without marking or adjustment chamber
50g ± 0.0001 g
F1 Stainless steel with a screw know
50g ± 0.00003 g
F2 Plated brass 50g ± 0.001 g
M1 Brass or cast iron with painted finish
50g ± 0.003 g
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
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
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
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 largerSRP -> Standard deviation for n replicate weighing d-> Readability of the balance
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
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 operationNominal Quantity to be weighed
Recommended Balance
Nominal Accuracy obtained
0.01 g Micro (6-figure) 0.000001 g (0.01 %)
0.1 g Analytical (5-figure) 0.00001 g (0.01 %)
1 g Analytical (4-figure) 0.0001 g (0.01 %)
100 g Top-pan (2-figure) 0.01 g (0.01 %)
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
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
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
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
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
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
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)
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)
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
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
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
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
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
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
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?
MASS MEASUREMENT
Thank you