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SOP for Operation and Calibration of Mettler Toledo Analytical Balance


Standard operating procedure to calibrate the Mettler Toledo analytical balance using standard weights.

1.0  PURPOSE

       To lay down a procedure for calibration & operation of analytical balance.

2.0  SCOPE

       This procedure is applicable to Mettler analytical balances of Quality Control (QC) installed in Microbiology Laboratory.

3.0  RESPONSIBILITY

       Microbiologist

4.0  ACCOUNTABILITY

       Head of Department

5.0  PROCEDURE

5.1  Installation

5.1.1  Place weighing pan on balance, the conical peg centers the pan in the opening in the base of the weighing chamber also place the draft cover into position.
5.1.2  The two leveling screws are adjusted so that the bubble is in the middle of the circle.
5.1.3  Connect the AC adapter to the specified power supply and plug the AC adapter’s other terminal into the socket in back of the balance.

5.2  Operation procedure

5.2.1  To switch the balance on briefly touch the “On/Off” key after the main power supply of balance has been switched on.
5.2.2  Check the level of the weighing balance; center the position of the air bubble.
5.2.3  To switch the balance off briefly touch the “On/Off” key and press it until off appears on the display.

5.3  Tarring

5.3.1  Open the sliding glass door by hand.
5.3.2  Place a container or a butter paper on the weighing pan.
5.3.3  Press the →O/T← key, the display changes to zero. Close the glass door and then opens again when tarring is complete.

5.4  Weighing In

5.4.1  Place the sample to be weighed up to the desired target weight (to read the weight accurately, the door must be closed)
5.4.2  Stability detector symbol O goes out when stability is achieved, The result is then stable and the same is printed.

5.5  Care and Maintenance

5.5.1  To switch the balance off briefly touch the “On/Off” key and press it until off appears on the display.
5.5.2  Use a moistened soft cloth or tissue paper to clean the weighing pan and the draft cover.
5.5.3  To remove residues from the weighing chamber use the small artist’s brush.
Note: Do not blow air through the chamber

5.6  Precaution

5.6.1  Protect from vibrations, thermal fluctuations, sunlight, moisture & air currents.

5.7  Auto Calibration

5.7.1  Switch ‘ON’ the power, then press gently the ‘ON/OFF’ switch on the touch screen to the switch on the balance.
5.7.2  Switch ‘ON’ the printer from the switch on the rear side of it.
5.7.3  Press and hold the CAL/MENU switch for few seconds. The balance would do internal calibration automatically, give out the print.
5.7.4  Message of CAL DONE would be displayed on the screen. The balance would come in the weighing mode.

5.8  Daily Calibration of balance

5.8.1  Calibrate the balance daily with the standard weights of 1g, 10g, 100g, and 200g individually.
5.8.2  When the balance stabilizes take the print out. Unload the weight and repeat the procedure for all the weights.
5.8.3  Record the observations along with the print out in the respective daily balance calibration data sheet.

5.9  Monthly Calibration of balance

5.9.1  Perform the electronic or self-calibration and check weight calibration as elaborated under “daily calibration check”.
5.9.2  In addition to the daily calibration check and perform the following calibration check

5.10  Calibration for uncertainty Measurement

5.10.1  Ensure that the standard weights used are within the validity period of its calibration.
5.10.2  The weights selected for uncertainty measurement are 1g and 50g are 200 g.
5.10.3  Weigh 1g of standard weight, for 10 times and take the print out of all the weighing.
5.10.4  Calculate mean and standard deviation (S.D.) for the weights taken above.
5.10.5  Calculate the uncertainty for all the selected weights by using following formula :
5.10.6                                                                                 3 x S.D.
                         Calculation for uncertainty = ---------------------------------------------------
                                                                             Standard weight (g) (as per certificate)
5.10.7  Follow the same procedure for other two selected weights.
5.10.8  Acceptance criteria :  Not more than 0.001 %.
5.10.9  Record the observations along with the printout in the respective monthly calibration data sheet.

5.11  External calibration

5.11.1  Ensure that the standard weights used are within the validity period of its calibration.
5.11.2  The weights selected for external calibration are 200 g, 100 g, 50 g, 10 g, 5 g, 2 g and 1g.
5.11.3  Load the weighing pan with 200g weight and let the balance stabilize. Take the printout.
5.11.4  Unload the weight and repeat the procedure for the weights mentioned in point No. 5.11.2
5.11.5  Record the observations along with the printout in the respective monthly calibration data sheet. 
5.12  Tolerance limit : 0.1 % of the mass of the calibrated standard weight.
Acceptance criteria : The balance should display the weight within the tolerance limits specified for each of the standard  weight range. In the case of non conformance, clean the standard weight with lint free cloth and reweigh, if the problem persists then call the service engineer for rectifying the problem. Do not use the balance, until the problem is rectified.  Recalibrate the balance after maintenance before use.

6.0 Abbreviations

SOP - Standard Operating Procedure
QC - Quality control
g - Gram
mg - Miligram
SD - Standard deviation
% - Percentage



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