VERIFYING CALIBRATION OF WEIGHING DEVICES USED FOR LABORATORY TESTING

Transcription

1 Test Procedure for VERIFYING CALIBRATION OF WEIGHING DEVICES USED FOR Texas Department of Transportation TxDOT Designation: Tex-901-K Effective Date: August SCOPE 1.1 All weighing devices shall meet the requirements of the test procedure using it. Some general-purpose scales and balances may not have sufficient accuracy for many tests. This will be especially true when large quantities of material are to be weighed to high degrees of accuracy. The test procedure may specify requirements for the weighing device, such as the capacity, accuracy, sensitivity, and readability. 1.2 Many laboratory balances have more than one range. Each range will have a different accuracy, readability, and sensitivity. The user must understand the test requirements and the capabilities of each weighing device. 1.3 The values given in parentheses (if provided) are not standard and may not be exact mathematical conversions. Use each system of units separately. Combining values from the two systems may result in nonconformance with the standard. 2. REFERENCE DOCUMENTS 2.1 AASHTO M ASTM E CLASSES OF WEIGHING DEVICES 3.1 Weighing devices are divided into two groups based on accuracy, readability, and sensitivity. Analytical balances are further divided into two classes (See Table 1) and general-purpose balances into five classes based on accuracy (See Table 2). 3.2 Analytical Balances: Analytical balances should conform to the accuracy, sensitivity, and readability requirements of a class specified in Table Place a label on each balance stating the class of the balance in a location easily seen by the user. CONSTRUCTION DIVISION 1 5 EFFECTIVE DATE: AUGUST 1999

2 3.2.3 Table 1 lists weight classes used for calibration, either as external or internal balance weights mechanically selected by the operator Verify counter weights in accordance with Tex-900-K. 3.3 General-Purpose Balances and Scales: General-purpose balances and scales should conform to the accuracy, sensitivity, and readability requirements of Table Place a label on each balance stating the class of the balance in a location easily seen by the user Select general-purpose weighing devices according to the sample weight. Do not use the weighing device for loads less than ten percent of the capacity of the range in use. Select a weighing device of a lesser capacity for small loads. 4. VERIFYING ACCURACY 4.1 Verify accuracy at a minimum of three points for each range or beam-arm of the weighing device, with a minimum of five points for the device. Checking more points may be necessary to determine if the instrument is accurate over its full range. This is especially true of two-pan weighing devices used with a variety of counter weights and with multiple beam-arm mechanical devices. 4.2 Apply verification test loads in order of increasing value. 4.3 For weighing devices with multiple ranges or beam-arms, verify each range and beamarm, at a minimum, with test loads of approximately 10, 50, and 100% of the range or capacity. The test load may be values presented by a minimum number of weights, not the exact value calculated from the capacity. EXAMPLE: For a range of 600 g capacity, the approximate 10 and 50% loads could be 50 and 300 g. Use a single 50 g weight and two or three weights to make the 300 g load. 4.4 For single range weighing devices, the minimum number of verification points should be at loads of approximately 10, 20, 50, 75, and 100% of the device capacity. The actual test load should be values presented by a minimum number of weights, not the exact value calculated from the capacity. EXAMPLE: For a scale of 4600 g capacity, the approximate 10% and 50% loads would NOT be 460 g and 2300 g. Use a single 500 g weight and several weights to make a 2500 g load. Verification at more than these five points is desirable. 4.5 Verify beam-arm hanger weights individually. Use a beam-arm slider to determine the hanger weight's deviation. CONSTRUCTION DIVISION 2 5 EFFECTIVE DATE: AUGUST 1999

3 4.6 Procedure: Clean the platform or pan, beams, display, etc. as needed. On laboratory balances, be sure that no debris or portion of a plastic cover restricts pan movement Level the weighing device If the weighing device has a platform or beam lock, use the lock each time placing a weight on the pan. When using an analytical balance with a lock, lock the balance before applying or removing any weight With the lock off, zero the load indicator or balance the beam at no load Make a test weighing with a load of from 50 to 100% of the capacity. If the indicated load is out of tolerance, record the values in an "as found" column. Adjust the device to meet the required accuracy When the test load is within specifications, verify the device on all ranges. Record the values in the "as found" column if the device was not adjusted and in the "adjusted" column if adjustments were made Report the weighing device as failing to meet the accuracy specifications if any range cannot meet the accuracy requirements for the class best describing that range Report the weighing device as failing to meet the specifications if it fails to operate properly (according to the manufacturer's instruction manual) on all ranges. 5. VERIFYING ECCENTRIC LOAD ACCURACY 5.1 Verify the eccentric or off-center load accuracy at a load of approximately 50% of the capacity of the weighing device. 5.2 Check additional load points if the device is suspected of being inaccurate over some portion of its load range. 5.3 Use the least number of weights practical for this test load. EXAMPLE: For a Torsion two-pan balance of 4500 g capacity, use a 2 kg (4.4 lb.) weight or stack two 1 kg (2.2 lb.) weights. 5.4 Compare the indicated value of the test load at the center of the loading pan or platform with the indicated value with the loads placed one-half of the distance from the center of the pan to the edge. Perform this test at four off-center points: one toward the operator one away from the operator one to the left, and one to the right. CONSTRUCTION DIVISION 3 5 EFFECTIVE DATE: AUGUST 1999

4 5.5 Procedure: Level the weighing device If the weighing device used has a platform or beam lock, lock the device before applying any weights. When using an analytical balance with a lock, lock the balance before applying or removing any weights With the lock off, zero the load indicator or balance the beam at no load Place the test load on the center of the pan or platform of the device and note the indicated weight Remove the load, zero the instrument (reset the balance), and then apply the load at one of the off-center loading points. Note the indicated weight. Repeat for the remaining three off-center loading points Report the weighing device as out of specification if the difference between the indicated weight for the test load centered and that for any off-center load position exceeds the tolerance for its class. 6. VERIFYING SENSITIVITY 6.1 Sensitivity tests determine the minimum weight change that the weighing device can detect. 6.2 Procedure: Level the weighing device With the lock off, zero the load indicator or balance the beam at no load Place a load of about 3/4 of the device capacity on the pan or platform and add small weights to the load until the weighing device detects a change in the reading. Do not lock the beam or platform while performing this part of the test. Note the minimum value of weights to affect a change in reading If the minimum change in the load to effect a change in the indicated load is greater than the required accuracy, report the device as failing the sensitivity test. 7. CERTIFICATION 7.1 Place a certification label on those balances or scales that meet the class requirements in this procedure Place the label so that the user can see it easily The serial number of the balance or scale should be on the label. CONSTRUCTION DIVISION 4 5 EFFECTIVE DATE: AUGUST 1999

5 7.1.3 The technician performing the verification should sign and date the label The technician should verify that a label states the correct class of the weighing device. 8. CALIBRATION REPORT 8.1 Issue a calibration report containing the following: Weighing device, brand, serial number, capacity, and class Whether the weighing device passes or fails the requirements for the stated class the cause of failure, if the weighing device fails. 9. TABLE REFERENCES Table 1 Classes of Analytical Balances Class Accuracy in grams Sensitivity and Readability in grams Class of Weight for Calibration Max. Load in grams (a) (b) (c) A ,2,3 200 B (a) Readability of digital readout or graduations on mechanical balances. (b) Suggested class of calibration weight (Tex-900-K, Table 1) (c) Approximate maximum capacity generally available with balances of this class. Table 2 Classes of General Purpose Balances and Scales Balance Class Readability and Sensitivity in grams Accuracy in grams (a) G ± 0.1 % G ± 0.1 % G ± 0.1 % G ± 0.1 % G ± 0.1 % (a) Accuracy throughout the range of use shall be equal to or less than the mass stated or 0.1 % of the test load, whichever is greater. CONSTRUCTION DIVISION 5 5 EFFECTIVE DATE: AUGUST 1999