Commonwealth of Pennsylvania PA Test Method No. 410 Department of Transportation October 2013 7 Pages

LABORATORY TESTING SECTION

Method of Test for

CHECKING THE ACCURACY OF BIN SCALES USED FOR PROPORTIONING

1. SCOPE

1.1 This method of test covers the procedures for checking the accuracy of bin scales used for proportioning of bituminous and concrete materials and is applicable to bin scales of all capacities. 2. APPARATUS

2.1 Weights- A minimum of ten 20 kg, (50 pound) weights meeting the National Institute of Standards and Technology Class F requirements. 3. PROCEDURE

3.1 Preliminary Inspection. This procedure shall be conducted prior to all scale checks.

3.1.1 Check the hopper to insure that it is empty and clean.

3.1.2 Check the scales weigh beam to insure that it is clean and that its

movement is not restricted.

3.1.3 Check the zero reading of the scale.* * The scale dial or reading may not be altered or adjusted either mechanically or electrically.

3.2 Complete Scale Check. This method shall be performed at the start of the season and after any significant modification of the weight system, or as deemed necessary by the Engineer.

3.2.1 Load the scale with the test weights without exceeding 10% of the nominal batch capacity of the scale (The nominal batch capacity shall be established by the Engineer). Record the scale reading and remove the weight.

3.2.2 Charge the hopper with a test load equal to the previous scale reading and record the reading.

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3.2.3 Place an amount of test weights equal to the test weights of Section 3.2.1 on the scale, record the reading, and remove the weights.

3.2.4 Repeat Sections 3.2.2 and 3.2.3 until the approximate midpoint of the nominal batch capacity is reached.

Note 1- Certified Weights may be used in lieu of aggregate test loads in Sections

3.2.2 and 3.3.2.

3.2.5 At the midpoint of the nominal batch capacity, check the repeatability of the scale by placing the test weights on the scale and recording the weight. Compare this reading to the original value obtained in Section 3.2.3 when the test weights were on the scale.

Note 2- If the repeatability check indicates a difference greater than 0.1% of the full scale capacity, the Engineer shall be notified.

3.2.6 Repeat Sections 3.2.2 and 3.2.3 until the nominal batch capacity is

reached.

3.2.7 The scale shall be considered accurate if the total error produced by the addition of the test weights is 0.5% or less of the nominal batch capacity.

3.2.8 In the event that this tolerance is not met, the Engineer shall be notified and the appropriate corrective measure(s) taken.

3.3 Incremental Scale Check

3.3.1 Incremental scale checks shall be performed at random intervals of the nominal batch capacity (i.e., 10%, 20%, 30%,...,100%) at the frequency of one different interval for each check as directed.

3.3.2 Aggregate Scales:

After assuring that the scale reads zero*, load the scale with the test weights without exceeding 10% of that portion of the nominal batch capacity represented by the aggregate(s) mass. Record the scale reading. Remove the test weights. The scale should again read zero. Charge the hopper with a load equal to the selected percentage of Section 3.3.1. Again add the test weights. Record the scale reading. Remove the weights. The scale shall meet the required accuracy of the incremental check if the total error produced by the addition of the test weights does not exceed 0.5% of that portion of the nominal batch capacity represented by the aggregates.

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3.3.3 Cement scales:

If a separate system is used to weigh cement, the system shall be checked in accordance with the procedure outlined above under “aggregate scales”. Again, the number of test weights used shall not exceed 10% of that portion of the nominal batch capacity represented by the cement. Again, the total error produced by the cement scale shall not exceed 0.5% of that portion of the nominal batch capacity represented by the cement, or 5 kg (10 lbs.), whichever is greater. Example 1: Metric: Incremental Scale Check (Aggregate Scale) An aggregate scale having a nominal capacity of 2500 kg is due for the bi-weekly metric scale check. 20% of the nominal capacity is selected at random (500 kg). The total error is computed by dividing the total error in kilograms (25 kg) by the nominal capacity for the scale.

Mat'l WT

Test WT

True WT

Scale Reading

Error

0 200 200 210 +10 500 200 700 715 +15

Total Error: +25 Percent Error:

(25/2500) x 100 = 1.0% Example 1: English: Incremental Scale Check (Aggregate Scale) An aggregate scale having a nominal capacity of 5500 lbs is due for the bi-weekly incremental scale check. 20% of the nominal capacity is selected at random (1100 pounds). The total error is computed by dividing the total error in pounds (25 lbs.) by the nominal capacity for the scale.

Mat'l WT

Test WT

True WT

Scale Reading

Error

0 500 500 505 +5 1100 500 1600 1620 +20

Total Error: +25 Percent Error:

(25/5500) x 100 = 0.45%

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Example 2: Attached are actual examples of complete scale checks entered on Form CS-4211C and performed on a scale having a full capacity of 4500 kg (10,000 lbs.) and a nominal batch capacity of 4000 kg (8,500 lbs). Scale increments are 5 kg (10 lbs).

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