Field inspection manual—Volumetric measuring devices

Category: Volume
Part: 4-STP
Section: 45
Publication date:

Part 4, section 45—Specialized test procedure—Reference scale test procedure

Application

This gravimetric procedure is intended to test a density or product reference scale prior to use in gravimetric testing of a volumetric measuring device. It is based upon standard test procedures applicable to non-automatic weighing devices (NAWDs); however, there may be minor deviations from these requirements.

Purpose

This procedure is used to determine the suitability of a density or product reference scale prior to use in gravimetric testing of a volumetric measuring device.

Equipment

  • A product reference scale suitable to hold the delivery vessel when full of product.
  • A delivery vessel of sufficient capacity and constructed with materials suitable for the product collected.
  • Local standards of mass in sufficient size and quantity to establish the suitability of the product reference scale and the density sample scale if used.

Procedure for scale performance testing

The most important aspects of the reference scale are its accuracy and ability to repeat indications for identical loads placed on the scale. If the reference scale does not meet requirements for repeatability, accuracy or sensitivity, it is not suitable for use for device evaluation purposes. Tests must be conducted throughout the range at which the reference scale is intended to be used. Eccentricity requirements are less important in this case, as typically the container will remain stationary on, or may even form part of, the load receiving element (LRE) and the product is free to flow through the container. This means that as long as the test weights are well distributed across the scale, eccentricity errors should not be relevant to the final readings. Containers should not be moved relative to the LRE.

Care must be taken to ensure environmental effects on the reference scale are minimized to every extent possible. This may include, but is not limited to, protection from wind, direct sunlight and variations in temperatures during use that may be caused by operational influences in the vicinity.

Warm-up

Before conducting any tests on the scale, it must first be powered-up, levelled and exercised. Follow the manufacturer's recommended warm-up instructions to ensure proper scale performance.

Repeatability test

The purpose of this test is to determine if the reference scale repeats the same indication when the same load is reapplied to the scale several times, in approximately the same position.

Procedure

Use a test load or test quantity equal to the total gross weight of the container (pycnometer or containment vessel) and the product when filled to capacity.

  1. Zero the scale.
  2. Apply the test load to the weighing element and note the indication.
  3. Remove the entire load.

Repeat this test two more times by applying the load to approximately the same area of the weighing element and in the same manner. Note the indications obtained.

Interpretation of results

The reference scale can only be used if the difference between the results obtained for the same load does not exceed ±1 graduation from the average of all readings. If the reference scale does not meet this requirement, contact your Volumetric Specialist for further options.

Sensitivity test

The sensitivity test is used to determine whether the device is capable of changing its indication accordingly when sensing a change in load equal to between 1.0 and 1.4 times the minimum graduation. This test is performed at zero load and at a load equal to the intended maximum load at which the reference scale is intended to be used.

Note: If applicable, the automatic zero mechanism should be disabled unless a small mass (approximately 5% of capacity) is added at zero load to place the scale outside of the automatic zero mechanism range.

Procedure

  1. Carefully add certified test standards equal to between 1.0 and 1.4 times the scale graduation.
  2. Record the indication.
  3. Repeat the test near or at the maximum load the scale is intended to be used.

Interpretation of results

The addition or removal of the sensitivity load must cause a change in indication by at least one graduation but not more than two graduations. If the reference scale does not meet this requirement, contact your Volumetric Specialist for further options.

Eccentricity (corner or section) test

The purpose of this test is to ensure that the reference scale weighs applied loads accurately despite their location on the scale. The device must be capable of weighing accurately regardless of changes to the position of the test load on the scale platform.

Note: This test is optional in situations where the containment vessel remains stationary for the duration of the device evaluation tests, since liquid products will always maintain an equivalent mass centre when introduced into the vessel.

Procedure

  1. Zero the reference scale.
  2. Apply a test load of approximately 25% scale capacity.
  3. Record the indication.
  4. Assume a four section scale; mentally divide the scale into four quadrants (north, south, east, and west). For other scale configurations refer to Field inspection manual – non-automatic weighing devices, STP-15 for appropriate loading patterns.
  5. Apply the same load sequentially to each quadrant of the scale platform.
  6. Record indication at each location.

Interpretation of results

The difference between the indications at each position must not exceed ±1 graduation from the average of all readings. If the scale does not meet this requirement, contact your Volumetric Specialist for further options.

Accuracy and return to zero tests

The purpose of these tests is to determine the scale's accuracy, linearity and hysteresis. The accuracy testing must be performed to at least the maximum range where the reference scale is intended to be used. At a minimum, the scale must be tested in the range that encompasses the gross weight of the filled container.

Note: Record a suitable number of certified test load indications for the increasing and decreasing tests for evaluations that will involve multiple successive product runs into the same container without emptying. The optimum test points should be equivalent to anticipated gross load amounts. If the scale/container capacity is equivalent to a single test run, the scale needs to be checked at only two points. The first point is the weight of the empty container (tare) and the second at a weight equivalent to full container (gross).

Procedure

  1. Zero the scale.
  2. Successively apply the appropriate number of certified test loads from zero to maximum test range.
  3. Utilize a substitution load if necessary as per procedure below.
  4. Record indications at each increasing load.
  5. Remove the loads individually in reverse sequence.
  6. Record indications at each decreasing load and at zero upon removal of final load.
  7. Remove the unknown load and place certified test standards back on the scale. Record indication.
  8. Remove certified test standards from scale; verify scale returns to zero.

Note: This procedure may only be used once. That is, the scale may not be used for any load greater than two times the amount of known local standards that are available. If this is insufficient capacity, more known standards must be used.

Interpretation of results

For each increasing/decreasing load test, the error indication must be no larger than ± 1 graduation. As well, the scale must be capable of returning to zero within ½ graduation and within 15 seconds. Any errors determined during the increasing/decreasing tests must be taken into account during reference weighments with the use of a scale calibration factor (SCF). If the scale does not meet this requirement, contact your Volumetric Specialist for further options.

Determining the scale calibration factor

Using the values obtained during the scale performance tests, determine the SCF at maximum load as follows:

SCF=M_ref⁄R_ref

where:

Mref = actual weight of known test load

Rref = scale reading with known test load

The SCF is applied to the indicated mass readings observed on the reference scale, to account for any offset in scale calibration at the load point(s) that the reference scale is intended to be used.

Note: An SCF must be determined for each scale being used and at each reference weight for the scale. This includes an SCF for the density scale as well as the reference scale. Care must be taken to ensure that these factors are applied only to the appropriate readings; they must not be interchanged.

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