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Division 8.2.3.10 Substitution weighing

United Kingdom

Original text. Later amendments are not shown.

Substitution weighing is an option and, if used, involves measurement of a reference weight before and after each weighing of a PM sampling medium (e.g. filter). While substitution weighing requires more measurements, it corrects for a balance's zero-drift and it relies on balance linearity only over a small range. This is most appropriate when quantifying total PM masses that are less than 0,1 % of the sample medium's mass. However, it may not be appropriate when total PM masses exceed 1 % of the sample medium's mass. If substitution weighing is used, it shall be used for both pre-test and post-test weighing. The same substitution weight shall be used for both pre-test and post-test weighing. The mass of the substitution weight shall be corrected for buoyancy if the density of the substitution weight is less than 2,0 g/cm3. The following steps are an example of substitution weighing:

  1. (a)

    Electrically grounded tweezers or a grounding strap shall be used, as described in point 9.3.4.6;

  2. (b)

    A static neutralizer shall be used as described in point 9.3.4.6 to minimize static electric charge on any object before it is placed on the balance pan;

  3. (c)

    A substitution weight shall be selected that meets the specifications for calibration weights in point 9.5.2. The substitution weight shall also have the same density as the weight that is used to span the microbalance, and shall be similar in mass to an unused sample medium (e.g. filter). If filters are used, the weight's mass should be about (80 to 100) mg for typical 47 mm diameter filters;

  4. (d)

    The stable balance reading shall be recorded and then the calibration weight shall be removed;

  5. (e)

    An unused sampling medium (e.g. a new filter) shall be weighed, the stable balance reading recorded and the balance environment's dew point, ambient temperature, and atmospheric pressure recorded;

  6. (f)

    The calibration weight shall be reweighed and the stable balance reading recorded;

  7. (g)

    The arithmetic mean of the two calibration-weight readings that were recorded immediately before and after weighing the unused sample shall be calculated. That mean value shall be subtracted from the unused sample reading, then the true mass of the calibration weight as stated on the calibration-weight certificate shall be added. This result shall be recorded. This is the unused sample's tare weight without correcting for buoyancy;

  8. (h)

    These substitution-weighing steps shall be repeated for the remainder of the unused sample media;

  9. (i)

    The instructions given in points 8.2.3.7 to 8.2.3.9 shall be followed once weighing is completed.