| 9.1 | The following points should be considered when selecting a weighing room: |
| (a) | The location of the weighing room should be in an uncontaminated, dust free environment and away from any activity or equipment that can cause vibration. |
| (b) | To avoid draughts and the ingress of dust, windows, if fitted, must at all times be kept closed (sealed) and the entrance to the weighing room must preferably be through an airlock or otherwise a self-closing door (any other condition that may affect a stabilised atmosphere must be addressed). |
| (c) | Commercial air conditioners are not acceptable unless they are of the type to maintain a constant humidity and temperature. |
| (d) | This room must be dedicated to the weighing and preparation of filter cassettes. |
| (e) | The following signs should be displayed at the entrance to the weighing room: |
| (iii) | No acclimatisation in progress (when applicable). |
| (iv) | Weighing in progress (when applicable). |
| 9.2 | The transparent weighing/stabilisation cabinet |
The transparent weighing/stabilisation cabinet may be used in conjunction with or in lieu of the weighing room and subject to the following:
| (i) | The criteria for filter weighing, as described in this supporting document must, as far as is compatible with the new method, be adhered to. |
| (ii) | The construction of the cabinet should conform o the following: |
The minimum dimensions shall be:
0,8m length
0,5m width
0,5m height
(Dimensions resulting in a smaller volume than that reflected above [0,2m3] will be problematic as it was found that the human body does affect the temperature and humidity inside the cabinet during the weighing process).
The cabinet body should be constructed of an angle strip metal frame housing not less than 3mm thick transparent Perspex (or glass) panels bolted or screwed to the metal frame, The metal used should be a good conductor so as to assist in discharging static electricity from the Perspex panels. The body should consist of four side panels and one top panel
| (a) | The cabinet shall be placed on an anti-static mat of slightly larger dimensions than the cabinet itself without any sealing material between the metal frame and the anti-static mat. |
| (b) | The metal frame of the cabinet shall be properly earthed independently of the anti-static mat, |
| (a) | The cabinet must not be exposed to wide variations in temperature and humidity. The following are examples of undesirable situations: |
| • | The cabinet must not be exposed to direct sunlight. |
| • | If an air-conditioning unit is operated in the same room it must not be switched on and off according to whim but must be kept running constantly to keep the temperature and humidity reasonably constant. The same principle applies to heaters during winter. |
| • | Kettles must not be boiled in the same room as it creates a sharp increase in the humidity. |
| (b) | Pollution of the surroundings of the cabinet by dust, cigarette smoke, etc, must be avoided. |
| (c) | The balance table, anti-static mat and cabinet must be dedicated solely to the purpose it was provided for. |
Both reference and field filters due to be weighed must be acclimatised for at least 12-hours. Steps must be taken to ensure that the cabinet is not lifted up or any of the apertures opened up during the acclimatisation period.
| 9.3 | Weighing room equipment |
A balance capable of weighing to the fifth decimal point of a gram is required
e.g. reading to the nearest 10 micrograms.
| (ii) | The following is required with regard to weighing room equipment and practice: |
| (a) | The balance, correctly levelled must stand on a rigid specially designed balance table on a properly earthed anti-static mat. |
| (b) | The balance must remain uncovered at all times and if possible the side doors left slightly open to allow air circulation during the acclimatisation period. |
| (c) | The balance must be regularly maintained and calibrated and a record kept. It is strongly recommended that a standard weight be used to check the accuracy of the scale in the interim periods. |
| (d) | An additional anti-static mat for disassembly of cassettes is required. |
| (e) | Sufficient storage space for filters and associated equipment must be provided. |
| (f) | A filter stabilization chamber or cabinet so designed as to allow free air circulation should be provided. |
| (g) | Carpets are not allowed in the weighing room because of problems of static build up unless they are manufactured and installed according to the South African Bureau of Standards' specifications. |
| 9.4 | Weighing room accessories |
| (a) | Suitable spring-loaded tweezers for handling filters. |
| (c) | Suitable marking pen. Ball point or filters. |
| (d) | Suitable means for recording weighing. |
| (e) | Non-static cloth for cleaning purposes, |
| 9.5.1 | Filter weighing procedure |
Switch on the balance and leave it to "warm up for at least 30-minutes. Certain balances, provided the power is on, remain in a "standby" mode and in such a case no warm up period is needed.
In order to facilitate the removal of filters from the weighing pan it is recommended that a small 'O'-ring of an impervious material be positioned in the centre of the pan. An added advantage of using art 'O'-ring is that all weighing takes place at a centralised position on the weighing pan so avoiding corner load errors.
Weigh the three reference filters simultaneously or individually, three consecutive times. These three filters remain allocated to the remainder of the field filters from that batch of field filters for mass correction purposes.
If weighed simultaneously the three reference filters should be placed evenly around the centre of the weighing pan.
Weigh each of the first 10 field filters three times consecutively.
Repeat the weighing of the three reference filters (this serves as a dual function: a check on the stability of the weighing room atmospheric condition and for mass correction on the following batch of field filters).
If the difference is 0,1mg or less per filter proceed to weigh the next 10 field filters. If the difference is greater than specified above, the weighing room environment is unstable and the reference filters must be weighed more often. Also re-weigh the previous 10 field filters.
NOTE: Samples once taken, are to be transported in suitable containers so as to minimise any possible particulate loss resulting from bumping, vibration or being transported in the inverted position. Filters should he transported with the dust deposit facing upwards.
| 9.5.2 | Particulate mass determination (mg) |
| (a) | Mass 1 (before sampling): |
| (i) | Mass of field filter and moisture content. |
| (ii) | Mass of each reference filter (normally three reference filters are used). If reference filters are weighed in batches of three, one third of the measured mass must be taken. |
| (b) | Mass 2 (after sampling): |
| (i) | Mass of field filter, particulates and moisture content. |
| (ii) | Mass of reference filter and moisture content. |
| (c) | Gross mass of particulates and moisture content on filter: |
Mass 2(b)(i) - Mass 1(a)(i).,
| (d) | Correction factor for moisture content change (derived from reference filters): |
Mass 1(a)(ii) - Mass 2(b)(ii),
This gives a + or - factor.
| (e) | Absolute mass of the particulates on the filter: |
Gross mass (c) + or - correction determined in (d)
NOTE: Any apparent negative value for the particulate mass must be examined critically and the reasons for this carefully analysed.
| (a) | The above procedure is the minimum requirement for particulate mass determination. |
| (b) | Wetted filter media, which have obviously been damaged physically, must be discarded. Others may be dried and weighed after the normal acclimatisation period. All filters, which appear to have been tampered with, must be discarded and air samples re-taken as soon as possible. Pre-weighed field filters together with their respective reference filters are preferably to be used within a period of three months. |
| (c) | Dust samples for radiation monitoring are conducted using the same methodology and quality assurance. |
