The first stage in calculating the ambient concentration of black smoke is to measure the darkness of the smoke stains obtained from the sampler. This is carried out using a photo-electric reflectometer. This instrument emits a steady light onto the smoke stain, which is reflected back from the smoke stain to a photo-sensitive element. The electrical response is then amplified to produce a meter reading. The darker the stain, the less light is reflected, so a low meter reading corresponds to a dark surface, and a high reading to a light surface. The reflectometer reads on a scale of 0 (black) to 100 (white). This reflectometer reading, together with the measured volume of air sampled, and the filter clamp size, is used to calculate the ambient concentration of black smoke from a standard calibration.

Three types of reflectometer are in use: the EEL Models 43 (analogue), the 43D (digital), and the Kemtronix MK 1 (digital conversion of the analogue EEL 43). These reflectometers are the reference instruments for the EC Directive 80/799/EEC for the measurement of smoke stain reflectance. All three have a measuring head, which comprises a source of light (a tungsten lamp) and a photo-sensitive element ( a selenium disc). The surface of this element is fragile and should not be touched, marked or exposed to direct sunlight. The head fits into a detachable mask which consists of a locating ring with a metal plate. The mask covers all the working area of the element except for an aperture 1.25cm in diameter, through which the stain is measured.

A standard grey and white tile is supplied for test purposes. This is used to set two fixed points before measurement. The grey part is marked with its reflectance value (usually around 34 to 36). The white part must be used as a backing for all other media handled, including setting to ‘100’ for a blank filter paper AND stain reflectance. The smoke stain must be formed on the smooth surface of a clean Whatman grade 1 (No 1) filter paper.

The operation of all three models is very similar. However, there are some differences, particularly in the setting up procedures. This section gives separate instructions for each type, followed by instructions for measurement of smoke stains, which are applicable to all three models. The relevant set of instructions can be photocopied and kept with the reflectometer.

The EEL Model 43 consists of a meter unit fixed in a case and a measuring head, which is connected to the unit. The unit also carries the mains switch and the sensitivity control.

1. The reflectometer must be used on a flat, level surface with its meter glass facing upwards.
2. Remove the measuring head from the case and ensure the lead is unwound.

Care must be taken when handling the measuring head. Never expose the underside of the head to bright light, electric lighting, daylight etc. For this reason, always keep the underside facing down. Also, make sure that light does not reflect upwards onto the underside of the head from shiny or light coloured surfaces below. It is advisable to cover the work surface with matt dark material. Bright light may damage the selenium disc, the increase in the electrical output may damage the meter, and the instrument calibration may alter.

3. Place the measuring head in the circular mask on the work surface. This mask must be used at all times when taking any measurement.

The head should fit freely into the mask. The mask is too tight if it remains attached to the head when it is raised. The mask should be kept in good condition. If the mask breaks, a proper repair is necessary - do not use any type of adhesive, tape or material which will alter the effective thickness of the plate.

4. Turn the meter to a convenient position so that the field of vision is sighted along the pointer and note the scale directly down below. Do not look sideways onto the pointer because of possible parallax error when reading the scale.
5. Check the pointer is exactly on zero position. If not, set to zero using the slotted adjuster seen on the face of the meter below the centre of the glass window. Turn the adjuster and gently swing the pointer. This can be done by gently rotating the reflectometer case side to side so that the pointer swings to and fro about the zero mark. Allow to settle, and repeat if necessary until the pointer rests on the zero mark.
6. Connect the measuring head to the meter: the plug is keyed and will only enter the socket in one position.
7. Connect the reflectometer to the mains supply, switch on and allow to warm up for at least 10 minutes.
8. Take the white and grey standard test tile, and wipe any dust from its glass surface. This tile is used to check the linearity of the reflectometer before use.
9. Place the circular mask over the white tile on the standard, insert the measuring head fully and let go. Take a reading from the meter and adjust the sensitivity accordingly, using the SENS control, until the pointer is on exactly ‘100’.
10. Move the circular mask over to the grey tile, insert the measuring head fully and let go. Note the reading on the meter : this must agree with the value on the cover glass of the tile (usually around 34 to 36) to within ± 1½ divisions.
11. Repeat the above actions several times until a constant grey tile value is obtained AND the pointer returns to exactly to ‘100’ each time.

Before measuring the smoke stains, the pointer must be set to ‘100’ on a clean, unused and undamaged Whatman No1 filter paper preferably from the same batch of filters used in the filter clamps.

12. Place a blank filter paper , smooth side uppermost, on the white tile. Place the circular mask on the blank filter, fully insert the measuring head and let go. Take a reading from the meter and adjust accordingly, using the SENS control, until the pointer is on exactly ‘100’. Keep adjusting the sensitivity until a stable reading of ‘100’ can be obtained.

1. The reflectometer must be used on a flat, level surface.
2. Remove the measuring head from the case and ensure the lead is unwound.

Care must be taken when handling the measuring head. Never expose the underside of the head to bright light, electric lighting, daylight etc. For this reason, always keep the underside facing down. Also, make sure that light does not reflect upwards onto the underside of the head from shiny or light coloured surfaces below. It is advisable to cover the work surface with matt dark material. Bright light may damage the selenium disc, the increase in the electrical output may damage the meter, and the instrument calibration may alter.

3. Place the measuring head in the circular mask on the work surface. This mask must be used at all times when taking any measurement.

The head should fit freely into the mask. The mask is too tight if it remains attached to the head when it is raised. The mask should be kept in good condition. If the mask breaks, a proper repair is necessary - do not use any type of adhesive, tape or material which will alter the effective thickness of the plate.

4. Ensure the measuring head is connected to the meter. The plug is keyed and will only enter the socket in one position; alternatively the plug may have been fixed in place by the manufacturer.
5. Connect the reflectometer to the mains supply, switch on and allow to warm up. The manufacturer recommends at least 30 minutes.
6. Take the white and grey standard test tile, and wipe any dust from its glass surface. This tile is used to check the reflectometer before use.

 

7. Place the circular mask over the white tile on the standard, insert the measuring head fully and let go. Press and release the "Auto Range" button so that the meter indicates a reading of 100.0. It may be necessary to do this more than once to obtain 100.0.
8. Move the circular mask over to the grey tile, insert the measuring head fully and let go. Note the reading on the meter : this must agree with the value on the cover glass of the tile (usually around 36) to within ± 1.5 divisions.

Before measuring the smoke stains, the pointer must be set to ‘100’ on a clean, unused and undamaged Whatman No1 filter paper preferably from the same batch of filters used in the filter clamps.

9. Place a blank filter paper , smooth side uppermost, on the white tile. Place the circular mask on the blank filter, fully insert the measuring head and let go. Set to 100.0 using the "Auto Range" button.

Having set up the reflectometer according to the appropriate set of instructions, and set a reading of 100.0 using a blank filter paper from the same batch as the exposed papers, proceed as follows for each smoke stain:

1. Place the filter paper on the white tile, ensuring the stain is uppermost and centred on the white tile. Place the circular mask centrally on the stain, and check carefully that none of the "clean" part of the filter is visible through the hole. Hold the mask so that it does not move while the measuring head is inserted and let go.

One of the common sources of errors is the smoke stain being measured on the wrong side. To avoid this happening, take the stain and hold up to the light. As a stain can be seen on both sides, turn the filter paper over until the darker of the two stains can be seen. The side with the darker stain is the one to be measured.

2. Note the reflectance. Record the reading to the nearest whole division on the edge of the filter paper (where the site name, number and date of exposure should already have been recorded).

Take care not to write on or mark either side of the stained area as this will make the stain unusable.

3. Every few stains measured, (typically around 10, but experience will show how many), recheck the ‘100’ using the blank paper as in the instructions above. If the setting is unchanged, continue with the stain measurement. If the ‘100’ setting has drifted, re-adjust to exactly ‘100.00’ and then remeasure the last stain or two, to ensure that no error has been made.

If a smoke stain has been formed on the rough surface of the filter paper because the paper was put in the sampler clamps the wrong way, put this stain aside while the normal smooth surface stains are measured. Then reset to ‘100’ as in 11 above, but using the rough surface of the blank, and complete the measurement of the rough surface stains.

4. After all the stains have been measured, switch off and allow the measuring head and bulb to cool.
5. Once cooled, repack the analogue reflectometer components carefully. Handle the cables gently so that the internal wiring is not broken. Before closing the lid, make sure that the plug and cable will not crush the meter glass.

Notes:

1. A quivering or oscillating pointer tip (analogue version) or an unstable reading (digital) may indicate that the measuring head bulb may need replacing.
2. Never touch the bulb glass with bare fingers: traces of sweat or grease will attract dust and cause the bulb to overheat.
3. Always disconnect from the mains before dismantling, and remember, internal components in the measuring head may be extremely hot to touch and are fragile.
4. Reflectometer test cards are available: the white tile must be used as a backing to the test card and ‘100.0’ must be set on the plain white area of the card. When measuring test card stains, you should record to the nearest tenth of a division ( you would normally read to the nearest whole division), and repeat the measuring sequence several times.
5. Stains must be stored carefully by site and pollution month and must be saved for a minimum of 18 months, in case they need to be remeasured by yourselves or by AEA Technology Environment for quality control purposes.

7.3 CALCULATION OF SMOKE CONCENTRATIONS

7.3.1 Use of "Look-up Tables"

Results can be determined using "look-up" tables (available from AEA Technology Environment), from which the equivalent concentration of standard smoke can be read off, using the volume of air sampled and the reading of the reflectometer. Results should be recorded to the nearest µg m^-3. The tables are given in Appendix 3.

Smoke concentrations in the UK have always been calculated by the use of the British Standard Smoke Calibration Curve (BS 1747: Part 2: 1991).

For reflectometer readings of 40 to 99 the following formula is used:

where
C = concentration in micrograms per cubic meter, British Standard calibration
V = volume of air sampled( for historical reasons this is in ft³: to convert from metric, 1 m³ =35.314667 ft³ ) .
F = a factor relating to the sampler clamp size, as follows:
0.288 for ½ inch clamp
1.000 for 1 inch clamp
3.680 for 2 inch clamp
12.80 for 4 inch clamp
R = reflectometer reading

This formula represents the calibration curve to within ± 1.3% over the range of the reflectometer readings between 40 and 90. When used to calculate concentrations from reflectometer readings between 91 and 98 the results may be underestimated by as much as 6%.

For darker stains with reflectometer readings between 40 and 20 the formula used is:

For stains with reflectometer readings of less than 20 this formula gives only an approximation to the concentration, the result being well below the true value. Reflectometer readings of less than 10 are impossible to assess accurately and hence the results are calculated as if the reading had been 10, which gives a minimum value.