IAN HANBY - CHARTERED ENGINEER

CALIBRATION & USE OF THE STICKY PAD READER


GUIDELINES FOR USE OF THE STICKY PAD READER


The use of Sticky Pads to monitor airborne dust was first devised and described by Beaman & Kingsbury - Assessment of Nuisance from Deposited Particulates Using a Simple and Inexpensive Measuring System - in Clean Air, Vol 11 No 2 pp77-81, 1981, copies of which can be obtained from NSCA, 136 North Street, Brighton BN1 1RG, UK. It is a cheap and easy way of assessing dust nuisance, but should not be regarded as a substitute for quantitative monitoring for health effects.


The primary purpose of this instrument is to assess the nuisance from deposited or wind-blown dust particles as described by Beaman and Kingsbury, but it can also be used to measure smoke stains on filter papers, although its suitability for this purpose is not guaranteed.


The method has certain drawbacks, such as susceptibility to losing dust in heavy rain, but as this is precisely what happens to cars, window sills, etc in similar situations, it can be regarded as a fairly good method of assessing perceived dust nuisance. If the sticky pad is exposed for too long, then it can become saturated with dust, since new dust will not stick on top of dust already trapped. Suitable exposure periods are usually between 2 and 7 days.
The sticky pads can be placed vertically to measure flux, horizontally (face up) to measure deposition or wound round a cylinder with a vertical axis to obtain directional information. Sticky pads 317mm x 148mm are available for sale, or can be made from Fablon self adhesive vinyl which is obtainable from most hardware shops; the white version of Fablon should normally be used. Suitable sizes of pad are about 150mm x 150mm for the horizontal or vertical methods or 317mm x 148mm, to fit a cylinder 100mm dia x 150mm long for directional work. The gauge is prepared by mounting the Fablon on flat boards approx 200mm x 200mm, or on the cylinder, with the protective layer outermost. Using a scalpel or craft knife, the backing paper is cut in such a way as to enable a small section of this backing to remain in place whilst removing the remainder. After exposure, the pad should be covered to prevent further pick-up and returned for analysis. It is recommended that the original backing paper be saved and used to cover the exposed sample, but care should be taken to use the correct side of the backing paper, or it will not release! The section which remained covered is eventually uncovered in the laboratory as a clean reference area.
Using the Sticky Pad Reader, calibrated as described, the Effective Area Coverage (EAC) is calculated by subtracting the instrument reading from the reference area reading, and then dividing by the number of days’ exposure to give %EAC/day. Directional results can be presented on a spreadsheet, an example of which can be seen here.
Beaman & Kingsbury found the following typical levels:

%EAC/day Situation
0.01 Rural
0.02 Suburban
0.3-0.4 Urban
0.5 Rural summertime
0.8-1.0 Industrial


and the following complaint thresholds, depending to some extent on the colour of the dust:

%EAC/day Response
0.2 Noticeable
0.5 Possible complaints
0.7 Objectionable
2.0 Probable complaints
5.0 Serious complaints

CALIBRATION OF THE STICKY PAD READER

A linear relationship between reflected light energy and meter reading has been achieved in the Sticky Pad Reader (SPR) by the conversion of photocurrent to voltage at zero photodiode voltage. Because of this linearity, a two-point calibration will set up the instrument correctly. However, it should be pointed out that the use of two points very close together, in terms of meter reading, will inevitably yield a poor calibration.


There are three approaches to the calibration method, depending on the requirements of the user, viz:
I. Emulation of an EEL Smokestain Reflectometer when samples which have been measured reliably on the EEL are not available
II. Emulation of a different instrument when samples which have been measured reliably on the different instrument are available
III. Setting up independently
Methods l or ll above should be chosen if correspondence is required between existing data measured using a different instrument, and future data using the SPR.
Wherever possible use the locating mask, as this enables the user to see clearly where on the sample the reading will be taken, and it also holds the sample flat. If it is not used (perhaps if it has been lost) then both the calibration and subsequent use of the SPR must be without the mask. The settings will be quite different with and without the mask.
The zero adjustment is on the LHS and the calibration adjustment on the RHS of the meter.
Calibration of the SPR should not be attempted using ceramic tiles; for example, method ll must use actual sticky pad samples. A calibration sheet is supplied with the instrument; prior to June 2009 the sheet had six square grayscale areas marked 99, 82, 67, 54, 42 and 33. Since that date, a different sheet has been substituted with three circular grayscale areas marked 99, 52 and 33.
When the instrument is being used after having been calibrated, the sample being assessed should be placed face up on the same piece of clean white paper as was used in the calibration procedure.


Method l. (Preferred Method)


This calibration procedure enables the SPR to produce similar readings to those produced by an EEL meter, so that reasonable consistency of data may be assumed between the two instruments, and should be used when the user does not have access to any samples previously assessed on the EEL meter.
To carry out the calibration, the user will need the calibration sheet supplied with the SPR (the actual sheet must be used; a photocopy will not work), an A4 sheet of clean white paper and a small screwdriver or trimtool.
· Switch on the instrument by inserting the plugtop power supply into a 230/240V 50Hz 13A socket. Allow several minutes (minimum 30) for the instrument to settle.


· Place the calibration sheet face-up on top of the clean white A4 sheet.
· Place the SPR on the ‘99’ area of the calibration sheet and note the meter reading as y1.
· Place the SPR on the ‘33’ area of the calibration sheet and note the meter reading as y2.
· Calculate the zero-error-corrected reading as follows:

y2’ = ( y1- y2) / 2
Round to the nearest integer.
· Adjust the zero control using the screwdriver or trimtool, with the SPR still on the ‘33’ area, until the meter reads y2’.
· Next, place the SPR on the ‘99’ area and adjust the calibration control until the meter reads 99.
· The SPR is now calibrated but should be checked by being replaced on the ‘33’ area, when the meter should read 33.
· Repeat the procedure, if necessary, to improve the accuracy, and finally check that the intermediate value(s) of 42, 54, 69 and 83 (or 52 only, on later sheets) give reading(s) correct to ±4. (The tolerance is more connected with the variability of the printer ink than the instrument)


Method ll.


This calibration procedure enables the SPR to produce similar readings to those obtained on different instruments (which can include the EEL meter), so that consistency of data may be assumed between the two instruments, and should be used when the user does have access to at least two samples previously assessed on the other meter.
The method does not specifically need a zero point.
To carry out the calibration, the user will need two reliably measured samples from the other instrument, an A4 sheet of clean white paper and a small screwdriver or trimtool.
It is essential that the user should be confident about the two values measured by the other instrument, and that they should be for values as far apart as possible.
· Switch on the instrument by inserting the plugtop power supply into a 230/240V 50Hz 13A socket. Allow several minutes (minimum 30) for the instrument to settle.
· Place the two samples face-up on the clean white sheet.
· Place the SPR on the cleaner of the two samples. Note the meter reading as y1 and the sample value as x1.
· Place the SPR on the other sample. Note the meter reading as y2 and the sample value as x2.
· Calculate the zero-error-corrected reading as follows:
y2’ = x2 * (y1-y2) / (x1-x2)
Round to the nearest integer. (Note that * denotes multiplication and / division).
· Adjust the zero control using the screwdriver or trimtool, with the SPR still on the x2 sample, until the meter reads y2’.
· Next, place the SPR on the x1 area and adjust the calibration control until the meter reads x1.
· The SPR is now calibrated but should be checked by being replaced on the x2 area, when the meter should read x2.
· Repeat the procedure, if necessary, to improve the accuracy.


Method lll.


This procedure enables the SPR to be calibrated in accordance with a user’s specific requirements. For example, if a specific type of dust is present on site, a piece of Fablon or other sticky pad material can be saturated with dust to give a zero value, and a clean piece of the same material can be regarded as having a value of 100. The following method does not require the points to have the values of zero and 100, although these figures are allowed.
To carry out the calibration, the user will need two samples, an A4 sheet of clean white paper and a small screwdriver or trimtool.
· Switch on the instrument by inserting the plugtop power supply into a 230/240V 50Hz 13A socket. Allow a several minutes (minimum 30) for the instrument to settle.
· Place the two samples face-up on the clean white sheet.
· Place the SPR on the cleaner of the two samples. Note the meter reading as y1 and the sample value as x1.
· Place the SPR on the other sample. Note the meter reading as y2 and the sample value as x2.
· Calculate the zero-error-corrected reading as follows:
y2’ = x2 * (y1-y2) / (x1-x2)
Round to the nearest integer. (Note that * denotes multiplication and / division).
· Adjust the zero control using the screwdriver or trimtool, with the SPR still on the x2 sample, until the meter reads y2’.
· Next, place the SPR on the x1 area and adjust the calibration control until the meter reads x1.
· The SPR is now calibrated but should be checked by being replaced on the x2 area, when the meter should read x2.
· Repeat the procedure, if necessary, to improve the accuracy.

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