A useful document on the theory of handwashing is available from the World Health Organization in the leaflet 'How to Handrub & How to Handwash
'. It reads: 'When washing hands with soap and water, wet hands with water and apply enough soap to cover all hand surfaces
'. This is actually the most precise information that I could find on the amount of soap needed for washing hands. Among others this implies that small hands need less soap than big hands, which supports the need for a variable supply soap dispenser
. One of the dispensers used for this Instructable read 'Apply a small amount of soap
', which I consider good corporate responsibility practice.
Let's now try to quantify the amount of soap that is being supplied at a single stroke of the handle from a small sample (n=4) of soap dispensers.
The soap dispensers were purchased in a regular shop, ranging in price from EUR 1.82 to EUR 2.56 (US$ 1.40 to US$ 1.97). In terms of specific costs, the four samples range from EUR 6.07 to 10.24 per liter (US$ 0.08 to 0.13 per cu in).
The amount of soap being dispensed at a single stroke of the hand-pump has been determined by counting the number of cycles required for pumping a total volume of 50 ml (3.05 cu in). This resulted in a range of 25 to 50 cycles.
The resulting soap volume dispensed in a single stroke can thus be determined at 1 to 2 ml (0.06 to 0.12 cu in per stroke).
Consequently, the price of a single handwash is 0.7 to 1.9 eurocent (US$ 0.006 to 0.015). This calculation does not take into account the costs for the water that is needed in the process and assumes that the quality of the soap is not a factor influencing the required quantity.
The chart's trendline does not really provide any particular insight to the correlation between 'supply volume per stroke' and 'specific costs' (R-square is very small). Readers of this Instructable are invited to populate the database with own observations. To do so, the Opendocument Spreadsheet (.ods) is available on request. Data communicated through a PM might be integrated and released in an update for this instructable.
The latter derived parameter, the specific costs, is most relevant but cannot be influenced by the user after having purchasing the dispenser. The first parameter however, the supply volume per stroke, can
be influenced by the user.
The next steps document approaches for reducing the amount of soap being dispensed and thus lowering the expenses of a single handwash (assuming that the quality of the washing process remains unchanged).