# Instructions for Calculator

The workbook needs to be opened on a computer rather than a phone or tablet, else the formatting won’t work; it’s quite a complex worksheet, and a bit messy inside.  The easy way to perform the calculation is based around the air changes, else it becomes recursive (and a spreadsheet lacks that sort of capability).  This means that the time column down the lefthand side will change depending on the capabilities of the equipment.  This makes it necessary to edit the spreaders and target columns in order to make a comparison between different MVHR specs.  It’s easy to get caught out by this, but provided the data is input correctly the worksheet will produce consistent output.  The could all be addressed by programming a proper app, rather than using a spreadsheet, but that’s beyond our capabilities. This worksheet is still capable of the job, it’s just a little fiddly to operate.

The seven green boxes along the top, and the two green columns are where the parameters can be entered.

The leftmost 4 green boxes grouped together on the left relate to the building, COVID-19, and the occupants.

The first box labelled cubic meters is where the volume of the space can be input in cubic meters.  This is presently set to 466m3 (the size of our venue), but can be changed.

The second and third green boxes both relate to Covid-19. The second box is ID50 quanta (the infectious dose measured in quanta), and this is set to 1 quanta by default, which is the best estimate based on the available evidence. The third box is the number of virons/resp (virons being exhaled in each respiration).  From a recently published research paper it is estimated that an infected person exhales around 16,166 virons per minute when resting, so at a typical respiration rate of 15 breaths per minutes works this works out at 1,078 virons being exhaled with every breath.

The fourth green box is where the mean respiration rate of the occupants in resp/min (respiration per minute) can be entered.  Please note that the worksheet assume only healthy adults in the room and a mean a tidal volume of 500ml. We suggest a rate of 18 for a bar or music venue type environment like ours.

The rightmost three green boxes labelled air change relate to the spec of the MVHR unit. The first of these boxes is the number of air handling units installed. This is default set to 1. Other than cost, the main limitation here is the size and weight of the units, and the space consumed by the ducting (the default Carma 9048 unit uses ducting with a 630mm cross section). The ducts can reduce in size as they branch off from the unit though. Our venue can accommodate a maximum of 1 such unit due to space and structural limitations.

The next box is each unit’s maximum throughput measured in m3/h (cubic meters per hour).

The final box relates to the air change efficiency. An air change period is time divided by the volume of the space divided by the throughput of the unit. So if a room is 1,000m3 and a unit is rated at 10,000m3/h the air will be changed once every 6 mins. However an air change is never 100% efficient, and how much air actually gets changed depends on how well the vents are laid out. A typical ventilation system will have an ACH efficiency of 63.2%, meaning only 63.2% of the air will actually be replaced in any one air change, so this is our default setting.

The elapsed time column on the left coincides with the air change periods, and the frequency of those vary depending on the capabilities of whatever MVHR unit is specified.  The table defaults to 100 air changes, but it’s relatively straightforward for someone with an understanding of formulas to alter this. Bear in mind that these time intervals will vary depending on the capabilities of the equipment…The two green columns on the left are also configurable.

The spreaders present column denotes how many spreaders are present during any given air change period.  By default this is set to show 1 spreader occupying the space the entire time (although in the photo above this is set to 4) . This column can be used to model various numbers of spreaders entering then leaving the space over the course of time.

The target present? column describes whether an uninfected person is present or not. This can be used to model how likely someone is to be infected depending on when they arrive/depart in relation to the spreaders; it takes a while for the aerosol to build up or be depleted, depending on the comings and goings. Only a “y” can be used to denote that a target is present (not case sensitive). Entering anything else in that column will mean that an uninfected person isn’t present during that air change period. This is presently set to show a someone entering the space after the spreaders have already been there for a while, then staying after the spreaders have left. Using this it’s possible to simulate people coming into the space at different times, before, after, or during the times that X number of spreaders have been breathing in there.

NOTE: Regarding both these columns… while a figure can be entered for each cell, by default the cells are all configured to equal the the value of the cell above (except for the topmost cell); this makes it quicker and easy to configure scenarios just by editing a few cells.