Engineering Question Regarding Perc

guyute65045

I have a question which may or may not be for engineers. I am studying to take the septic designers test in my state and need to wrap my brain around this process.

Determining the Percolation Rate, The rate of drop for each run is plotted, on graph paper with logarithmic scales on both axes (log/log graph paper), against the cumulative time of the seven runs, including the refill times. The best straight line is fitted to the seven data points and extrapolated out to one (1) day (1440 minutes) of cumulative time The rate of drop after 1440 minutes is the percolation rate

I know how to run a perc test and have done many, up until last year we used a simple equation to figure out MPI, now this graphing thing is a bit confusing sounding to me. Am I still just figuring the average and dividing that number by 1440 to get the MPI? I don't even think I know what logarithmic graphing is, I know basic graphing. Hell we don't even really use perc tests anymore, mostly it is using soil profiles to determine hydraulic loading rate for the soil using charts from the U of Wisconsin (Converse paper)>

If anybody has a source that might explain the basics of logarithmic graphing to me that would be great. I did an internet search but it was a lot of computer stuff. I learn best when I can apply the theories to actual scenarios I know, ie septic design etc.......

Thanks a ton,

Tim Doyka

No!

ch0mpie

Not sure about the calc for a perc test, we ussually do variable head tests. The middle of the attached page shows an example of a semi-log plot of head ratio vs time. I think this is similar to what you have to plot for a perc test. The idea of a log plot is to represent data thats not liner with straight line by manipulating the scale.

Attached Files

Dm701 123.pdf (35.0 KB, 51 views)

abm2958

A logarithmic scale means that it increases by a factor of ten between points (exponentially) instead of linear.

Blas

I disagree, it's a very good game, but i think Donkey Kong is the best game ever.

__________________
"It is what it is"

Steve Manning

I'm probably not going to be much help. I would say septic design is very much a state by state case and likely a county by county design issue. Around here you have to have a soil scientist come out and pull a plug of soil from the ground and classify it based on the visual clues as to the type of NRCS soil it is. The local health department then uses a chart based the upon number of bedrooms for residential and the NRCS soil classification of drainage rate to tell us how many feet of drain field we will need. If it is commericial they have some equation they use but it is well hidden from public view.

I noticed you can obtain your classification from the U of Wiss. maps. If these are the NRCS maps you can likely obtain this data on line now at this web site below. The last health dept. guy around here that I worked with on a drain field used to do field work to produce these maps earlier in his life for the USGS and said they were not that accurate. That is why they require a field determination around here.

//websoilsurvey.nrcs.usda.gov/app/

As for logarithmic graphing I have not used that in ages just because of a lack of need or want.

Sorry I couldn't be of more help. I hope you find the website useful.

guyute65045

Thank you all for the replies, I think that I just need to crack a book. I can figure it out without the graph, but for the test I need to plot it out and show the work. In our state all that you need to do to be a septic designer is pass the test, and I have been working on the surveying and soils classes to pass it for two years. I guess in the end it is only a couple of questions on a very long test.