Recent content by Usjes

Thanks to the above reference I have been able to confirm that the sum of n IID Chi_squared(k) RVs will be distributed as: Chi_squared(n*k) or equivalently Gamma(nk/2,2) I still see a problem with the Wikipedia entry though, it asserts that the mean of n IID Chi_squared(k) RVs will be...

Hi, I am trying to understand the degrees of freedom parameter in the Chi_squared distribution and I have found two references from the same source that appear, to me, to contradict one-another. Can anyone explain what is going on ? In https://onlinecourses.science.psu.edu/stat414/node/171 it...

Hi, It has been a long time since I have worked with pdfs so perhaps someone can help. According to Wikipedia (http://en.wikipedia.org/w/index.php?title=Chi-squared_distribution#Additivity) the pdf of the addition of n independend Chi_squared distributed R.V.s is also Chi_squared distributed...

Hi, I am trying to figure out what the result is when adding two sinusoids of the same frequency but with different phase and amplitudes. Specifically I want to know if the result is always another sinusoid of the same frequency. For the case of the the same amplitude I have: cos(wt) +...

Hi, Does anyone have a formula for the freezing time (ie time taken for it to freeze) of a fixed volume of water (1mL) at a given temperature (Ts) and pressure (1 Atmosphere) when the surrounding temperature is set to below 0 Celsius (Ta < 0C) ? ie. I'm looking for Time to freeze if you...

Okay so I did a triall run with a small cooler bag + Gel packs (2) and your predictions proved eerily accurate xts. See the attached plot. The temperature did bottom out at about -5 Degrees. Clearly there is a very steep drop in temperature at the start which will ruin my sample so now I am...

I'm trying to minimize the overall volume of the container to be as portable as possible, hence my preference for the highest R insulator. Also the contents won't be homogenous, ie. I will have a small vial of liquid (1mL) in a 350mL thermos and then some form of frozen gel-pack. The vial can...

Thanks for the R-values Naty1, these seem to be the most relevant info and they basically confirm that I was thinking, that for a given coolant (eg. adding ice or water or frozen gel-pack) the best insulator will be a thermos flask. Judging by the listed values it will be significantly better...

Hi, I am trying to keep a small vial of liquid cold (> 2 celsius but < 8 celsius) for 10 hours when the ambient temperature is 20 -> 30 celsius. So if I'm making a container to hold it I'd like to know what is the best material to use. My initial thought was to simply place it in a thermos...