I'm having trouble with a simple application with metRology. I need to
estimate the uncertainty of the density thickness of seven sheets of film.
This is calculated from measurements of mass, length and width of
rectangular samples of film.
It's not too hard to calculate the whole thing with a little Monte Carlo
loop. I get about 0.07 with this:
#sample area
L<-5*2.54 #cm
W<-8*2.54 #cm
#sample mass
m<-0.2543*1000 #mg
#uncertainties
L.u<-(1/16)*2.54 #cm (nearest 16th inch)
W.u<-(1/16)*2.54 #cm
m.u<-0.006*1000 #mg scale calibration data
denth<-c(0,0,0)
singth<-c(0,0,0)
for(i in 1:1e5) {
#denth[i]<-7*dt+sum(rnorm(7,0,dt.u))
for(j in 1:7)
singth[j]<-(m+rnorm(1,0,m.u))/((L+rnorm(1,0,L.u))*(W+rnorm(1,0,W.u)))
denth[i]<-sum(singth)
}
sd(denth) #0.07279882
Now with the metRology package, I get a much higher number, which is
actually the seven times the uncertainty of one layer:
require(metRology)
d.set<-list(mass=m,Length=L,Width=W)
d.set.u<-list(m.u,L.u,W.u)
dent<-expression(7*(mass/(Length*Width)))
uncert(obj=dent, x=d.set, u=d.set.u, method="GUM")
I've tried other ways of defining the expression and every method option.
I suspect the expression is the problem area, but if so, I really need to
know how to get it right for harder problems. Thanks in advance for any
suggestions.
- Mark
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