deltavar() always returns 0

Question

I am using the deltavar() function in emdbook to carry the variance of whale dive times, surface times, and the amount of time a whale at the surface would be in a plane's field of view through the availability equation written below.

z_jan <- 533.27531  47.87911  52.62564 #vector of means for January.  The first number is the mean dive time, followed by the mean surface time, and the mean field of view.  
sigma_jan <- 690226.840   7550.909   2677.737 #vector of variances for January.  The first number is the variance of the dive times, followed by the variance of surface times, and variance of field of view.

fun <- (s/(s+d)) + (d*(1 - 2.71828^(t(-1)/d))/(s+d)) ## availability equation I need to carry the variance through
fun<- as.character(fun)
vars <- c("d", "s", "t") ## variables in the function (d = dive times, s = surface times, t = field of view time)

delta_jan <- deltavar(fun = fun, meanval=z_jan,vars = vars,Sigma = sigma_jan)

The code runs, and it seems like everything should work, but every month of data I try returns 0. Any thoughts?

Answer 1

Try the following:

delta_jan <- deltavar(fun = s/(s+d)) + (d*(1 - 2.71828^(t(-1)/d))/(s+d),
                      meanval=z_jan,
                      vars = vars,
                      Sigma = sigma_jan)

I think that the "fun" cannot be a char, only a function.