diff --git a/NAMESPACE b/NAMESPACE
index e69ff53..55cee95 100644
--- a/NAMESPACE
+++ b/NAMESPACE
@@ -47,4 +47,7 @@ importFrom(lazyeval,lazy_eval)
 importFrom(ncdf4,nc_close)
 importFrom(ncdf4,nc_open)
 importFrom(ncdf4,ncatt_get)
+importFrom(ncdf4,ncvar_add)
+importFrom(ncdf4,ncvar_def)
 importFrom(ncdf4,ncvar_get)
+importFrom(ncdf4,ncvar_put)
diff --git a/R/convert_sim_var.R b/R/convert_sim_var.R
index 5724fd9..b37c1d8 100644
--- a/R/convert_sim_var.R
+++ b/R/convert_sim_var.R
@@ -10,6 +10,7 @@
 #' @param longname the longname for the new variable
 #' @export
 #' @importFrom lazyeval lazy_dots lazy_eval
+#' @importFrom ncdf4 ncvar_def ncvar_put ncvar_add
 #' @examples 
 #' \dontrun{
 #'sim_folder <- run_example_sim(verbose = FALSE)
@@ -17,12 +18,13 @@
 #'convert_sim_var(nc_file, tempF = temp/5*9+32, unit='degF',longname='temperature degrees Farenheit')
 #'plot_var(nc_file, 'tempF')
 #'convert_sim_var(nc_file, crazy_var = temp-u_mean*1000)
-#'plot_var(nc_file, crazy_var)
+#'plot_var(nc_file, 'crazy_var')
 #'
 #' }
 #' 
 convert_sim_var <- function(nc_file='output.nc', ..., unit='', longname=''){
   
+  sim.vars <- sim_vars(nc_file)$name
   message('convert_sim_var is untested and in development')
   
   # // here, vals would be defined by the function passed in by `...`. Probably captured w/ lazyeval?
@@ -32,10 +34,16 @@ convert_sim_var <- function(nc_file='output.nc', ..., unit='', longname=''){
     stop('not yet ready to handle multi-var expressions')
   
   var.name <- names(convert)
+  if (var.name %in% sim.vars)
+    stop(var.name, ' cannot be added, it already exists.', call. = FALSE)
   
   fun.string <- deparse(convert[[1]]$expr)
   variables <- strsplit(fun.string,"[^a-zA-Z_]")[[1]]
   variables <- variables[variables != '']
+  vars.not.in <- variables[!variables %in% sim.vars]
+  if (length(vars.not.in) > 0)
+    stop(paste(vars.not.in, collapse=', '), ' do not exist in simulation vars', call. = FALSE)
+  
   data <- lapply(variables, function(v) get_raw(nc_file, v))
   names(data) <- variables
   vals <- lazyeval::lazy_eval(convert, data=data)[[1]]
diff --git a/man/convert_sim_var.Rd b/man/convert_sim_var.Rd
index 994f88b..c9bc3ad 100644
--- a/man/convert_sim_var.Rd
+++ b/man/convert_sim_var.Rd
@@ -26,7 +26,7 @@ nc_file <- file.path(sim_folder, 'output.nc')
 convert_sim_var(nc_file, tempF = temp/5*9+32, unit='degF',longname='temperature degrees Farenheit')
 plot_var(nc_file, 'tempF')
 convert_sim_var(nc_file, crazy_var = temp-u_mean*1000)
-plot_var(nc_file, crazy_var)
+plot_var(nc_file, 'crazy_var')
 
 }
 
diff --git a/tests/testthat/test-convert_sim_var.R b/tests/testthat/test-convert_sim_var.R
new file mode 100644
index 0000000..250d4f8
--- /dev/null
+++ b/tests/testthat/test-convert_sim_var.R
@@ -0,0 +1,24 @@
+context("convert variables in netcdf output")
+
+test_that("errors with poor matches", {
+  sim_folder <- run_example_sim(verbose = FALSE)
+  nc_file <<- file.path(sim_folder, 'output.nc')
+  expect_error(convert_sim_var(nc_file, temp = temp*2), "temp cannot be added, it already exists.")
+  expect_error(convert_sim_var(nc_file, temp.new = garbage), "garbage do not exist in simulation vars")
+})
+
+test_that("can add variable", {
+  convert_sim_var(nc_file, temp.new = temp*2)
+  expect_true('temp.new' %in% sim_vars(nc_file)$name)
+  expect_is(get_var(nc_file, var_name = 'temp.new'), 'data.frame')
+})
+
+test_that("errors when you try to add it again", {
+  expect_error(convert_sim_var(nc_file, temp.new = temp*2))
+})
+
+test_that("can modify variable with more than one variable function", {
+
+  convert_sim_var(nc_file, crazy_var = temp-u_mean*1000)
+  expect_true('crazy_var' %in% sim_vars(nc_file)$name)
+})
\ No newline at end of file