--- layout: post title: ITS-90 temperature scale tags: [oce,R] category: R year: 2015 month: 5 day: 10 summary: Oce will soon be getting functions to convert between the IPTS-68 and ITS-90 temperature scales. This posting investigates how much difference this might make in practical work. --- # Introduction Recently, oce has been gaining flexibility in terms of conductivities stored in data files. This is necessitated by the fact that RBR files store conductivity in mS/cm, whereas calculations for seawater properties use the unitless conductivity ratio. With the CTD code under examination for this work, it might make sense to also handle temperatures stored in files. The two choices for that seem to be the IPTS-68 and ITS-90 conventions [1,2], and a natural question is whether using ITS-90 temperatures in formula designed for IPTS-68 will yield errors of practical significance. # Functions The following are functions for the conversion, as suggested in [1]. ```{r} T90toT68 <- function(T90) 1.00024 * T90 T68toT90 <- function(T68) T68 / 1.00024 ``` # Test of inferred density First, define some base quantities ```{r,results="hide",message=FALSE} library(oce) S <- 35 T90 <- 20 p <- 100 ``` and then do some calculations, e.g. ```{r} T90toT68(T90) ``` This temperature difference, `r abs(T90-T90toT68(T90))`, is several times larger than the SBE911plus initial accuracy of 0.001 C as stated at [3]. (It is about double the stated stability over a year of drift.) Another test is of density: ```{r} swRho(S, T90, p) # incorrect swRho(S, T90toT68(T90), p) ``` Finally, the following tests the amount that salinity would need to be adjusted to compensate (in density terms) for a temperature misapplication. ```{r} rho0 <- swRho(S, T90toT68(T90), p) uniroot(function(S) swRho(S, T90, p) - rho0, lower=34, upper=36)\$root ``` In a practical application, one might compare this salinity difference, `r S-uniroot(function(S) swRho(S, T90, p) - rho0, lower=34, upper=36)\$root`, with expected inaccuracies in measurement, or perhaps with the inter-sample "noise". # References and resources 1. [Seabird Electronics application note on temperature conversion](http://www.seabird.com/sites/default/files/documents/appnote42Feb14.pdf) 2. [Saunders 1990](http://www.nodc.noaa.gov/woce/wdiu/wocedocs/newsltr/news10/news10.pdf) article on IPTS-68 to ITS-90 conversion, in WOCE newsletter Sept 1990 number 10, page 10. 3. [Seabird Electronics SBE911plus specifications](http://www.seabird.com//sbe911plus-ctd) 4. [Oce website](https://dankelley.github.io/oce/) 5. Jekyll source code for this blog entry: [2015-05-10-ITS90-temperature-scale.Rmd](https://raw.github.com/dankelley/dankelley.github.io/master/assets/2015-05-10-ITS90-temperature-scale.Rmd)