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 IPTS68 and ITS90 conventions [1,2], and a natural
question is whether using ITS90 temperatures in formula designed for IPTS68
will yield errors of practical significance.
Functions
The following are functions for the conversion, as suggested in [1].

T90toT68 < function(T90) 1.00024 * T90
T68toT90 < function(T68) T68 / 1.00024

Test of inferred density
First, define some base quantities

library(oce)
S < 35
T90 < 20
p < 100

and then do some calculations, e.g.

T90toT68(T90)

## [1] 20.0048
This temperature difference, 0.0048, 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:

swRho(S, T90, p) # incorrect

## [1] 1025.199

swRho(S, T90toT68(T90), p)

## [1] 1025.198
Finally, the following tests the amount that salinity would need to be adjusted to
compensate (in density terms) for a temperature misapplication.

rho0 < swRho(S, T90toT68(T90), p)
uniroot(function(S) swRho(S, T90, p)  rho0, lower=34, upper=36)$root

## [1] 34.99833
In a practical application, one might compare this salinity difference,
0.0016675,
with expected inaccuracies in measurement, or perhaps with the intersample “noise”.
References and resources

Seabird Electronics application note on temperature conversion

Saunders 1990
article on IPTS68 to ITS90 conversion, in WOCE newsletter Sept 1990 number 10, page 10. 
Jekyll source code for this blog entry: 20150510ITS90temperaturescale.Rmd
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