Rinko DO - compensated for temperature using the Rinko temperature sensor
RINKO formula revision
In November 2013, JFE Advantech introduced a revised version of the equation relating sensor voltage output to dissolved oxygen saturation. The revised equation is referred to as the (B) version; this example describes the older, original equation. For a description of the (B) version, refer to Example 11.
Sensors calibrated according to the (B) version of the equation should be labelled as such, so if the sensor has no label it probably uses this original equation. However, there is always the possibility that this label was not applied or is now missing; for example, older sensors returned to the manufacturer to be recalibrated will be processed using the (B) equation, and may not be re-labelled. The most reliable way to determine which equation applies is to check the most recent calibration certificate from the manufacturer.
It is obviously important that the correct equation is used by the logger, which requires the correct channel type to be set in the logger's configuration (see the channel command, and the Section Supported Channel Types). The original equation described in this example requires channel type "doxy02". If you believe that the logger configuration is not correct for your sensor, please contact RBR Ltd for assistance.
Let us consider the RBRconcerto³ C.T.D.DO, where the DO (dissolved oxygen) channel is a Rinko-III sensor from JFE Advantech. This sensor also has its own temperature output, used to compensate the temperature dependence of the raw DO output, and to monitor this requires a fifth channel in the instrument, Tr. See the section corr_rinkotemp - Correction of Rinko Dissolved Oxygen using logger Temperature sensor for details of this temperature channel.
The equation for the temperature corrected output of the DO sensor, as provided by JFE Advantech, is:
where
A, B, C, D, F are coefficients provided by JFE Advantech
N is the DO sensor output voltage in Volts
Tr is the compensating temperature, also from the Rinko-III sensor
The form of this equation used by the instrument is:
where
x0 , x1 , x2 , x3 , x5 map directly and trivially to the coefficients A, B, C, D, F
value( n0 ) is the compensating temperature
Note carefully that in this example the value of n0 would be 5, using the temperature measured by the Rinko-III sensor itself. The temperature measured by the logger on Channel-2 is not suitable for direct use, because it does not have a time response which matches that of the Rinko-III DO sensor.
DOtcomp is then the input to a simple linear equation, which according to JFE Advantech can be further compensated for pressure effects as follows:
where
E, G, H are coefficients provided by JFE Advantech
P is pressure in Mpa
DOcorr is the fully corrected dissolved oxygen output in percentage saturation
The G, H coefficients can be modified by the user to update the calibration of the sensor; the remaining coefficients should not need to be modified.
Casting the equation into the form used by the instrument gives:
where
c0 , c1 are G, H
x4 is E
n1 is the index of the pressure channel, in this example 3
value ( n1 ) is the final output value of the pressure channel in dbar
The instrument correctly accounts for the fact that value( n1 ) is in dbar but the value of coefficient x4 (E) is determined assuming the pressure to be in MPa.