Example 11: corr_rinkoB - correction of Rinko dissolved oxygen using 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, and is described in this example. For a description of the older, original equation, refer to Example 5.

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 the 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 (B) version of the equation described in this example requires channel type 'doxy08'. If you believe that the logger configuration is not correct for your sensor, please contact RBR Ltd for assistance.

Consider an RBRconcerto³ C.T.D.DO logger, 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 logger, 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 as follows:

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 logger 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 logger 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 logger 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.

An RBRconcerto³ C.T.D.METS has a METS methane sensor from Franatech in addition to the usual C, T and D channels. The METS sensor also has its own temperature output, used to compensate the temperature dependence of the raw methane output, and to monitor this requires a fifth channel in the logger, Tm. See the section corr_metstemp - temperature measured by a METS (methane sensor) for details of this temperature channel.

The equation for the temperature corrected concentration output of the methane sensor, Cm, as provided by Franatech, is:

where

• c0...c6 are coefficient values provided by Franatech,

• Vm is the voltage in Volts from the sensor's methane output,

• Vt is the voltage in Volts from the sensor's temperature output.

Franatech's calibration documentation may have no formal naming convention for the coefficients, in which case the values will be simply shown in an equation on the calibration sheet. The terms c0...c6 are those used by the logger's calibration command to report or set the values: if updating, be careful to assign the correct values to each coefficient.

The sensor output voltage for methane concentration, Vm, is related to the logger's reported voltage ratio R by a simple linear equation, the coefficients of which (x0,x1) are determined by RBR Ltd at the factory.

The sensor output voltage for temperature, Vt, is an intermediate variable in the calculation of temperature from the supporting channel, and can be back-calculated from that result:

where

•  ct0, ct1 are the primary coefficient values for the supporting temperature channel,

• Tm is the temperature output of that channel in °C, value(n0).

Examples

>> calibration 4 type
<< calibration 4 type = doxy08

Confirm the channel type.

>> calibration 4 datetime = 20171201000000, c0 = 0.346, c1 = 1.08873

Set the "user" coefficients.

>> calibration 4 datetime = 20171201000000, x0 = -41.7148, x1 = 25.425, x2 = -0.08097, x3 = 0.0021, x4 = 4.5e-5, x5 = 0.0

Set the coefficients provided by JFE Advantech.

>> calibration 4
<< calibration 4 type = doxy08, datetime = 20171201000000, c0 = 0.346, c1 = 1.08873, x0 = -41.7148, x1 = 25.425, x2 = -0.08097, x3 = 0.0021, x4 = 4.5e-5, x5 = 0.0, n0 = 5, n1 = 3

Request confirmation of all calibration coefficients.