Example 20: corr_cond2 - conductivity corrections for CT cell

CT cell formula revision

In September 2018, RBR Ltd. introduced a new equation for the combined CT cell. Instruments manufactured after the 1st September 2018 are using this equation. The difference between the two corrections is a move from a first- to third-order polynomial for pressure dependence.

Continuing with the RBRconcerto³ C.T.D.pH logger, the conductivity reading from Channel-1 without corrections is given by a simple linear function:

where R is the normalized voltage ratio from Channel-1 monitoring conductivity, c0,c1 are the core coefficients of the linear equation, and Craw is the uncorrected conductivity output, reported in mS/cm for RBR marine instruments.

The equation which corrects the output for the effects of both temperature and pressure on the conductivity cell is:

Casting the equation into the style used by the logger would give:

where

• Craw is the uncorrected conductivity, c0  +  c1 × R,

• x0, x1 correspond respectively to the temperature compensation constants "Kc1", "Kc2"

• x2, x3, x4 correspond respectively to the pressure compensation constants "Kp1", "Kp2", "Kp3"
  

• x5 is the calibration temperature "Tcal" in °C,

• x6 is the calibration pressure "Pcal" in dbar,

• n0 is the index of the internal temperature of the conductivity cell channel, in this example 8,
  value(n0) is the final output value of the internal temperature of the conductivity cell channel in °C,

• 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,

• Ccorr is the corrected output in mS/cm. 

It is quite common to have a logger monitoring conductivity and temperature without a pressure channel, typically deployed at a known, constant depth. In this case, n1 would be set to "value", and so value(n1) would be substituted by a default value (see the "settings pressure" command).

Examples

>> calibration 1 type
<< calibration 1 type = cond11

Confirm the channel type.

>> calibration 1 datetime = 20171201000000, c0 = 0.2346, c1 = 153.4873

Set the core coefficients.

>> calibration 1 datetime = 20171201000000, x0 = 0.2003, x1 = 0.2943, x2 = 0.005, x3 = 0.085, x4 = 0.0001, x5 = 0.0000, x6= 0.0000, x7 = 15.028, x8 = 10.0025

Set the cross-channel correction coefficients.

>> calibration 1
<< calibration 1 type = cond11, datetime = 20171201000000, c0 = 0.2346, c1 = 153.4873, x0 = 0.2003, x1 = 0.2943, x2 = 0.005, x3 = 0.085, x4 = 0.0001, x5 = 15.028, x6 = 10.0025, n0 = 8, n1 = 3

Request confirmation of all calibration coefficients.