Logger3 command reference rev. C
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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:

$Craw = c_0 + c_1 \cdot R$

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:

$Corr = \dfrac{Craw - Kc_1 \cdot (T - Tcal)}{1 + Kc_2 \cdot (T - Tcal) + ( Kp_1 \cdot (Pcorr - Pcal) + Kp_2 \cdot (Pcorr - Pcal)^{2} + Kp_3 \cdot (Pcorr - Pcal)^{3}) }$

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

$Corr = \dfrac{Craw - x_0 \cdot (value(n_0) - x_5)}{1 + x_1 \cdot (value(n_0) - x_5) + (x_2 \cdot (value(n_1) - x_6) + x_3 \cdot (value(n_1) - x_6)^{2} + x_4 \cdot (value(n_1) - x_6)^{3})}$

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 temperature channel, in this example 2,
    value(n0) is the final output value of the temperature 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 = 15.028, x6 = 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 = 2, n1 = 3

Request confirmation of all calibration coefficients.

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