With the RBRlegato³ C.T.D logger, the conductivity reading from the conductivity channel without corrections is given by a simple linear function:
$Craw = c_0 + c_1 \cdot c_2 \cdot R$where R is the normalized voltage ratio from the channel monitoring conductivity, c0,c1 are the core coefficients of the linear equation, along with c2 which is a geometrical factor (K-factor) reflecting the final installation of conductivity cell, 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:
where
- Craw is the uncorrected conductivity, c0 + c1 × c2 × 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). Confirm the channel type. Set the core coefficients. Set the cross-channel correction coefficients. Request confirmation of all calibration coefficients.Examples
<< calibration 1 type = cond13
<< calibration 1 type = cond13, datetime = 20171201000000, c0 = 0.2346, c1 = 153.4873, c2 = 1.0001 x0 = 0.2003, x1 = 0.2943, x2 = 0.0051, x3 = 0.0850, x4 = 0.0001, x5 = 15.0280, x6 = 10.0025, n0 = 2, n1 = 3