How to Convert Conductivity to Salinity

The equation that converts conductivity to salinity is quite lengthy. Fortunately, online calculators are available to simply the calculation, or at least to check your work after doing the lengthy calculations. The main stumbling block, aside from all the arithmetic, is getting your measurements into the proper units. Note, for example, that the SI units for conductivity are siemens per meter (S/m). A siemen is the reciprocal of an ohm, also called a "mho."

Instructions

1. Calculate the conductivity ratio R by dividing the conductivity by 4.2914 S/m.

2. Calculate r-sub-t, a function of temperature "t," by calculating c0+c1xt+c2xt^2+c3xt^3+c4xt^4, where the caret "^" indicates exponentiation. Let "t" be the solution temperature in Celsius. The parameters "c" are, from c0 to c4 respectively: 0.6766097, 2.00564x10^-2, 1.104259x10^-4, -6.9698x10-7, 1.0031x10^-9.

3. Calculate R-sub-p, a function of pressure "p" in decabars and temperature in Celsius, as follows: 1 + p x [e1+e2xp+e3xp^2] / [1+d1xt+d2xt^2+(d3+d4xt)xR]. Note that you calculated R in Step 1. Here, the "e" parameters are, respectively: 2.070x10^-5, -6.370x10^-10, 3.989x10^-15. The "d" parameter are, respectively: 3.426x10^-2, 4.464x10^-4, 4.215x10^-1, -3.107x10^-3.

4. Calculate R-sub-t, a function of R, R-sub-t and r-sub-t as follows: R/(R-sub-p x r-sub-t).

5. Calculate "S as follows: [t-15]/[1+k(t-15)] x [b0 + b1x√(R-sub-t) + b2xR-sub-t + b3xR-sub-t^(3/2) + b4xR-sub-t^2 + b5xR-sub-t^(5/2)]. Here, the parameters "b" are, from b0 to b5, respectively: 0.0005, -0.0056, -0.0066, -0.0375, 0.0636 and -0.0144. Also, remember that k=0.0162. Here, √ refers to the square root of the argument.

6. Calculate the salinity "S," in PSUs, by calculating a0 + a1x√R-sub-t + a2xR-sub-t + a3xR-sub-t^(3/2) + a4xR-sub-t^2 + a5xR-sub-t^(5/2) + "S. PSU stands for "practical salinity unit." One PSU refers to 1 gram (g) of salts per 1,000g of solution. More intuitively, it is about 1g per liter of seawater. In this equation, the parameters "a0" to "a5" are, respectively 0.0080, -0.1692, 25.3851, 14.0941, -7.0261, 2.7081.

Tips & Warnings

• Since there are so many opportunities for arithmetic errors, double-check your work against an online calculator.

• The equations are valid over the temperature range -2 to 35 degrees Celsius. The valid range of "S" for these equations is 0.2 to 42 PSU.

References

• United Nations Educational, Scientific and Cultural Organization
• Johns Hopkins Applied Physics Lab Calculator