what is the difference between sg and salinity?

[EB847]

who uses a salinity test kit?

 

[pwj1286]

Specific Gravity is the measure of density of a material and can change acorrding to temperature.

Salinity is the measure of dissolved salt content in water.

They are proportional to each other to a certian extent.
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Miscarriage / Still Birth Forum

 

[EB847]

so do i need to test for salinity or just use a refractometer/hydrometer?

 

[Len]

You can test for either for reef-related purposes. A refractometer will usually give you the results in both s.g. and salinity. Hydrometers will only give you readings in s.g.

 

[pwj1286]

Cheap way would be to by a hydrometer, but they are amazingly inacurrate.

Refractormeter would be more accurate then anything else.
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Pain Management Forum

 

[EB847]

what is the best sg level?

 

[Len]

1.023 to 1.030 is okay. I keep mine at 1.025-1.026. Stability, IME, is most important.

 

[Anonymous]

Specific Gravity is the measure of density of a material and can change acorrding to temperature.

Salinity is the measure of dissolved salt content in water.

They are proportional to each other to a certian extent.

Pretty good. For almost all intends and purposes, it is fine. But if you want to know more, read on:
1, both SG and salinity is dimensionalless, but oftentimes you will see S, ppt, or psu for salinity measurement.

2, SG usually assume that the pressure is normal, that is, at sea level. Another assumption is that the ratio (density) to the pure water when the pure water is around 4C (~40F).

2, Salinity usually is derived from conductivity or chlorinity measurement. It is a rather difficult concept. To save myself some embrassment, I will just quote the following:
http://oceanworld.tamu.edu/resources/oc ... r06_01.htm

A Simple Definition
Originally salinity was defined to be the "Total amount of dissolved material in grams in one kilogram of sea water." This is not useful because the dissolved material is almost impossible to measure in practice. For example, how do we measure volatile material like gasses? Nor can we evaporate sea-water to dryness because chlorides are lost in the last stages of drying (Sverdrup, Johnson, and Fleming, 1942: 50).

A More Complete Definition
To avoid these difficulties, the International Council for the Exploration of the Sea set up a commission in 1889 which recommended that salinity be defined as the "Total amount of solid materials in grams dissolved in one kilogram of sea water when all the carbonate has been converted to oxide, the bromine and iodine replaced by chlorine and all organic matter completely oxidized." The definition was published in 1902. This is useful but difficult to use routinely.

Salinity Based on Chlorinity
Because the above definition was difficult to implement in practice, because salinity is directly proportional to the amount of chlorine in sea water, and because chlorine can be measured accurately by a simple chemical analysis, salinity S was redefined using chlorinity:

S = 0.03 + 1.805Cl (6.1)

where chlorinity Cl is defined as "the mass of silver required to precipitate completely the halogens in 0.328 523 4kg of the sea-water sample."

As more and more accurate measurements were made, (6.1) turned out to be too inaccurate. In 1964 UNESCO and other international organizations appointed a Joint Panel on Oceanographic Tables and Standards to produce a more accurate definition. The Joint Panel recommended in 1966 (Wooster, Lee, and Dietrich, 1969) that salinity and chlorinity be related using:

S = 1.80655 Cl (6.2)

This is the same as (6.1) for S = 35.

Salinity Based on Conductivity
At the same time (6.2) was adopted, oceanographers had began using conductivity meters to measure salinity. The meters were very precise and relatively easy to use compared with the chemical techniques used to measure chlorinity. As a result, the Joint Panel also recommended that salinity be related to conductivity of sea water using:

S= -0.08996 + 28.2929729R15 + 12.80832 R215
-10.67869R315 + 5.98624R415 - 1.32311R515 (6.3a)
R15= C(s,15,0)/C(35,15,0) (6.3b)


where C (S, 15 , 0) is the conductivity of the sea-water sample at 15°C and atmospheric pressure, having a salinity S derived from (6.4), and C (35 , 15 , 0) is the conductivity of standard "Copenhagen" sea water. Millero (1996) points out that (6.3) is not a new definition of salinity, it merely gives chlorinity as a function of conductivity of seawater relative to standard seawater.

Practical Salinity Scale of 1978
By the early 1970s, accurate conductivity meters could be deployed from ships to measure conductivity at depth. The need to reevaluate the salinity scale led the Joint Panel to recommend in 1978 that salinity be defined using only conductivity, breaking the link with chlorinity. All water samples with the same conductivity ratio have the same salinity.

The Practical Salinity Scale of 1978 is now the official definition:

Spsu= 0.0080 - 0.1692 R1/215 + 25.3851 RT + 14.0941 R3/2T
-7.0261 R2T + 2.7081 R5/2T + DS (6.4a)
RT= C (S,T,0) / C (KCl,T,0) (6.4b)
DS = [(T - 15) / (1 + 0.0162(T - 15))] + 0.005 - 0.0056 R1/2T - 0.0066 RT (6.4c)
- 0.0375 R3/2T + 0.636 R2T - 0.0144 R5/2T
2 £ S £ 42

where C (S, T, 0) is the conductivity of the sea-water sample at temperature T and standard atmospheric pressure, and C (KCl, T, 0) is the conductivity of the standard potassium chloride (KCl) solution at temperature T and standard atmospheric pressure. The standard KCl solution contains a mass of 32.435 6 grams of KCl in a mass of 1.000 000kg of solution. An extension of (6.4) gives salinity at any pressure (see Millero 1996: 72).