Q.What is the difference between load line hydrometer and draught survey hydrometer ?

Well as the name suggests, 

Draft survey hydrometer is used to know the density of the sea water that will be used during draft survey for calculating the weight of cargo loaded.

And Loadline hydrometer is used to know the density of sea water that will be used for calculating the compliance with the loadline convention.

But isn't the density of water same ? So if these two hydrometers are supposed to give different densities for the same water, is one of these incorrect ?

Absolutely not !!!

Draft survey measures the actual density of the water. Whereas Loadline hydrometer measures the Apparent density (also called specific gravity) of the water.

Let us see what is the difference !!!

Specific gravity is the density in relation to something. When we measure the specific gravity of water, it is in relation to the fresh water at 15 C.

So if specific gravity of sea water as measured with loadline hydrometer is 1.025, it means that the density of water in relation to the fresh water at 15 C is 1.025.

Specific gravity is a ratio, a Number and does not have any unit.

Now what is the actual density of water if the specific gravity is 1.025.

The fresh water density at 15 C is 0.999 Kg/L in vacuum. So the actual density of water with specific gravity 1.025 will be 

1.025 x 0.9991 = 1.024 Kg/L in vacuum

The correction for converting this density from vaccum to the air is 0.0011. So actual density of sea water (with specific gravity of 1.025) in air will be

1.024-0.0011 = 1.023 Kg/L in air

Now If we measure the density of the same sea water with draft survey hydrometer, it will give the reading of 1.023 Kg/L in air.

When to use which Hydrometer ?

If you want the dock water density to know how much you can submerge the load line and still comply with the load line convention, use loadline hydrometer. This will give you the specific gravity (apparent density) of the sea water. 

But when calculating the weight of cargo loaded by draft survey, we want to know the actual sea water density (and not specific gravity). In this case we must use draft survey hydrometer.

How to distinguish between Loadline and Draft survey hydrometer ?

Both the hydrometers can be distinguished by the marking on the hydrometer.

As loadline hydrometer measures specific gravity or Relative density, it will have the marking of notation RD or Sp. Gr.

Draft survey hydrometers have the unit of the density and temperature marked on these. For example it may have the marking as Kg/L at 15 C.

Ship staff must use the correct hydrometer for the intended use. If another hydrometer is used, proper corrections must be applied to it.

Re question how to recognize the hydrometers.

A) Zeal's load line hydrometers are brass standard 60F hydrometers, while Carltons load line hydrometers are 15.6C glass hydrometer i.e. both have readings i.e. both have - readings = SG60F = SG15.6C = 1 in pure water at 60F = 15.6 C and density = 0.999 T/m3;

-readings = SG60F = SG15.6C = 1.0001 in pure water at 15C and density = 0.9991 T/m3.

Temperature of calibration is given on the stem of hydrometers.

B) draft survey hydrometers are glass hydrometers calibrated at 15C and in pure water at 15C they have readings = SGair = density in air = density - 0.0011 T/m3 = SGvac - 0.0011 T/m3 = 0.998 T/m3;

c) density hydrometers are usually calibrated at 20C (OIML standard) or at 15C (oil industry standard). In pure water at 20C they have readings = 0.9982 T/m3 , while at 15C they have readings =0.9991 T/m3.

Consequently in FW refered to since 1930 by Conventions on load lines as "fresh water of unit density":

a) load lines hydrometers have readings = SG60F = SG15.6C = 1.001;

b) density hydrometers have readings = SGvac = density = 1 T/m3;

c) draft survey hydrometers have readings = SGair = density in air = 0.9989 T/m3.

Kind regards,

A. Bratek

bratek_a@op.pl 

 

Hi,

According to OIML R33 there are two results of weighing of mass:

a) mass = true mass = weight in vacuo;

b) conventional mass = mass in air = apparent mass = weight in air.

Other way the result of weighing of the same mass = 1 m3 of "fresh water of unit density" equals:

a) mass = 1 T;

b) weight in air = 0.998905 T/m3.

 

Consequently "fresh water of unit density" has:

a) density = 1 T/m3 = reading of standard metric hydrometer;

b) SGair = 0.998905 T/m3 = reading of imperial standard hydrometer = draft survey hydrometer.

 

In relative metric weight system the same water has SG = 1 and pure water at 60F (https://en.wikipedia.org/wiki/Gallon ) has SG = 0.998859 i.e. is 1.001142303 lighter than "fresh water of unit density" and has density 1.001142303 times lesser than "fresh water of unit density".

In relative imperial weight system pure water at 60F has SG60F = 1 and SGair = apparent density = 10 lb/UK gallon = 0.997765 T/m3, while water of SG60F = 1.001142303 has:

a) density = SG60F * density of pure water at 60F = 1 T/m3 = reading of standard metric hydrometer;

b) SGair = SG60F * SGair of pure water at 62F = 10.01142303 lb/UK gallon = 0.998905 T/m3 = reading of draft survey hydrometer.

Other way density = apparent density + 0.001094 T/m3. In oil industry however density = apparent density + 0.0011 T/m3 and to match that industry draft survey hydrometer has reading = standard metric hydrometer - 0.0011 T/m3.

 

In relative imperial weight in air = apparent mass system standard water of SG62F = 1 is pure water of SGair = 10 lb/UK gallon. If one assumes that such water is "fresh water of unit density" and SGair = 0.997765 T/m3, then he has to assume that SW is water of SG62F = 1.025 and SGair = 10.25 lb/UK gallon = 1.02271 T/m3 = 63.845 lb/ft3.

Except for the world of fools SW was however water of density = 64 lb/ft3 = 1024 oz/ft3 and SG60F = 1.026208 in imperial world, while water of density = 1.025 T/m3 and SG= 1.025 in metric world i.e.: water of SG62F = 1.025 for stability purposes has specific weight = density = 1.025 * 0.998859 T/m3 = 1.02383 T/m3 = 63.915 lb/ft3 and is dock water of:

a) SG = 1.02383;

b) SG15 = 1.02475;

c) SG60F = 1.02485;

d) SG62F = 1.025;

e) SG20C = 1.025673.

 

According to Messrs Zeal their load line hydrometer is standard 60F hydrometer i.e.:

a) in imperial SW of density = 64 lb/ft3 has reading = 1.0262;

b) in metric SW of density = 1.025 T/m3 has reading = 1.02603. 

 

If one understands what 1 Tonne means, then he has no problem to understand what is mass and what is "fresh water of unit density" and consequently SW for stability purposes.

A. Bratek

bratek_a@op.p