reefclown
29-09-05, 16:36
There apperas to be an awfull lot of confusion about these terms and what they mean with relation to specific instruments, would be nice if we could get a collective understanding and put something usefull in the FAQ section.
My fave related quote "kim "
If people don't know to which index of SG their instrument (hydrometer or refractometer) is calibrated, they can't possibly know their density to an accuracy any better than +/- 0.004 g/cc approx. So measuring to 0.001 is really silly.
That uncertainty is +/- 5 % on the salinity scale (we are targetting about 35 %), so it is quite considerable. Could easily lead to low alkalinity, for example.
Haven't heard of many tanks crashing for this reason, but fusspots like me would sleep a little less well. My moral....know your SG first ! Targeting an SG without knowing which one the instrument reads is careless."
In general, if you don't know your salinity then it's pretty much pointless trying to get other paramaters into a particular range.
A snippet to bring the point home from AndyH
The really bad thing about hypo-salinic tanks (ie. less that 1.0264 @ 25C) is that it screws up all the other measurements for example Ca, alk, Mg, iodine (in its various forms) etc etc etc and people are seemingly obsessed with having 400 ppm Ca in their tanks whereas at 31 ppt (S=31) one would only expect the Ca level to about 350ppm.If you want to shoot for 31ppt then at least expect all the ions to be 12% under normal values.
Firstly, typicall instruments
Hydrometers
-----------
Sea Test - Swing arm
deep six - Swing arm
TMC Glass Jobbie
e.t.c
Kim on Hydrometers
It is to be used at 25 C (ie that is the temperature at which it is calibrated). The reading is a specific gravity, and in this case the SG is computed as the density of the sample divided by the density of water at 4 C. This is the most common definition of SG quoted in relation to oceanographical work, apparently.
Since the density of water at 4 C is pretty much 1 g/cm, the SG measured by the hydrometer is, in fact, close to being numerically equal to the density of sample.
If you want to be fussy, then because the density of water at 1 atm and 4 C is in fact 0.99997 g/cm, to get the true density just subtract 0.00003 g/cm from the hydrometer reading. But I would guess that this is finer than the instrument is capable of measuring so it's easy to see why this was described as a density hydrometer.
simon on Hydrometers
"Density is as its says density of water.
SG is an expression of density in relative to pure water at a reference temperature. Natural seawater has a SG of 1.024-1.026 and is equivalent to 35ppt salinity
If you are saying your reading by that meter is 1.024 I would say your sg is approx =1.028
If you had said it was 1.026 then would be sg 1.030 aprrox
My meter reads same as my iks probe set to density when both say 1.022 my salinity is 35ppm again by probe."
simonh "Using the TMC hydrometer at 25C a specific gravity of 1.0264 is equivalent to a natural seawater salinity of 35 PSU."
Can we come up with a simple instruction set and a set of reference tables that will allow users to use the above instruments within reason?
Refractometers?
It is an ATACO seawater model (not saltwater=brine) model? Salt water refractometers read about 0.15 "units" out vs seawater. Atago S/Mill-E and the ATC S/Mill-E (ATC = temperature compensated) at £200 is a tad expensive though.
the general blurb "RHS-10ATC dual scale portable Salinity Refractometer in a blue, foam-lined hard case. The RHS series of portable refractometers is designed to measure the salinity of sea water and saline solutions of similar concentration. One scale checks the NaCl levels with the range of 0-100 ppt (with 1 ppt scale divisions) and the other scale gauges Specific Gravity with a range of 1.000 to 1.070 (+/- 0.001 accuracy). Both enable the direct determination of salinity in water that contains dissolved salt and little or no other dissolved solids.
Hydrometers are nice, but they can be inaccurate. The RHS-10ATC is designed to be very accurate to protect your investments!
All RHS models use ambient light, no battery or power source is required, making them truly portable. Models with the “ATC” suffix are equipped with “Automatic Temperature Compensation” for accurate measurements without recalibration after shifts in ambient working temperature (field use).
Included are: a pipette for dropping test fluids onto the prism (never dip the refractometer into the test fluid!), a mini screwdriver for scale calibration, and a Users Guide"
As a general rule, can we agree on the values that will represent s=35 (35ppt) on a brine refractometer (as it's what most have).
Electronic Gizmos
IKS
AndyH "I set the reading to salinity rather than density so have no idea if that causes problems. Since both density and SG are temperature dependent I don't see the point in either of those units of measure. "
andyh "Similarly with conductivity. NSW should be 53mS/cm. "
So, if using an IKS, stick with reading your measurements in Salinity!
Tunze, the reference tables supplied by Tunze are inaccurate ?
Kim provides a more accurate table
If you convert your SG to density, then for "standard" seawater at 25 C:-
Density...........Conductivity.........Salinity
1.01207..............32.17.................20
1.01582..............39.26.................25
1.01957..............46.21.................30
1.02334..............53.02.................35
1.02712..............59.74.................40
Density (g/cm3), conductivity (mS/cm), salinity (g/kg).
(taken from Kaye and Laby, Physical and Chemical Constants, 14th Edition).
this is close to the IKS supplied table.
If using a Tunze, use the table above.
No real issues in this area. i.e 53ms/cm = s=35.
What is SG
by Kim ?
My point about SG is that there are many definitions of it. I can't remember them all, but let me illustrate with an example, rather than waving arms and confusing myself too..
Definition 1
SG is the density of the sample divided by the density of pure water at 4 C. (Density of pure water at 4 C is 1.0000 g/cc (almost).)
Definition 2
SG is the density of the sample divided by the density of pure water at 25 C.
Density of pure water at 25 C is 0.9970 g/cc)
Assume that you have a hydrometer (or refractometer) which measures the SG of water at 25 C, and that your water has a salinity of 35 %. That is a density of 1.0233 g/cc at 25 C. Note that the temperature at which a hydrometer works correctly is usually not the "Definition Temperature" - it's just a result of the design.
Hydrometer 1 will read 1.0233, hydrometer 2 will read 1.0264. They are both correct - they are measuring different things. So it's imporatnt to know what they measure ! If you don't know this, arguing about 0.001 is just not useful.
Hope that helps. If nothing else, hope it suggests that SG is just a ridiculous concept anyway. Invented by brewers on a Friday afternoon....
what is Salinity?
kim
ppt is weight per weight, but if you want to be fancy, seawater salinity is quoted as (eg) 35, not 35 ppt, and it is no longer defined in terms of weight per weight but electrical conductivity relative to a chemical standard. Hence no units....it is like a percentage. Not a lot of people know that......Even fewer care.)
what's the point by Glenn.
I think the problem i have with the debate over the benifits of a refractometer are the principles of the equipment.
Basically any measuring instrument is made to a degree of accuracy and precision, typically related to the cost of the kit.
Accuracy is how close to a measured value the instrument can get and precisiosn being the ability to reproduce the results.
Simply being a refractometer does not mean an insturment is good at both these concepts.
It may be that the refractomer suffers from fewer inherent problems than the swinarg thing but it does not necessarily mean its either more accurate or precise because its a refractometer.
For the record i dont think that swing arm devices are particulalry well made, accurate or precise.
I do think that a hydrometer has every chance of being both accurate and precise and there is no reason why they should be discounted as a reliable means for measuring the specific gravity of out tanks.
Finally i do wonder what we are going to do with the degreee of accuracy that we often see spoken of.
IF you have a 150 gallon system like mine, the top off adds roughly 500ml per addition, so i dont suppose the sg varies much. If i didnt have an auto top off i would add around 2 gallon per day and the percentage change would be larger.
If you translate these kind of issues to smaller tanks the percentage change over a day can be quite dramatic in the.
So why do we need to worry if we measure the sg to within say 0.001 ?
i.e. even if we have an accurate measure, what can we do with it, if it can't be used reliably to maintain s=35 then what's the point?
and finally
andyh "About the only thing we can rely on is a calibrated (with certificate) hydrometer and thermometer."
So once you've all got Kim and Gleen to translate the above into something usefull we might finally have a single reference point for the future :whistling: :D
over to you
My fave related quote "kim "
If people don't know to which index of SG their instrument (hydrometer or refractometer) is calibrated, they can't possibly know their density to an accuracy any better than +/- 0.004 g/cc approx. So measuring to 0.001 is really silly.
That uncertainty is +/- 5 % on the salinity scale (we are targetting about 35 %), so it is quite considerable. Could easily lead to low alkalinity, for example.
Haven't heard of many tanks crashing for this reason, but fusspots like me would sleep a little less well. My moral....know your SG first ! Targeting an SG without knowing which one the instrument reads is careless."
In general, if you don't know your salinity then it's pretty much pointless trying to get other paramaters into a particular range.
A snippet to bring the point home from AndyH
The really bad thing about hypo-salinic tanks (ie. less that 1.0264 @ 25C) is that it screws up all the other measurements for example Ca, alk, Mg, iodine (in its various forms) etc etc etc and people are seemingly obsessed with having 400 ppm Ca in their tanks whereas at 31 ppt (S=31) one would only expect the Ca level to about 350ppm.If you want to shoot for 31ppt then at least expect all the ions to be 12% under normal values.
Firstly, typicall instruments
Hydrometers
-----------
Sea Test - Swing arm
deep six - Swing arm
TMC Glass Jobbie
e.t.c
Kim on Hydrometers
It is to be used at 25 C (ie that is the temperature at which it is calibrated). The reading is a specific gravity, and in this case the SG is computed as the density of the sample divided by the density of water at 4 C. This is the most common definition of SG quoted in relation to oceanographical work, apparently.
Since the density of water at 4 C is pretty much 1 g/cm, the SG measured by the hydrometer is, in fact, close to being numerically equal to the density of sample.
If you want to be fussy, then because the density of water at 1 atm and 4 C is in fact 0.99997 g/cm, to get the true density just subtract 0.00003 g/cm from the hydrometer reading. But I would guess that this is finer than the instrument is capable of measuring so it's easy to see why this was described as a density hydrometer.
simon on Hydrometers
"Density is as its says density of water.
SG is an expression of density in relative to pure water at a reference temperature. Natural seawater has a SG of 1.024-1.026 and is equivalent to 35ppt salinity
If you are saying your reading by that meter is 1.024 I would say your sg is approx =1.028
If you had said it was 1.026 then would be sg 1.030 aprrox
My meter reads same as my iks probe set to density when both say 1.022 my salinity is 35ppm again by probe."
simonh "Using the TMC hydrometer at 25C a specific gravity of 1.0264 is equivalent to a natural seawater salinity of 35 PSU."
Can we come up with a simple instruction set and a set of reference tables that will allow users to use the above instruments within reason?
Refractometers?
It is an ATACO seawater model (not saltwater=brine) model? Salt water refractometers read about 0.15 "units" out vs seawater. Atago S/Mill-E and the ATC S/Mill-E (ATC = temperature compensated) at £200 is a tad expensive though.
the general blurb "RHS-10ATC dual scale portable Salinity Refractometer in a blue, foam-lined hard case. The RHS series of portable refractometers is designed to measure the salinity of sea water and saline solutions of similar concentration. One scale checks the NaCl levels with the range of 0-100 ppt (with 1 ppt scale divisions) and the other scale gauges Specific Gravity with a range of 1.000 to 1.070 (+/- 0.001 accuracy). Both enable the direct determination of salinity in water that contains dissolved salt and little or no other dissolved solids.
Hydrometers are nice, but they can be inaccurate. The RHS-10ATC is designed to be very accurate to protect your investments!
All RHS models use ambient light, no battery or power source is required, making them truly portable. Models with the “ATC” suffix are equipped with “Automatic Temperature Compensation” for accurate measurements without recalibration after shifts in ambient working temperature (field use).
Included are: a pipette for dropping test fluids onto the prism (never dip the refractometer into the test fluid!), a mini screwdriver for scale calibration, and a Users Guide"
As a general rule, can we agree on the values that will represent s=35 (35ppt) on a brine refractometer (as it's what most have).
Electronic Gizmos
IKS
AndyH "I set the reading to salinity rather than density so have no idea if that causes problems. Since both density and SG are temperature dependent I don't see the point in either of those units of measure. "
andyh "Similarly with conductivity. NSW should be 53mS/cm. "
So, if using an IKS, stick with reading your measurements in Salinity!
Tunze, the reference tables supplied by Tunze are inaccurate ?
Kim provides a more accurate table
If you convert your SG to density, then for "standard" seawater at 25 C:-
Density...........Conductivity.........Salinity
1.01207..............32.17.................20
1.01582..............39.26.................25
1.01957..............46.21.................30
1.02334..............53.02.................35
1.02712..............59.74.................40
Density (g/cm3), conductivity (mS/cm), salinity (g/kg).
(taken from Kaye and Laby, Physical and Chemical Constants, 14th Edition).
this is close to the IKS supplied table.
If using a Tunze, use the table above.
No real issues in this area. i.e 53ms/cm = s=35.
What is SG
by Kim ?
My point about SG is that there are many definitions of it. I can't remember them all, but let me illustrate with an example, rather than waving arms and confusing myself too..
Definition 1
SG is the density of the sample divided by the density of pure water at 4 C. (Density of pure water at 4 C is 1.0000 g/cc (almost).)
Definition 2
SG is the density of the sample divided by the density of pure water at 25 C.
Density of pure water at 25 C is 0.9970 g/cc)
Assume that you have a hydrometer (or refractometer) which measures the SG of water at 25 C, and that your water has a salinity of 35 %. That is a density of 1.0233 g/cc at 25 C. Note that the temperature at which a hydrometer works correctly is usually not the "Definition Temperature" - it's just a result of the design.
Hydrometer 1 will read 1.0233, hydrometer 2 will read 1.0264. They are both correct - they are measuring different things. So it's imporatnt to know what they measure ! If you don't know this, arguing about 0.001 is just not useful.
Hope that helps. If nothing else, hope it suggests that SG is just a ridiculous concept anyway. Invented by brewers on a Friday afternoon....
what is Salinity?
kim
ppt is weight per weight, but if you want to be fancy, seawater salinity is quoted as (eg) 35, not 35 ppt, and it is no longer defined in terms of weight per weight but electrical conductivity relative to a chemical standard. Hence no units....it is like a percentage. Not a lot of people know that......Even fewer care.)
what's the point by Glenn.
I think the problem i have with the debate over the benifits of a refractometer are the principles of the equipment.
Basically any measuring instrument is made to a degree of accuracy and precision, typically related to the cost of the kit.
Accuracy is how close to a measured value the instrument can get and precisiosn being the ability to reproduce the results.
Simply being a refractometer does not mean an insturment is good at both these concepts.
It may be that the refractomer suffers from fewer inherent problems than the swinarg thing but it does not necessarily mean its either more accurate or precise because its a refractometer.
For the record i dont think that swing arm devices are particulalry well made, accurate or precise.
I do think that a hydrometer has every chance of being both accurate and precise and there is no reason why they should be discounted as a reliable means for measuring the specific gravity of out tanks.
Finally i do wonder what we are going to do with the degreee of accuracy that we often see spoken of.
IF you have a 150 gallon system like mine, the top off adds roughly 500ml per addition, so i dont suppose the sg varies much. If i didnt have an auto top off i would add around 2 gallon per day and the percentage change would be larger.
If you translate these kind of issues to smaller tanks the percentage change over a day can be quite dramatic in the.
So why do we need to worry if we measure the sg to within say 0.001 ?
i.e. even if we have an accurate measure, what can we do with it, if it can't be used reliably to maintain s=35 then what's the point?
and finally
andyh "About the only thing we can rely on is a calibrated (with certificate) hydrometer and thermometer."
So once you've all got Kim and Gleen to translate the above into something usefull we might finally have a single reference point for the future :whistling: :D
over to you