D.O. Probe calibration

[Mr31415]

There used to be an article on here somewhere on how to calibrate a D.O. probe - but I cannot find it. Any links?

[Boomer]

There are no articles on actually calibrating DO meters but there have been a couple of discussions on it.

What do you need to know and who's DO meter is it ?

[Mr31415]

My Hanna .

I want to calibrate DO and ORP. I guess ORP is easy - I just need to buy the calibration fluid.

However DO is another story. I can calibrate it using DO saturation or DO Concentration. For DO saturation it almost seems like the meter can do a 2 point calibration but that is not so. It never asks for the second point.

So I took 1L of RO water, heavily aerated it and used a small powerhead for surface agitation. I filled my little membrane with electrolyte, shook out the bubbles and fitted it to the probe. It read 3.2mg/L. And stayed there. So I repeated the procedure about 3 times.... The last time of fitting the membrane to the probe I managed it to read about 10mg/L - so finally I could actually calibrate the unit to 7.6mg/L (the value pure water at 20C and our barometric pressure should be when fully saturated). The other times the unit said "invalid point".

I then took my Hanna 0 oxygen solution - but the probe only read 35% saturation. I saw that the expiry date on the fluid was 04/07 - that could have had something to do with it, right?

I measured my tanks yesterday night - about 4 hours after lights out. Reef - 4.6mg/L. Predator - 4.8mg/L. QT - 5.0mg/L. Tiny 6l freshwater tank with airstone and 3 small mollies (don't ask) - 6.2mg/L. All tanks are at approx. 27C.

So... Have I done it correctly?

PS: I used the Air Saturated Water method

[Mr31415]

Regarding the 0% fluid - it was bad. I just purchased a new bottle and it measured 0% properly. The guy at the lab used normal air to calibrate the 100% point - not so sure whether this is correct as I am very sure 100% in air is not equals to 100% in seawater.

[Mr31415]

I now took 1 L RO water, aerated it and used a small pump to move the water for about 5 minutes. I used the DO Concentration calibration mode and chose 6.5mg/L as calibration point (my water was much warmer).

I tested my predator tank about 6 hours after lights on - it was at 5.6mg/L saturation.

So I decided to try another method. I took 200ml of tank water, aerated it heavily for 5 minutes then turned it off and swirled the probe in it. I set the value on which it stabalised to 100% (this time I used the DO Saturation calibration method), then I used 0 DO fluid and calibrated the 0 point to that.

When I tested my aquarium I got 105.6% saturation. Does this seem more reasonable? I have a *huge* skimmer on that tank - VERY good flow (about 45000l/h), and strong lights (950W MH and 216W of T5).

So is this the correct value?

I had some troubles with my pressure calibration (as this affects the DO quite a bit). I am at 1.5km above sea level - according to my weather station I am at 762.8mmHg. The Hanna shows a factory calibrated value of something like 646mmHg. I cannot calibrate it to that level as it says it cannot adjust by more than 40 units. So I hope this has not influenced the calibration. As I understand it when using saturation then temp and pressure does not matter as long as both are the same as when you calibrated the meter - correct?

[Boomer]

Mr

Ok, the meter has to be adjusted for Salinity otherwise it will give false readings. And yes you often just calibrate it to normal air but then need to set it to Seawater @ 35 ppt, 53,000uS or 53mS, It looks fine for FW. The new fluid is fine you say but it still has to be calibrated to seawater. It is the Chloride ion in Seawater that throws it off. Here is a link to a FW or Seawater DO at sat. Just type in Salinity, temp and elevation

http://www.aquanic.org/images/tools/oxygen.htm



Form the link last link

CALIBRATION Specific-conductance standards, 10 to 50,000 uS/cm at 25 C

Did you do the procedure for seawater using the EC meter part. Often on meters like yours you just dial in a EC value like 53,000 uS or 35 ppt. Some meters ask you for Salinity but may not, so you just set or dial it in.

Seawater is 53,000 uS.

[Mr31415]

Eish you are talking a bit above me. My hanna cannot (AFAIK) be calibrated for DO in any other way than using DO Saturation 2-point at 100% and 0%, or Concentration 1-point 4mg/L - 50mg/L

How about my final attempt? According to your calculator saturated seawater at 36ppt and 27C should be at 6.5mg/L - so could I aerate the sea water and then do a point calibration using DO concentration at 6.5mg/L?

[Mr31415]

From the manual:

"If D.O.% saturation range is calibrated, D.O. concentration range will be cali-
brated too, and viceversa.
The D.O. % saturation value is the result of a comparison between the concen-
tration of oxygen in air (100 % ) and in water, so it is suggested to calibrate the
probe near the area where the measurements will be taken in order to have the
best accuracy.
Note also that the D.O. concentration values are based on D.O. % saturation,
temperature, salinity and atmospheric pressure so it is recommended to use a
standard solution or a reference D.O. meter to compare readings during cali-
bration.
D.O. % saturation calibration can be performed at 1 or 2 standard points (0 %
and 100 %), or at a single custom point (50 to 500 %).
D.O. concentration calibration can be performed at a single custom point (4 to
50 mg/L). "

[Mr31415]

And also - what about what I wrote on the pressure? Even your calculator (like the tables I used from my link) has pressure in it. Why can I not set my meter to the pressure at my altitude above sea level? Is this a problem on the DO value?

[Boomer]

Le me call then for you, so I can get the whole 9-yards. This meter, the HI 9828, Multiparameter Water Quality Portable Meter, should do altitude and salinity automatically by a key pad stroke on the meter.

so could I aerate the sea water and then do a point calibration using DO concentration at 6.5mg/L?

Yes but how are you going to know if it is undersaturated, saturated or above saturation ???

[Boomer]

Ok they are going to call back. I found a pdf manual on your meter amd yes it can be set to seawaer. Here it is;

http://www.hannainst.com/manuals/manHI_9828.pdf

D.O. concentration
It is possible to select: ppm or mg/l.
Conductivity and Actual Conductivity
It is possible to select: Auto (autoranging both
μS/cm and mS/cm),1 μS/cm, 0.001 mS/cm,
0.01 mS/cm, 0.1 mS/cm, 1 mS/cm, Auto mS
(autoranging in mS/cm unit).
NOTE: Actual conductivity is the conductivity
reading without temperature compensation.
Resistivity
It is possible to select: Ω/cm, kΩ/cm or MΩ/cm.
TDS
It is possible to select: Auto, autoranging both
ppm(mg/l) and ppt(g/l),1 ppm (mg/l), 0.001 ppt
(g/l), 0.01 ppt (g/l), 0.1 ppt (g/l), 1 ppt (g/l),
Auto ppt (g/l) (autoranging in ppt or g/l unit).
NOTE: For setting ppm or mg/l, see paragraph
3.2 System Setup.
Seawater specific gravity
This value is a widely used parameter for seawater; it is similar to density measurement
and it is an expression of salts content in water. It depends on atmospheric
pressure, temperature and salinity.
In the seawater specific gravity menu it is possible to select the reference temperature:
σt, σ0, and σ15. (Current temperature, t=0 °C, t=15 °C respectively).
Atmospheric pressure
It is possible to select: atm, kPA, mmHg, inHg,
mbar, psi.
NOTE: A maximum of 12 measurements can
23
be enabled simultaneously on display.
A warning message appears if trying to
enable more than 12 measurements

[Mr31415]

Hmmmm... I know that (about my meter). I have already set it to sigma-15 (based on a looooooong discussion we had a couple months ago).

But what has my conductivity probe to do with my D.O. probe?

I am so much confused. At the end of the day I really only like to know one thing. How do I calibrate my D.O. probe so that I can get accurate values when used in my reef tank? I.e. 27C and 1.026 SG. I don't know how to build a "Air-Calibration Chamber in Water" - so I need a rather accurate, accessible way of calibrating my probe...

The manual and those links do not help me. I do not know how to construct a

[Boomer]

But what has my conductivity probe to do with my D.O. probe?

It may not on your meter you can just set the DO parameters and "type" in Salinity as ppt or as Sg in the DO parameters menu as seen on the pdf. The meter then compensates for seawater AUTOMATICALLY. In some meters you just input the EC signal reading into the DO meter software. EC is how you measure salinity, 53,000 uS =53mS = 35 ppt = 1.0264 @ 25 C. I would use Salinity as in ppt or 35 ppt. t=15 °C is not t=25 °C, which is more to the temp of your tank. An if you are in the low 80's it would be t=27 °C.

How do I calibrate my D.O. probe so that I can get accurate values when used in my reef tank? I.e. 27C and 1.026 SG.

Calibrate with the solution and then in the DO parameters menu enter 35 ppt once calibrated and pressure. Most meters like this auto correct for barometric pressure. Yours lets you enter pressure, just like salinity. Altitude is not a good way to do it as pressure can change with no change in altitude, due to storm fronts. Just get the reading off of a barometer.

It is all explained in the Pdf

Chapter 3 - SETUP MODE

A few parameters have to be set before taking measurements. In the main menu there are two setup items: Measurement and System setup.

Measurement setup allows to set the displayed readings and their units; system setup allows to set the system parameters, e.g. interface language, date and time, LCD contrast, acoustical signals, etc..

I will know more tomorrow on how it really works

[Mr31415]

I feel very stupid as I still cannot see how any of this has relevance on calibrating my DO probe

I cannot set DO concentration to anything other than ppm or mg/L. And DO saturation is always %.

I have enabled the Salinity as well as SG modes of the EC probe. SG is using sigma-15, since that is the oceanographic standard as we discussed last time - NOT 25C. Sigma-15 corresponds perfectly with my refractometer as well as my Aquatronica probes. SG and salinity both reads accurately based on calibration fluids and comparison to my refractometer and other three aquatronica EC probes.

There is NO place where I can configure the DO menu to include SG, Density, Salinity or any such thing - if it uses this it is all automatically.

The DO can only be set to ppm or mg/L, or I can use % saturation as I explained.

In the calibration I can ONLY either set the 100% and 0% points, OR set a fixed concentration value.

Like I said - " DO parameters menu enter 35 ppt once calibrated and pressure. " is not possible with my meter. I have gone through the PDF 100 times and through each and every menu setting in the unit 200 times - this is not possible.

DO concentration ranges from 4 - 50 ppm - that is a far cry from 35ppt if that is what you meant (which also does not make sense cause the one is dissolved oxygen concentration the other salinity).

Chapter 3 - SETUP MODE - There is nothing I can see that is applicable. My problem is with:

"DISSOLVED OXYGEN CALIBRATION
If D.O.% saturation range is calibrated, D.O. concentration range will be cali-
brated too, and viceversa.
The D.O. % saturation value is the result of a comparison between the concen-
tration of oxygen in air (100 % ) and in water, so it is suggested to calibrate the
probe near the area where the measurements will be taken in order to have the
best accuracy.
Note also that the D.O. concentration values are based on D.O. % saturation,
temperature, salinity and atmospheric pressure so it is recommended to use a
standard solution or a reference D.O. meter to compare readings during cali-
bration.
D.O. % saturation calibration can be performed at 1 or 2 standard points (0 %
and 100 %), or at a single custom point (50 to 500 %).
D.O. concentration calibration can be performed at a single custom point (4 to
50 mg/L).
4.3.1 Procedure
After choosing the D.O. calibration mode in the
main calibration menu, select the type of calibra-
tion with the arrow keys and then press OK.
D.O. % saturation
This calibration starts by default with 100 %.
• Fill the probe beaker with approximately 4 mm
(5/32”) distilled water and screw it on the probe.
• The message NOT READY appears until the
value is stabilized.
• To change standard calibration point, press CAL.POINT.
To insert a different calibration value, after
pressing CAL. POINT, press also CUSTOM and
insert the desired value with the keyboard.
• When the reading is stable READY and CON-
FIRM appear. Press CONFIRM to accept the
calibration point.
• After the first calibration point is confirmed, put the probe in a D. O. 0% satu-
ration standard solution and wait until the measure is stable.
• Press CONFIRM to accept the calibration point.
• At the end of calibration the following messages appear: “Storing data on
probe, please wait...”, “Updating GLP data, please wait ...” and “Calibration
completed”.
• Press OK to return to the calibration menu.
• To return to the main menu, from the calibration menu, press ESC repeatedly.
NOTES: The user can perform a single point calibration with standard values
To abort the calibration after first point is accepted, press ESC.
If the D.O. input is not within the acceptable range, the message “IN-
VALID INPUT” will be shown.
D.O. concentration
To calibrate the D.O. concentration, it is necessary a solution with known con-
centration.
• In D.O. calibration menu, select DO concen-
tration.
• Insert the known value and press OK.
• When the reading is stable, CONFIRM ap-
pears; press CONFIRM to accept the value.
• When the messages “Storing data on probe, please wait...”, “Updating GLP
data, please wait ...” and “Calibration completed” appear the calibration is com-
pleted and the user can return to the main calibration menu by pressing OK or
to the main menu by pressing ESC repeatedly."

I do NOT understand how to do that with sea water - or for that matter a known saturated sea water sample.

[Boomer]

I went back and looked closer at the pdf and it does not appear you can not correct for Salinity. IMHO that is a rip-off for a meter like this and the first one I have see, if this is so, for a meter of this caliber . Normally the meter asks you for the salinity, on some there is a dial you just turn to the salinity you want and others you can imputrom the EC reading or plug in the EC reading.

I will know for sure tomorrow.

SG is using sigma-15, since that is the oceanographic standard as we discussed last time - NOT 25C.

No, not for you . In this t=15 °C mode it will give the the SG of a 15 C hydrometer correct to 25 C. For natural seawater @ 35 ppt that will be 1.0244 and NOT 1.0264

You should set this meter to ppt, which will not put the meter in a "simulated" 15 C hydrometer mode. WE would want a t= 25 °C or t =27°C to be correct.

This pretty much prove my point. NSW is 35 ppt

Type in 15°C calibration, tank temp@ 25 ° and "observed reading of 1.0243 = 35 ppt. And if your observed reading was 1.0264 = 37.8 ppt.

Now do that for a 25° C calibration, tank temp @ 25 C = 35 ppt


http://www.reefkeeping.com/issues/20...conversion.htm

perfectly with my refractometer

Then something is very wrong . Refract are set at the factory to a sample of table salt solution of NaCl. Seawater is not the same. The light does not bend/refract the same in a table salt solution as it does in seawater. So, a well made refract will always read 1.5 ppt to low or , .013 Sg to low. That means the salinity is really 33.5 ppt and not at 35 ppt, even though the refract reads 35 ppt or 1.0264. This is why we have people using PinPoint'ss 53mS for calibrating their refracts, so they are accurate. We even have an article on it. Know one makes a hand-held refract for seawater, it is all a myth

Refractometers and Salinity Measurement
http://reefkeeping.com/issues/2006-12/rhf/index.php

[Mr31415]

I think this time I (for once in my life) can prove you wrong.

I refer you to our nice discussion a year and 2 months ago where you said sigma-15 is correct:

http://reefcentral.com/forums/showthread.php?s=&threadid=914676&highlight=sigma

But I'll wait for later today when you wake up to solve my dilemma.

[Mr31415]

If we want to target 35ppt salinity (which is temperature independent), it means we want to target a specific gravity of 1.0279 if the water was at the reference temperature of 3.98C, 1.0259 if the water was at the PSU reference temperature of 15C. However the water is warmer - if it is 25C then we want to target 1.0234 or at 27C we want to target 1.0227 (since warmer water is less dense so density goes down hence SG). I assume these values are relative to the density of fresh water a reference temperature of 15.6C (the reference temperature of the PSU) - however the impact of this is smallish as fresh water changes density less with changes in temperature than sea water.

Since some of our refractometers and other meters are temperature compensating, it complicates things. And I think this is where my confusion comes in. How I understand it is if my Hanna is set to sigma-t it would show exactly the values above for a salinity of 35ppt - the true density at the current temperature relative to some reference freshwater density - maybe 3.98C or 15.6C - I don't know.

However setting it to sigma-15 means the sample reference temperature is 15C. So for a water sample @ 15C it would show 1.0259 (same as the true value), however at 25C it will perform temperature compensation and interpolate the conductivity (which is the thing it directly measures and then converts to Salinity (I presume)) to what the density is supposed to be at the measured 25C and the measured pressure (for now lets ignore pressure and assume surface water pressure = 1 atm).

What confuses me is what is measured directly and what is interpolated. If the sample temperature is at 25C then what it measures must be originally at 25C. So in that case 15C is the "interpolated" value. Sigh I am confusing myself.

A hydrometer is so easy. If a hydrometer is calibrated for 15C, it simply means it assumes the sample is at a fixed temperature of 15C and measures changes in density based on that. More dense, the higher it will be pushed out of the water and vice versa. Nothing complex here. You read from tables the actual density since water is less dense in our reefs at 25C, thus we need to bring in the second variable T.

But the electrical conductivity meter.... It measures actual conductivity - it cannot measure anything else. So it converts to Salinity (PSU) by taking the actual conductivity it measured, converting that to the temperature compensated conductivity for sea water at 15C and divides that in to the conductivity of a potassium chloride solution also at 15C to get the ratio K, which it then uses in the formula:

S = 0.0080 - 0.1692 K^1/2 + 25.3853 K + 14.0941 K^3/2 - 7.0261 K^2 + 2.7081 K^5/2

to get the Salinity. Fine I get that. So to show my sigma-t value, it merely takes the calculated S (or conductivity directly) and uses the Sea Water Equation of State calculator to calculate the actual density based on the currently measured temperature of the sample - ???. The sigma-15 value does not make sense. How can one temperature compensate based on a reference temperature of 15C when the measured sample is at a different temperature?

For me it would make sense to say since reefs are 27C (lets simplify and keep to this value) and 35ppt, we need to either target 27C and 35ppt on our refractometer or electrical meter (which will fixate the density by dfinition), or if we want to use density we need to set our meters to sigma-t and target 1.0227 - the actual density of seawater relative to the density of freshwater @ some unknown reference - maybe 15C, with the sample at 27C.

[Boomer]

I just got the call from Hanan and you have been sending them e-mails. They tell me for you to call them . They are telling me the DO will automatically compensate for Salinity once Salinity is measured. Just like I said I thought it should

The density of seawater DOES NOT equal the Specific Gravity of Seawater. At a 25 C, D = 1.0233 and Sg = 1.0243. Density and Sg are Not the same thing. This meters sigma t is for density and not Sg.


I think this time I (for once in my life) can prove you wrong.

Nope


I forgot about our other discussion. Although it says Sg in the manual it is really Density. So when you set it to t=15C it is correcting to density and not Sg even though they are telling you Sg. And I forgot to mention that in the other post that t=15C is Density and not SG. So I made an error in my own post that nobody caught, even me, till now.

I finally said 25.3 sigma-15.
It is pretending to correct to a SG meter that is = to a calibrated point at that temp of 18.90C or 25.00C or what ever the sample temp is. Where you want it.

That word SG meter should read Density meter. Here is proof it is Density and not SG

http://www.es.flinders.edu.au/~matto...lecture03.html

Type in 35 ppt @ 25 C

t=23.43 and that is density and it even says density on the button. "calculate seawater density"

and Sg would = 24.34 (1.02434)

type in 15 C cal water temp 25 C and Sg 1.02343 and it equal 35 ppt.

http://www.reefkeeping.com/issues/20...conversion.htm

So. a Density of 1.02343 = Sg 1.02434 = 35 ppt @ 25 C, if one was using a Density hydrometer or a Sg hydrometer set to 15 C and used in a sample of water at 25 C.



So you have to remember that value you are getting as SG is really D and not SG. If you had a lab grade hydrometer set to 15 C it will read a Sg of 1.0243 and not 1.02343.



How can one temperature compensate based on a reference temperature of 15C when the measured sample is at a different temperature?

The wonder of electronics and math equations. The meter measures the temp, say 25 C. It has software in the meter for 15C. It looks at what the Density would be at 15 C and then calculates form an equation in the software what the Density should be at 25 C. Just like this one does

http://fermi.jhuapl.edu/denscalc.html

or this one

http://www.phys.ocean.dal.ca/~kelley...roperties.html

[Mr31415]

I have rediscovered the answer again I am so proud of myself.

Salinity is not temperature dependent purely because it was defined at 15C based on that reference temperature, pressure and conductivity of KCl. Salinity is dimensionless.

Density is the mass of certain elements per unit volume of liquid. Density is dependent on the temperature and pressure (assuming we are talking about the density of sea water), and has a dimension of g/cm³.

Specific Gravity is the ratio of density of sea water (in our case) to the density of fresh water. And this is where most people (including myself) fall out of the bus. Ignoring pressure for now - since density is dependent on temperature, then it follows that SG is dependent on two temperatures - the temperature of the sea water and the temperature of the fresh water. One reference temperature for fresh water is 3.98C (the lowest density water can have under normal atmospheric pressure I think). At this reference temperature fresh water's density is 1.0000 g/cm³. Which makes SG = Density of Sea Water as the numerator is 1. However there are other standards such as 20C where fresh water's density becomes 0.998206g/cm³. To simplify, lets ignore this for now and assume 3.98C - 1g/cm³.

The second variable is the temperature of the sample sea water. So to find SG, we need to find the density of sea water at the sample's temperature. And this is where the other evil twin shows its head. Oceanographers got lazy I guess when they needed to share information. The guy in the Arctic circle submitted a study on water currents and had to include in his work statements such as "and at a SG of sigma-t = 26.5 with T = 10C I deduce that...". The guy in the Bahamas had to write "and at a SG of sigma-t = 22.9 with T = 25C"... and that confused them since they were actually referring to the same Salinity - 34.4. So to standardise they decided to base all temperature measurements on 15C (and other researchers use other standards such as 3.98, 20C etc.). So now the arctic guy still measures sigma-t = 26.5, but he recalculates it to a reference temperature of 15C, in effect stating: The SG of the sea water (which I measured at some temperature) would have been 25.5 if the temperature of the sea water was 15C. The bahama guy still measures 22.9 but reports the value as 25.5 if the temperature would have been 15C, which it is not.

So even though the *actual* densities, hence the actual SG are different, they report the same value because the temperatures of 10C and 25C were taken into account to work back to a reference value of what the density would have been at 15C. Now both report sigma-15 = 25.5. I guess if temperature is important then they will include T.

I just don't get why we need to convert back to a standard temperature value. Why not always use Salinity when we want a common temperature independent value, and use SG when we want to show the effects of temperature?

So to summarise - at a Salinity of 34.4, no matter what the temperature of the water - be that 3.98, 15, 20, 25, 30C - S stays 34.4.
SG (assuming fresh water @ 3.98C) measured as sigma-15 will show 25.5 (1025.5) independent of water temperature - be that 3.98C, 15, 20, 28C.
SG (assuming fresh water @ 3.98C) measured as sigma-t will show 26.5 (1026.5) if the water is at 10C, 25.5 (1025.5) if the water is at 15C, and 22.9 (1022.9) if the water is at 25C.

For hobbyists to communicate with each other, they cannot say "keep your aquarium at 1022.9 provided your temperature is at 25C, but keep it at 1021.9 if you run your tank at 28C, and ....

So they either say "Run your tank at a Salinity of 34.4 ppt", or they say "Run your tank at a SG of 1.0255" with the (and here comes the BIG confusion) implied reference of (possibly) fresh water at 3.98C, and sea water at either 15C or 20C. For 35ppt, if the reference temp was 15C then it would be 1025.97, if it was 20C then it would have been 1024.76 and if it was 25C, then it would have been 1023.34.

My Hanna yielded these results:
Salinity: 35ppt (Tropic Marine Pro Reef Salt + RO water)

Temp      sigma-t       sigma-15      Refractometer

15C       1.0260        1.0260        1.0265 (35ppt) 20/20

20C       1.0247        1.0260        

25C       1.0234        1.0260

27C       1.0227        1.0260        1.0265 (35ppt), 20/20

which agrees with what a technical lab guy at Hanna USA told me. My Hanna shows you the value the density would have been had the temperature been 15C, when in sigma-15 mode. Which is in my opinion correct. Salinity 35ppt = SG 1.026 at reef temperatures (based on 15C reference)...

(This was more a way for me to write out what I know and make myself understand this better - not attacking you!! - so sorry that this is so long...)

[Mr31415]

Hmm ok I have been working the whole day on this post... Performing measurements etc. Only now saw your post - thanks for that.

But there is one thing still to discuss... How do I calibrate my DO sensor?

How do I calibrate the saturation point?

[Mr31415]

Here is a nicer table - I botched the other one up:

Temp   Density(SWEoS)   sigma-t   sigma-15	Refractometer

15C    1025.97          1026.0    1026.0	1026.5 (35ppt) 20/20

20C    1024.65          1024.7    1026.0 

25C    1023.34          1023.4    1026.0

27C    1022.70          1022.7    1026.0    1026.5 (35ppt), 20/20

Where Density (SWEoS) is the Density (sigma-t) value based on the Seawater Equation of State calculation using Salinity, Temperature and pressure. sigma-t is my Hanna's measured value when in sigma-t mode. sigma-15 is my Hanna's measured value when in sigma-15 mode. Refractometer is what my refractometer measured.

[Boomer]

Salinity is not temperature dependent

Density is the mass of certain elements per unit volume of liquid

Specific Gravity is the ratio of density of sea water (in our case) to the density of fresh water

Yes and we are now getting you there.

Temp   Density(SWEoS)   sigma-t   sigma-15	Refractometer

15C    1025.97          1026.0    1026.0	1026.5 (35ppt) 20/20

20C    1024.65          1024.7    1026.0 

25C    1023.34          1023.4    1026.0

27C    1022.70          1022.7    1026.0    1026.5 (35ppt), 20/20

Yes, the sigma-t is the actual Density of the water and the sigma- 15 is where it is correct back to the std of 1026.

For hobbyists to communicate with each other, they cannot say "keep your aquarium at 1022.9 provided your temperature is at 25C, but keep it at 1021.9 if you run your tank at 28C, and

If there are using this meter. Most use refracts or hydrometers so this would not fit them. Chemical oceanographers never use Sg but D , Ec or Salinity in ppt. Most in this hobby use SG. This is where a lot of the confusion comes in. Often people in this hobby just take D tables and by some kind of magic just change the word D to Sg which is incorrect. Even Hanna has done this. That manual should say D and not SG

But there is one thing still to discuss... How do I calibrate my DO sensor?

How do I calibrate the saturation point?

It is in that pdf or just call them


4.3 DISSOLVED OXYGEN CALIBRATION
4.3.1 Procedure
After choosing the D.O. calibration mode in the
main calibration menu, select the type of calibration
with the arrow keys and then press OK.
D.O. % saturation
This calibration starts by default with 100 %.
• Fill the probe beaker with approximately 4 mm
(5/32”) distilled water and screw it on the probe.
• The message NOT READY appears until the
value is stabilized.
• To change standard calibration point, press CAL.POI


You did good on your own so far, just like last time

[Boomer]

  



2.5     3.5     8.9 

Well, one thing for sure you taught me how to do this

[Mr31415]

Yeah a software engineer can teach an old bomb technician / chemical wizard some tricks - even if it can't be on his own turf

Ok so let me rephrase "How to calibrate my DO" to: Is it correct if I add RO water to my little plastic beaker, insert the probe (making sure the probe is not touching the water - but just above it), then accepting the stable value as 100%? And the meter will compensate automatically when I test sea water? Based on the EC and temp and pressure it measures? In other words it "knows" it is in sea water and the saturation point is now different so it compensates?

Last question - what is up with the 4mm of water in the beaker? Why not just calibrate it in the air? Or is the idea that water vapor is 100% saturated with O2?

[Boomer]

You should call these guys as to their whys