Mindstream Aquarium Monitor

Electrobes · Jul 30, 2015

Would it be hard to compute using all four versus the two parameters? Would it require that much computing resources?

sonnus · Jul 30, 2015

Randy Holmes-Farley said:
The question is how much the values from a perfect measurement of pH and CO2 to get alk deviate from reality based on the range of salinity and temperature that reefers encounter. I've not run the calculations to know.

I might be misunderstanding your point but Mindstream is using CO2, pH, temp and salinity all in their alkalinity calculation. Apparently, their measurements are going to be more accurate than anything currently available so maybe their alkalinity measurement will be useful. Honestly, if it's +/- 10% I'll be happy with that. I keep my alkalinity high enough so that a 20% drop won't cause a catastrophe.

Randy Holmes-Farley · Jul 30, 2015

It might require a look up table for the constants, although there may be ordinary equations that relate them to salinity and temperature.

The ultimate equation is fairly simple:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog

Carbonate alkalinity (Ac) is mathematically defined as:

1. Ac = [HCO3-] + 2[CO3--]

Where [X] means the concentration of species X. Carbonate (CO3--) is counted twice because it contributes two units of alkalinity for each unit of concentration. The relationship between the amount of bicarbonate and carbonate to pH is well defined if the system is at equilibrium with the atmosphere. Substituting these known relationships into equation 1, we get:

2. Ac = (K1KHPCO2 / [H+]) + 2(K1K2KHPCO2 / [H+]↑2)

where pCO2 is the concentration CO2 in the air. Current carbon dioxide concentrations are around 350 ppm and have been rising with time, from less than 300 ppm in preindustrial times. KH is the Henry's law constant for CO2(which describes the solubility of CO2 in water from the air to form carbonic acid, H2CO3), K1 is the first dissociation constant for H2CO3 (describing the dissociation of H2CO3 to HCO3-), and K2 is the second dissociation constant for H2CO3 (describing the dissociation of HCO3- to CO3--).

sonnus · Jul 30, 2015

Also, Mindstream mentions that the fluctuation in pH after adding supplements will effect the alkalinity calculation and this will take up to an hour to stabilize. We might have to change the dosing frequency of the aquarium or at least carefully consider the placement of the Mindstream in order to get reliable measurements.

Electrobes · Jul 30, 2015

sonnus said:
Also, Mindstream mentions that the fluctuation in pH after adding supplements will effect the alkalinity calculation and this will take up to an hour to stabilize. We might have to change the dosing frequency of the aquarium or at least carefully consider the placement of the Mindstream in order to get reliable measurements.

They said they use pH and Carbon Dioxide, they make no mention of using the other two parameters.

Shep · Jul 30, 2015

Delete

ReefMadScientist · Jul 30, 2015

Randy Holmes-Farley said:
It might require a look up table for the constants, although there may be ordinary equations that relate them to salinity and temperature.

The ultimate equation is fairly simple:

Chemistry And The Aquarium: The Relationship Between Alkalinity And pH ? Advanced Aquarist | Aquarist Magazine and Blog

Carbonate alkalinity (Ac) is mathematically defined as:

1. Ac = [HCO3-] + 2[CO3--]

Where [X] means the concentration of species X. Carbonate (CO3--) is counted twice because it contributes two units of alkalinity for each unit of concentration. The relationship between the amount of bicarbonate and carbonate to pH is well defined if the system is at equilibrium with the atmosphere. Substituting these known relationships into equation 1, we get:

2. Ac = (K1KHPCO2 / [H+]) + 2(K1K2KHPCO2 / [H+]↑2)

where pCO2 is the concentration CO2 in the air. Current carbon dioxide concentrations are around 350 ppm and have been rising with time, from less than 300 ppm in preindustrial times. KH is the Henry's law constant for CO2(which describes the solubility of CO2 in water from the air to form carbonic acid, H2CO3), K1 is the first dissociation constant for H2CO3 (describing the dissociation of H2CO3 to HCO3-), and K2 is the second dissociation constant for H2CO3 (describing the dissociation of HCO3- to CO3--).

Well...

sonnus · Jul 30, 2015

Electrobes said:
They said they use pH and Carbon Dioxide, they make no mention of using the other two parameters.

I just realized that on their Kickstarter page! I don't know where I got the temp/salinity part but I remember reading it somewhere, maybe another board.

Shep · Jul 30, 2015

On a side note this is the post viewed and commented thread I have ever posted lol

Randy Holmes-Farley · Jul 30, 2015

So, assuming one were to calculate alk from ONLY pH and CO2... (which they may or may not do)

My quick and dirty calculation suggests that carbonate alkalinity calculated from pH and CO2 alone could be off by 20% or more even if the measurements are perfect, within the range of temp and salinity values reefers might encounter.

This article provides the numbers:

http://www.gfdl.noaa.gov/bibliography/related_files/millero0201.pdf

If we take a reasonably wide range for temp 72 to 84 deg F (22.2 to 28.9 deg C), K1 varies by 20% and K2 varies by as much as 40%.

For salinity, if we look at 30-37 ppt, K1 varies by about 7% and K2 by 17%.

So in looking at the equation:

Ac = (K1KHPCO2 / [H+]) + 2(K1K2KHPCO2 / [H+]↑2)

If K1 and K2 are off by 25 and 45% at the extremes, the alkalinity will be off by perhaps 25-50%.

If they pick an "average" value somewhere in the middle, they might be off by 10-30% at the extremes (and not off in the middle), depending on how close your tank was to the values they choose for salinity and temp in the equation.

Randy Holmes-Farley · Jul 30, 2015

sonnus said:
I might be misunderstanding your point but Mindstream is using CO2, pH, temp and salinity all in their alkalinity calculation. Apparently, their measurements are going to be more accurate than anything currently available so maybe their alkalinity measurement will be useful. Honestly, if it's +/- 10% I'll be happy with that. I keep my alkalinity high enough so that a 20% drop won't cause a catastrophe.

How do you know they use all 4 to get alkalinity? It's certainly good if they do.

Never mind, I see you next post.

Shep · Jul 30, 2015

Randy Holmes-Farley said:
So, assuming one were to calculate alk from ONLY pH and CO2... (which they may or may not do)

My quick and dirty calculation suggests that carbonate alkalinity calculated from pH and CO2 alone could be off by 20% or more even if the measurements are perfect, within the range of temp and salinity values reefers might encounter.

This article provides the numbers:

http://www.gfdl.noaa.gov/bibliography/related_files/millero0201.pdf

If we take a reasonably wide range for temp 72 to 84 deg F (22.2 to 28.9 deg C), K1 varies by 20% and K2 varies by as much as 40%.

For salinity, if we look at 30-37 ppt, K1 varies by about 7% and K2 by 17%.

So in looking at the equation:

Ac = (K1KHPCO2 / [H+]) + 2(K1K2KHPCO2 / [H+]↑2)

If K1 and K2 are off by 25 and 45% at the extremes, the alkalinity will be off by perhaps 25-50%.

If they pick an "average" value somewhere in the middle, they might be off by 10-30% at the extremes (and not off in the middle), depending on how close your tank was to the values they choose for salinity and temp in the equation.

So how accurate are your average test kits?

Randy Holmes-Farley · Jul 30, 2015

Shep said:
So how accurate are your average test kits?

That obviously varies, but my DIY is much better than that.

Shep · Jul 30, 2015

Randy Holmes-Farley said:
That obviously varies, but my DIY is much better than that.

Ok so lets just say someone is using the Redsea or API (they seem to be the two most common), would they be about as accurate as the MAM?

ReefMadScientist · Jul 30, 2015

Here is my .2 cents.

The price for the Mindstream compared to other tests makes me want to stick with my old ways for now. With the calculations that Randy has provided, it shows that it will be very difficult for the Mindstream to be accurate. I can be wrong...but for all the replacements for the year and so on - Ima keep it simple.