# Air pocket in drop checkers



## Alastair (13 May 2011)

I was just pondering to myself about my drop checkers today as you do when it's all new to us, and was just thinking, if the air pocket inside a glass drop checker was much less, would this make the reaction time slightly quicker as there is less space for the gas to build up inside before reacting with the solution?? Hmmmm  :?


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## greenink (13 May 2011)

Alastair said:
			
		

> I was just pondering to myself about my drop checkers today as you do when it's all new to us, and was just thinking, if the air pocket inside a glass drop checker was much less, would this make the reaction time slightly quicker as there is less space for the gas to build up inside before reacting with the solution?? Hmmmm  :?



Surface area much more important i think - and with most glass drop checkers that means getting the bulb half full and as much air as possible. But could be wrong!


My 'see through' tank challenge http://ukaps.org/forum/viewtopic.php?t=15878


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## Alastair (14 May 2011)

But if there's much more air, wouldn't it take longer for the gases to travel to the solution?


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## Johno2090 (14 May 2011)

Not since the surface area is larger towards the base allowing mire gas exchange. It's the chemical reactions in the solution that take the time to register the change in carbic acid formation ( I think IRS carbic acid )


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## ceg4048 (14 May 2011)

Alastair said:
			
		

> I was just pondering to myself about my drop checkers today as you do when it's all new to us, and was just thinking, if the air pocket inside a glass drop checker was much less, would this make the reaction time slightly quicker as there is less space for the gas to build up inside before reacting with the solution?? Hmmmm  :?


Hi,
   In order to grasp the mechanism of the dropchecker try imagining a dropchecker filled entirely with water and zero air. We would then be left with a water/water interface, wherein the tank water is in direct contact with the reagent water. 

CO2 is 10,000 times less soluble in water than it is in air. So the movement of the gas from the tank water to the dropchecker water is inherently slow. CO2 easily out-gasses to atmosphere. That is why we have to keep pumping it into the tank for hours on end. 90% or more of the gas you inject escapes to atmosphere immediately. The air pocket inside the checker is therefore your "mini atmosphere" which facilitates escape of the gas from the tank water into the dropchecker. Therefore, you want more air in the checker. That air bubble acts as hoover, pulling CO2 from the tank water in the same way that the atmosphere above the tank pulls CO2 out of the tank's water. As the partial pressure of the CO2 in the bubble increases it then diffuses into the reagent water.

Likewise, if the CO2 in the tank water falls, then the partial pressure of the CO2 in the air space falls, and the higher partial pressure of the CO2 in the reagent water quickly vents into the air bubble, thereby facilitating it's movement back into the tank. The bubble therefore serves two purposes, it physically isolates the tank water from the reagent water so that only gases can move between the two waters and it acts as a hoover to move the gasses more easily between the two waters. As a result, it serves no purpose to minimize the volume of air in the checker. If you do this then you will only be making the checker even more slower to respond than it already is.

When you think about CO2, or any solute in a solvent, you should always think about the solutes pressure. 30 part per million (ppm) of CO2 in water is really a measurement of the pressure of CO2 within the total pressure of the water. 15 ppm of Nitrate is really an indication of the pressure of those Nitrate molecules being exerted against the body of water in which it is dissolved. When the gas escapes from the water it's pressure within the solvent collapses in the same way that a balloon collapses when the air escapes. The only way to prevent the gas from escaping from the water is to have an equal or higher pressure of that gas above the water.

When you inject CO2 you are injecting it at a higher pressure than what is present in the atmosphere. That's why you regulator gauge reads something like 1.3 PSI, which means that the gas is entering the water at 1.3 PSI higher than atmospheric pressure. This pressure forces the gas into solution and raises the CO2 partial pressure in the water, but since the atmospheric pressure of CO2 is much lower just above the tank water, the CO2 pressure within the water escapes has a tendency to fall immediately.

Hope this helps.

Cheers,


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## Alastair (14 May 2011)

That's helped greatly thanks clive :0)...on the subject of drop checkers, I've got two either end in my tank, and in one have just started using 4dkh bromo blue from a sponsor. I'm having trouble in that the solution is really really dark. About half way through the day I get a nice lime green in the other, however in the 4dkh bromo it's a very deep green, quite hard to see. Flow rate is spot on all around the tank, and have upped bps but still no lime green from this one???


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## ceg4048 (14 May 2011)

Hi Alastair,
                Well the first thing you need to do is to check the validity of the bromo blue. This is easily don by putting a few drops with water in a glass or vial and then adding a drop or two of vinegar. The solution should immediately go yellow. If not then the reagent is a dud. If it does go yellow immediately, then you know beyond a shadow of a doubt that there is something wrong with your injection or distribution technique.

Are you adhering to the 10X flow rule?
Are you using a spraybar or some other method to get good distribution?
I assume this is a gas cylinder. Is it full?

It's difficult to assess from a distance. Check the bromo blue and if it passes the test perhaps you can show us some photos of the setup.

Cheers,


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## Alastair (14 May 2011)

Hi Clive, I've Only just changed to this solution as have run out of my old one and thought I'd buy an all in one. Previously it was fine. 
Its 450 litre tank, given wood etc about 380 litres, a tetra tec ex2400 and 1200 with spraybars along the back pointing straight forward as you recommend in one of your articles. 
Co2 is currently diffused via diffusers under the intake of each filter, and the tiny bubbles coming out of the spray bars are getting distributed all over the tank. 
The bromo mix does change colour slightly but is always extremely dark, even when I place a White ruler behind. Have just done the vinegar test and yup went yellow. So it works, but surely the solution shouldn't be so dark??


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## AverageWhiteBloke (14 May 2011)

I have had a few bad experiences with certain types of reagent the ones that came free with drop checkers from Asia. Even though they passed the vinegar test they still seemed to never go lime green whatever amount of co2 I put in the water. There is a good article in here somewhere about the different reagents and their names. Essentially bromo blue was the best as it was most accurate in the ranges we work with tanks at between 6.5 and 7. 
I'm not sure where people get there bromo from to guarantee it is actually Bromo blue and not one of it's alternatives  other than obviously specific long term co2 kits or premixed from the sponsors. I premix my own 4dkh solution and use Hagen PH low range test kit as my reagent. Reading the box it has only Bromo Blue as it's ingredient. The high range one uses some other chemical.

BTW interesting post on the air bubble, I have often wondered what would be best


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## Alastair (14 May 2011)

hi average white bloke, im glad you said that, ive got a few of the reagents from asia which i havent used yet but after that ill not bother. 
mine is premixed from a sponsor, and even with my co2 turned up really high it doesnt seem to get any lighter


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## AverageWhiteBloke (15 May 2011)

I managed to find the stuff I was on about regarding the various types of reagent. http://www.funsci.com/fun3_en/acids/acids.htm

from this post with a similar problem I experienced.
http://ukaps.org/forum/viewtopic.php?f=20&t=11198


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## Alastair (15 May 2011)

then i guess the 3 bottles of co2 regent i have are pretty useless then. What do you think to me adding a little 4dkh to the AE 4dkh bromo once its in the drop checker to lighten it up, i guess that would help to show the colour more??
i did email them but was advised to crank uop the co2 which over an 8 hour period i did but my fish started showing signs of distress, but the colour was no way near lime green.


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## Alastair (15 May 2011)

and the other question i have, if i were to say purchase bromo blue seperately online, is the BROMOTHYMOL BLUE 0.04% what is used or can be used in drop checkers??


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## AverageWhiteBloke (15 May 2011)

Check this post of mine http://ukaps.org/forum/viewtopic.php?f= ... 8&start=10 I did a little experiment with the free reagent I got with my DC and even though the vinegar test worked the free reagent stayed blue whatever happened in the tank.
I'm not sure what was in the reagent but it certainly didn't react in the parameters of the fish tank. I'm not sure what problem your having here is that you can't see through reagent to get a reading or it's just not changing colour at all?

I'm also not sure of the Bromo 0.04% I've often wondered where members source their stuff. I use nutrafin low ph like I mentioned because the ingredient is bromo blue. I would imagine the Sera long term co2 re-fills do as well. The only problem I had with AE 4dkh and bromo pre-mixed was it was very opaque and quite difficult to get a reading off with out putting something white behind it. I tried once add some bromo to the premixed stuff to get a better colour but this made the reagent go slightly green, not sure if the different chemicals reacted with each other or there was contamination of some kind.

You could thin it down with some 4dkh to get a better reading but I find it unusual that the colour is too dark to read usually its the other way round, if you have two DC's try putting them next to each other and see if they get the same results.


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## Alastair (16 May 2011)

I've placed them both next to one another, one is a light green, whilst the AE mix is ridiculously hard to tell what colour. It's very very dark, it does have a green tint to it but I find myself having to take it out and hold it up to the window to get a colour. It's more that I can't see through the solution to get a reading. 
I actually tried a couple of drops of one of the reagent that came with the drop checker, which was an orangey yellow colour and when mixed with 4dkh went blue. Used in the tank it did turn green after a bit but read that some of the regents that come with them are inaccurate


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## Alastair (16 May 2011)

Problem solved. Richard from aqua essentials is sending me a more diluted bottle. How's that for customer service. Top guy :0)


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## AverageWhiteBloke (16 May 2011)

Good for them, hopefully this will solve your problem. DC's are very hard to get a reading from at the best of time. The tank lights reflecting off greenery often make it impossible to see what colour is in there. My DC has a white sucker on as opposed to clear so I view it from the opposite end of the tank using the sucker as the background or alternatively the white plastic ruler is a winner.


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## Alastair (16 May 2011)

Ahhh I never thought of that..mine have White suckers on them too. Not sure what to do with the regent that came with the drop checkers though originally. They do turn blue in 4dkh and change colour in the tank...


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## AverageWhiteBloke (16 May 2011)

There's chance for them yet Alistair, I'm not saying there all bad just I had a bad experience with my particular one. It's fair to say that in theory the AE stuff is pretty accurate. You could set up a AE DC next to one using one of the other reagents if you get the same result with both I would say your good to go.

There is also a test you can do to see if the reagents match real world by getting a sample of water and testing the PH then aerating the sample and testing again which should tell you the amount of co2 that had left the sample after aeration.

But as Clive pointed out to me at some point the DC itself is not exact science. I have stopped trying pin things down to exact parameters. The DC give you a general idea of what's going on but the plants and fish give better clues. The DC suffers from delay so what your seeing now is more likely to be the state in the tank from an hour or two ago. Take the DC out of the tank and see how long the colour goes to turn blue again to give you some idea and that reaction is quicker in the open air than it would react using the small pocket of air used in the tank.

An example would be my 4dkh solution is home made using a set of digital scales and measuring jugs and tested to within half a degree with a standard kh test kit,not what you would call lab standard   So my solution may not be 4dkh which in effect would give a slightly different reading than if it were. But that isn't important, if I run the tank in green and co2 related problems occur then I need to either improve the flow or run the tank in yellow. Vice versa if the DC is saying green but my fish are starting to gasp then I know running in green is as far as I need to go.

I guess what I'm saying is match the DC to your particular tank.


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## Alastair (16 May 2011)

I'm going to purchase some seperate 4dkh from one of the sponsors, and give the regent the drop checkers came with a shot next to the AE solution. Like you say, if they both seem to show similar colours then I may aswell use those too. But with the darker solution from AE, and the new one he's sending, I think I'll have enough to last a good few years ha ha ;0)


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## AverageWhiteBloke (16 May 2011)

Making your own 4dkh is interesting and quite simple with minimum outlay. You just need ro water and some bicarbonate of soda. You never know you could knock some up and get the same results as your AE stuff. Win Win.


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## Alastair (16 May 2011)

Yeah I've been looking at how to make it, might invest in some digital scales and give it a shot....


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