Simple DIY Drop Checker (V2 is Alive! 3/31)

[Loop]

That's cool. I never thought about cutting the funnel. Would it be pretty easy to cut the glass without breaking it? I've never tried cutting any glass before.

One idea I had was to just make something along the lines of this.



Sorry for the crude drawing, but I was just thinking of using some vinyl tubing again with some sort of cap on the end to hold the fluid in. Not sure if it would work out perfectly, but I was assuming that heating it, bending it, and cooling it would make it keep the "U" shape.

[Hoppy]

Originally Posted by Loop

That's cool. I never thought about cutting the funnel. Would it be pretty easy to cut the glass without breaking it? I've never tried cutting any glass before.

One idea I had was to just make something along the lines of this.



Sorry for the crude drawing, but I was just thinking of using some vinyl tubing again with some sort of cap on the end to hold the fluid in. Not sure if it would work out perfectly, but I was assuming that heating it, bending it, and cooling it would make it keep the "U" shape.

This, or some variation of it, would work well. For example, the loop could be made from a piece of acrylic tube - heat it and bend it - with the vinyl tubing used to join them.

Cutting glass is simple in concept, but takes some practice to do well. Cutting a bottle is very easy, and perhaps not fool proof, but still easy. Cutting the funnel might be a challenge.

[smannell]

I got to thinking about some of the various designs suggested here last night when I couldn't sleep, and a couple of important design considerations occurred to me. Provided I'm remembering my various engineering classes properly, using a funnel or flared tubing will not have a significant impact upon the reaction time for a drop checker. Theoretically, it could even slow the reaction down. Second, the diameter of the tubing is not the most important factor in determining the response time. Let me try and explain.

The rate at which a gas dissipates from or is absorbed into a liquid depends upon the following factors.

1. The concentration of the gas present in the liquid (in this case ppm of CO2)
2. The partial pressure of the gas in contact with the liquid
3. The surface area of the liquid(s) in contact with the gas
4. The Reynold's number of the fluids involved (remember that gas is a fluid)
(I'll save this one for last as it is complicated and will only have a small affect)

Item number 1 is what we are trying to measure. For a closed system such as a drop checker, #2 is dependent almost entirely upon #1. Neither of these is affected by the physical design of the drop checker. Item #3 is dependent upon two dimensions: the first is the ID of the top of the tubing (the part in contact with the gas); the second is the ID of the jar minus the OD of the tubing. A funnel at the bottom of the tubing does not impact either of these.

When the drop checker is placed into the tank, CO2 begins to diffuse out of the aquarium water and into the air trapped in the drop checker. The rate at which this occurs depends mostly upon the concentration of the CO2 in the tank and the surface area of the water in contact with the air, ie. the tubing ID. This reaction will continue until the system reaches steady state (equilibrium). The time to reach steady state will depend mostly upon the volume of air trapped in the drop checker. The more air in the system, the more CO2 it can hold at a given temperature/pressure.

At the same time, CO2 is being absorbed by the indicator fluid from the trapped gases. Once again the rate of this reaction is primarily dependent upon the surface area of the indicator fluid. As before, this will continue until the concentration of the CO2 in the indicator fluid matches that of the aquarium water, assuming that CO2 has the same permeability for both liquids. The total time for this reaction to reach steady sate will depend mostly upon the volume of the indicator solution.

In simpler terms, in order to get the quickest response time a drop checker should have the following characteristics:

1. Equal areas of indicator fluid and aquarium water in contact with the trapped air.
The smaller of the two areas will be the limiting factor for the rate of gas diffusion.
(This means a bottle with an ID roughly 1.4 times the diameter of the mouth).
2. A large surface area to volume ratio for the indicator fluid.
3. A large surface area to volume ratio for the gas with respect to both liquids.
Essentially you want a short, fat jar with the above diameter to mouth ratio.

Finally, the Reynold's number. For those of you that have never studied any type of fluid mechanics, Reynold's number is a dimensionless number (meaning it has no units such as meters, grams, seconds, etc.) that is used to indicate various properties of moving fluids. It depends upon density, viscosity, flow rate, turbulence, etc. For this application the important factor is the flow/turbulence in the water column of the tubing. The indicator fluid and the trapped gas are essentially motionless, so the only way to speed up the dissipation of CO2 is to get the water in the tubing moving.

Placing a funnel on the end of the tubing might help a tiny bit if it is oriented properly with respect to the water flow in the aquarium. On the other hand, if it is positioned improperly it could impede the flow of water below and therefore inside the tubing. The best way to help increase flow would be to place a "Tee" or "Y" shaped plumbing fixture at the bottom of the tubing, and then turn one end into the flow and the other end away from it. If you are using a "Tee", placing a funnel on the incoming end of the fixture would increase the flow rate through the fixture and thus help stir the water in the vertical section of the tubing. Overall, I'm guessing the water flow is a minor factor in the reaction time compared to the surface areas and volumes involved.

This is probably far more information than anyone wanted to know about a simple drop checker, but that's what happens when an engineer suffers from chronic insomnia. I hope this was in some way helpful (cutting a glass funnel sounds like a pain in the ass). Let me know if you have any questions or couldn't understand my techno-babble.

Sean

[Loop]

smannell

Thanks for the fancy tecno-babble. It was an interesting read. I've never studied anything relating to fluid mechanics before, but alot of the properties your stating are things I had assumed from basic science, but have not yet been able to produce. I've been looking for a container of some sort to design one more like your describing, but I don't think I'm going to find anything like that just sitting in a store somewhere. I'm pretty sure it's something that will have be created for this specific purpose, from scratch, by someone willing to put in more thought/design than most of the companies in this small market want to invest. Even if they did, I'm not sure I would be willing to pay the price they would demand for their investment. Luckily for us financially challenged people there is always a cheap knockoff company waiting to steal designs and sell on ebay for $5. Until that day comes we have Hoppy's design for the best/quickest, and some other easy ones to get by with.

You're an engineer, why don't you give it a shot at something professionally designed to sell?

[Hoppy]

You can experiment with different geometries of drop checkers and if you do you quickly learn that the fastest response time comes from high area of tank water to air bubble surface, low air bubble volume, high area of air bubble to indicator fluid surface, and low volume of indicator fluid. A flared opening like a funnel, does reduce the response time considerably. The biggest effect is having a large surface area to volume ratio for the indicator fluid - a shallow dish of fluid as opposed to a tube of fluid. Reynold's number isn't involved at all. The speed of movement of the dissolved CO2 in water is very high in any case, so much so that it has little effect - you can assume that the CO2 concentration changes instantaneously at any local spot in the tank whenCO2 is added. (It doesn't, but compared to one hour response times, it might as well be instantaneously.)

[smannell]

Nice to know someone has actually experimented with these things. Theory and practice don't always coincide. Any idea why the funnel matters when it doesn't change the surface area? Does CO2 tend to rise towards the top of an aquarium (this is something I hadn't considered)? Is it simply causing more water movement in the tubing? I believe you, I'm just trying to figure out the principle involved.

I probably should have simply said water circulation as opposed to Reynold's number since we are dealing with very low flow rates where turbulence is minimal and viscosity doesn't matter (I was simply looking for a term that indicated how much movement was occurring at the gas/liquid interface). My fluid mechanics class was 19 or 20 years ago. If CO2 dissipates that quickly then water circulation probably doesn't matter much at all.

Sean

[Loop]

I don't understand why you're saying a funnel doesn't change the surface area. It seems to me it obviously would. Is there something I'm missing here?

Little opening = small surface area of trapped air to water
Bigger opening = larger surface area of trapped air to water

Right? Assuming the funnel shape doesn't increase the volume of the trapped air too much that it offsets the gain.

Unless this is exactly what you mean, that the funnel increases the ratio of air volume to surface area too much. Which I'm definitely too lazy to try and calculate to see.

[larams67]

Originally Posted by Loop

I don't understand why you're saying a funnel doesn't change the surface area. It seems to me it obviously would. Is there something I'm missing here?

Little opening = small surface area of trapped air to water
Bigger opening = larger surface area of trapped air to water

Right? Assuming the funnel shape doesn't increase the volume of the trapped air too much that it offsets the gain.

Unless this is exactly what you mean, that the funnel increases the ratio of air volume to surface area too much. Which I'm definitely too lazy to try and calculate to see.

The surface area that matters is the area above the ph solution. A funnel shape at the aquarium water side doesn't change that.

[psalm18.2]

What is the "solution" refering to? Is this for ferts?

[Hoppy]

That's why the funnel shape is beneficial. The size of the air to DC liquid surface area is also important. The bigger, the better, but the lower the volume of liquid in the DC bulb, the better too.

[Loop]

Originally Posted by larams67

The surface area that matters is the area above the ph solution. A funnel shape at the aquarium water side doesn't change that.

I was thinking that both would matter quite a bit. The way I was thinking is that you want CO2 from aquarium water to get to the ph solution. In order for this to happen it needs to get into the trapped air in the drop checker first, and a larger surface area would speed this up. The faster you can get CO2 into that trapped air, the faster CO2 can enter the ph solution, which you would also speed up by having a large surface area.

I do also see having the least amount of solution and air in there being beneficial. Seems like the lower volume of both, the faster it can reach an equal level of CO2 with the tank water.

I don't mean to sound argumentative about it, I'm just trying to understand the reasoning behind the ideas.

[Loop]

Originally Posted by psalm18.2

What is the "solution" refering to? Is this for ferts?

The solution is refering to the liquid inside the drop checker.

[smannell]

OK, after looking at your drawing I understand the reason for the confusion. I was assuming that the aquarium water would rise up far enough that it was past the funnel and into the tubing (it's hard to see from the pictures). Obviously this would depend upon the height of the funnel and how far the drop checker was submerged. You are absolutely correct that using a funnel would increase the surface area and decrease the reaction time provided the interface between the trapped air and the tank water was within the funnel as opposed to farther up into the tubing. I apologize for not making my explanation more clear, and sorry if I caused a lot of confusion. Next time I'll try and figure out how to draw picture and include it in the post (worth a thousand words).

I also thought of how to utilize a fat, and hopefully short jar even if it has a large mouth. Provided the jar has a plastic lid that seals well, you could drill a hole the proper size for the tubing (a spade bit works well for this). With a plastic lid and plastic tubing, super glue makes a quick, convenient sealant. I use it for air tubing all the time and never had a leak. If the jar/bottle is too tall, you could try melting candle wax or paraffin and using it to fill the jar part way to decrease the volume of air it would hold. I don't think it would react with or absorb any CO2 (paraffin is used for home made jams and jellies and doesn't affect the taste).

Sean

[Loop]

What if you made something shaped like this



Not sure what you would make it out of, maybe use something like petri dishes and some acrylic tube (the tube would be curved at the top, not square like in the pic). Keep the tubing as short and thin as possible to limit the volume of air. Imagine a more streamlined shape for a finished product. The white square inside the solution would be something that would take up some of the space to keep the solution volume low, but let you keep a large surface area on top of it. You would view it from the angle the red arrow is pointing so that you still had a decent size viewing area with a white background. The shallow squared off shape instead of a funnel should also give less volume of air inside.

Just an idea that I had trying to think of something that fit the criteria of high surface areas and low volumes.

[larams67]

Originally Posted by Loop

What if you made something shaped like this



Not sure what you would make it out of, maybe use something like petri dishes and some acrylic tube (the tube would be curved at the top, not square like in the pic). Keep the tubing as short and thin as possible to limit the volume of air. Imagine a more streamlined shape for a finished product. The white square inside the solution would be something that would take up some of the space to keep the solution volume low, but let you keep a large surface area on top of it. You would view it from the angle the red arrow is pointing so that you still had a decent size viewing area with a white background. The shallow squared off shape instead of a funnel should also give less volume of air inside.

Just an idea that I had trying to think of something that fit the criteria of high surface areas and low volumes.

The part I'm not sure about is the diameter of the section that connects the surface area chambers. It would seem to restrict the exchange of gases.

I don't see how the chinese made glass styles for instance would be any good. A big funnel reducing down to a pinhole at the bend before opening back up for the ph solution chamber can't be effective.