Lab cooling bath

[simon...@gmail.com]

Hi, I am a UCC PhD student in Plant Science trying to build a cooling bath. Any suggestions?  I have no Idea how to do it, I am a plant scientist with really scarce aptitude towards physical/ electrical stuff...

Thanks in advance

Simona 

[Cathal Garvey (Phone)]

Hey Simona!
I'm based in cork, and a member of Nexus Makerspace in the old tax office, Sullivan's Quay. I'd be happy to help out.

How cool do these baths get? :)

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[Glyn Evans]

Could an old mini fridge be modified to use as a cooling water bath?

What temperature does it have to go down to? Will it be lower that 0DegC? If so it will require an ethanol bath. This will have safety implications and will make it more difficult to build so that its EX rated.

 

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[simon...@gmail.com]

The bath should reach the lowest T you can get in winter time in Irish freshwaters. (I'd say at least few degrees below 0). 

Cathal, your help would be very appreciated, would you have time to meet? 

[Cathal Garvey]

Sure, later this week if you like? I'm a bit up the walls today and
tomorrow. You can reach me on (vodafone prefix)-6363185

Instead of Ethanol you could probably use propylene glycol (nontoxic
"antifreeze") mixed with water. I'm *fairly sure* it's nonconductive and
nonflammable, but worth double-checking. I have plenty of the stuff left
over after an experiment with agarose purification..

For active cooling, you'd probably be best using a minifridge as glyn
suggests, though I'm not sure if those things run on their sides or if
they need the correct orientation for coolant evaporation and
condensation..?

The other option would be a big-ass peltier thermal pump and put the
whole thing in a fridge to help the peltier do its job; they're really
only useful for making a gradient of ~5-10 degrees. They take huge
energy to run though.

[Glyn Evans]

If you need to only go a couple of degrees below 0 then as Cathal says the glycol would work fine. I was thinking ethanol if it was below -20.

The idea of a mini fridge then wont work as it will only cool to 3 or 4 degrees.

Would a small chest freezer be a feasible idea? the temperature can be controlled using a PID temp controller and set a plastic box into it as your bath. Use a small aquarium pump to circulate the water around to  get an even distribution of temperature.

Not sure if the chest freezer option is too bulky though?

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[simon...@gmail.com]

I can't just put a box in a fridge or chest freezer because I must grow duckweeds (aquatic plants) in the bath, they need light... That's why I was thinking of growing them in flasks, place the flasks in a bath and place the bath in the growth room.
I'll try to figure out how to use the propylene glycol, I need to read something about it. 

Cathal thanks, later in the week is grand, no rush at all.

Thanks so much to both of you guys, I'll let you know if I manage to do something good!

On Tuesday, January 21, 2014 2:19:49 PM UTC, simon...@gmail.com wrote:

[Glyn Evans]

Ill keep thinking about it and if i come up with anything feasible ill let you know. My friend works in the refrigeration industry so ill ask him if he has any ideas too.

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[Nathan McCorkle]

On Tue, Jan 21, 2014 at 7:54 AM, Glyn Evans <glyndw...@gmail.com> wrote:
> If you need to only go a couple of degrees below 0 then as Cathal says the
> glycol would work fine. I was thinking ethanol if it was below -20.
>
> The idea of a mini fridge then wont work as it will only cool to 3 or 4
> degrees.

Many mini-fridges designed for small apartments or University
dormitories also include freezer sections. These run on the same
cooling unit, often the cooling is directed to the freezer section
first, then into the fridge section.

Often these days, these mini-fridges use Peltier coolers, no
compressor or refrigerants.

If you were to use a mini-fridge, you'd likely need to take off the
door and replace it with some type of insulation, maybe a piece of
styrofoam (expanded polystyrene) or something. Since the Peltier
cooler is often in the rear of the normally-upright mini-fridge, you'd
need to add some leak-sealant since the Peltier would be on the bottom
if you turned the fridge on it's back-side and used it as a water
bath.

[Cathal Garvey]

It sounds like the system needs to be open or at least transparent on
top (to let light into the weed), but it's a volume of water so it
shouldn't matter too much if it's open to the air.

To avoid having to rigorously waterproof the inside of the fridge,
although some sealing is recommended, could find a well-fitted metal
container and sit that inside, and use cheap thermal grease to mate the
lower face of the metal container to the cold area at the back of the
minifridge?

I'm assuming here that the bath needs to be unfrozen but the plants
won't be immersed directly in the bath, because in that case you'd need
to look up how plants react to propylene glycol and control for that in
the experiment. :)

[simon...@gmail.com]

Hi guys, I procrastinated the start of my experiment, but now I really need to sort out the cooling bath.  Made a little research today based on your previous suggestions. The idea of the mini fridge is not bad, but I am also considering the idea to buy one or more thermoelectric Peltier coolers like this:

 http://www.ebay.com/itm/1-2-5-10pcs-New-12V-60W-TEC1-12706-Thermoelectric-Cooler-Peltier-HK-/141166719909?pt=LH_DefaultDomain_0&var=&hash=item20de3133a5 

and use it to cool down the water in a box and place my flasks in the box. Do you think it might work? How many unit you think I need to cool down to -5/-10 degrees a volume of water approximately 50 x 40 cm and 8/10 cm deep? Another idea was to find a big peltier cooler and place my flasks on it, avoiding the bath, but it seems they only sell small units.

What do you think? can I avoid the mini fridge and just use the Peltier? any different idea is welcome

Thanks so much guys

Simona

On Tuesday, January 21, 2014 2:19:49 PM UTC, simon...@gmail.com wrote:

[Cathal Garvey (Phone)]

Hi Simona,
I don't know specifics, but I caution you to look up the power requirements (Voltage sure, but Current more importantly) of peltiers and ensure you can match that current WHILE controlling it.

For example, most transistors which you'd otherwise use to control devices according to temperature will rapidly burn out switching a peltier: you'd have to use a transister-controlled relay instead.

[Nathan McCorkle]

You don't need to PWM a peltier, but they do last longer if you control them that way. A peltier I got from a mini fridge, when hooked up to a power supply, produced a bit of ice on the surface, so it really depends how quickly you want to cool the samples and if they're producing heat or not.

A solid-state relay is just a transistor and a transistor driver, any MOSFET with a gate driver should be fine, the gate driver is required to boost the instantaneous current to the gate of the MOSFET , which sucks up charge like a capacitor. The reason you need the driver is so the charge up time is minimal (happen quickly) so you avoid the in-between time of not being fully on or off. The MOSFET is less efficient in that region of being turned on, so to consume less power you need to switch fast. 20khz or more should be fine for a PWM frequency. The idea is that you change the temperature in a little burst, then give it the off-duty time to diffuse the heat a bit, then repeat.

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[Cathal Garvey (Phone)]

+1 for using a solid state relay, but you have to get a DC one, NOT AC. Many AC SSRs rely on polarity flip to reset, so if used with DC (like a peltier) they mightn't switch off.

..or so I gather. I've only ever used the AC kind, and with AC, so no personal experience.

[Steve]

Hi Cathal, Simona,
I left a USB mini fridge in the Nexus some time back... very low power required.
May be hackable, I'd originally intended to use it for transporting samples with battery powered cool box cooler.
worth a look?
Steve

[simon...@gmail.com]

 No idea what and where Nexus is, but I'd like to have a look. Maybe a mini fridge is the easiest way to do it after all. Can we arrange a view?

Thank you

S.

On Tuesday, January 21, 2014 2:19:49 PM UTC, simon...@gmail.com wrote: