As un-electronic a question as possible - cheapo homemade black dye?

[Delta]

Does anyone have an suggestions as to the cheapest way I could dye water black?
It is not for human (or animal) consumption, just to fill plastic bottles with.  (The idea is to stack them up in my greenhouse, such that they absorb some heat through the day, then release it in the evening.  Here in Scotland we need to make as much use of what little sun we get.)

I know this is not a "water dyeing forum" - although there probably is one on t'internet somewhere! - but I know there are some ingenious types on here!

[IanB]

Go to a suitable shop (John Lewis?) and buy some black fabric dye?

[Mechanical Menace]

Food colourings would be my suggestion.

Edit: Cheap and readilly available. You're pretty certain to end up with a solution and not a suspension too.

[Zero999]

Black fountain pen ink.

[Lightages]

I would just drop some black paint inside the dry bottle, swish it around to coat the inside, let it dry, and then fill with water. Cheap, fast, and you don't have to worry about the die degrading and falling out of suspension.

[Tinkerer]

Just paint the outside of the bottle black.

[T3sl4co1l]

Use brown beer bottles.  Here, I'll help...*hic*

Tim

[tautech]

Black oxide powder as used in the masonry/concrete industry.
Should be able to source it from any big HW store.

[mikeselectricstuff]

would colouring the water black make any difference?
I'd think reflection off the bottle surface means that painting the bottle black would be more effective at capturing radiated heat

[Tomorokoshi]

How about mix the water with powdered charcoal?

[grantbob]

would colouring the water black make any difference?
I'd think reflection off the bottle surface means that painting the bottle black would be more effective at capturing radiated heat

I agree and think just painting the containers flat black would give you good absorption of heat to transfer to the water. In fact you'd probably only need to paint the side that would get the sunlight. It would be a pretty simple test to start with two identical containers, fill one with "black water" and paint the other flat black, put them in the sun and check the temps (or get some thermocouples and do some data logging!)

-grantbob

[Delta]

Great shout about painting the outsides of the bottles, I hadn't considered that the shiney plastic might not be the best for heat absorption!

I'll get some matt black paint, and do a black water / black bottle comparison - but I think we know which will win!

Just need to wait for a weather control machine to pop up on KS or IGG, then I can turn the sun up a bit!

[babysitter]

To keep eevblogs history of overengineering, get a thermal camera and fill different bottles with water of the same temp and look which one emits and absorbs best!

Then go on and protect the water from fouling, you might try hydrogen peroxide, potasium permanagnate, parabene for example (Not at the same time).

Or is there a storage medium which is better but not much more expensive than water? Sometimes you want to melt during heating and freezing during cooling phase. Something like candle wax. Or Gallium. Oops this might be on the too expensive side...

[pickle9000]

Aluminum beer kegs painted black.

2 drops of bleach per liter of water will keep it from greening up.

[KJDS]

The Victorians would use piles of fresh horse manure that releases heat as it rots. Once it's well rotted then use it as fertilizer. If there aren't any stables near you then you'll need to buy a horse, which won't be cheap but could be cheaper than a thermal camera.

[hamster_nz]

Painting the inside might work better than painting the outside black. Hot black paint in contact with the water but behind the insulating plastic will heat better than cold water and hot paint seperated by the thermal resistance of the plastic.

Segway back to electronics....

You could model this with resistors, capacitors and a voltage source... Resister to ground for heat loss to the air. A resistor to represent the thermal propertie of the bottle, a cap to represent the thermal capacity of the water and then a relatively high voltage source and high value resistor to represent the incoming/outgoing  radiation.

The voltage source has to be relatively high... The sun is quite hot,  and when the sun isnt shining the bottle will radiate IR out too.

Or maybe I am over thinking it...

[technix]

Four letters: soot.

This is how Chinese ancients made their inksticks - in fact the best Chinese ink is still made from wood soot. Mix soot into some kind of adhesive material (The best Chinese ink used animal glue with a little bit of herbs in it, but any strong adhesive can be used here to make functional ink) and you get some Chinese ink, more than enough for this purpose (and if you have some remained, you can also try your hands at Chinese fine arts if you want to)

[PeterFW]

Let me pile up on the answers: E150 Food coloring.
(Caramel color, if that is the correct english translation).

Sold in liter bottles in commercial food shops for very little money.
Fabric dye is not that opaque, a bunch of other things recommendet are not that water soluable.