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On Sat, 15 Apr 2023 08:51:39 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Saturday, April 15, 2023 at 10:26:24?AM UTC-4, John Larkin wrote:
On Sat, 15 Apr 2023 07:10:18 -0700 (PDT), Fred Bloggs
bloggs.fred...@gmail.com> wrote:
On Saturday, April 15, 2023 at 1:09:21?AM UTC-4, Anthony William Sloman wrote:
On Saturday, April 15, 2023 at 6:01:11?AM UTC+10, John S wrote:
On 4/14/2023 1:17 PM, Lamont Cranston wrote:
I have ordered four 12V 12W Flexible Polyimide Adhesive Hot Foil Heating Film. If I wire two in series and then put that in parallel with two more in series and run it on 19V, (solving my 19V problem) I will have about 30 Watts of heating. I\'m heating the air of a 2 gallon plastic bucket ~0.25cuft. I\'m thinking (guessing) this will be plenty of heat to keep the bucket at 80*F with a minimum ambient temperature of 68*F.
I do have a temperature controller to shut off the heater when the desired temperature has been reached. It does have a small insulating air gap between the two buckets.
Your thoughts?
Mikek
As added info, I\'m building a small bean sprout grower with adjustable watering schedule and temperature control. it is all set inside a two gallon bucket, that is slipped into a second bucket.
The temperature difference is 80F - 68F = 12F.
The mass of air in 0.25 cubic feet is 0.020175 lb.
The amount of heat required to raise the temperature of 0.020175 lb of
air by 12F is:
0.24 BTU/lb °F x 0.020175 lb x 12 F = 0.05796 BTU
Converting BTU to watts:
0.05796 BTU/hour x 0.293 watts/BTU = 0.017 watts (rounded to 3 decimal
places)
So, approximately 0.017 watts of power would be required to heat 0.25
cubic feet of air from 68F to 80F, assuming the air is contained in a
well-insulated container.
This is idiotic.Watts are units of energy flow rate. The answer to the question has to be Joules, not Watts which are Joules per second.,
0.25 cubic feet of air is 7.1 litres. At room temperature one mole of air occupies 24 litres so that is about 0.3 mole air molecules are diatomic so they store both rotational and translational energy so their heat capacity is 5/2R where R is the universal gas constant 8.314 J?K?1?mol?1.
So the heat capacity of a mole of air is 29.8 joule per mole per degrees Celcius.
So 0.25 cubic feet air has a heat capacity of 3.46 joule per degree Fahrenheit and you\'d need 41.6 joules to heat it from 68F to 80F.
What actually matters is the heat you\'d have to supply to keep it at 80F, which depends on the amount of insulation you wrapped around the whole set-up which will have a much higher heat capacity than 0.25 cubic feet of ari
It doesn\'t need to be that complicated. The air is nothing, unless he\'s blowing fresh air through it, which doesn\'t sound like he is. The heater only has to make up for conduction through through the boundary of the bucket. Most of these little plastic buckets are HDPE, which from what I can find has a thermal conductance of k = 0.45-0.5 W/(m-C).
Air is about 0.03. The bucket is not sitting in a pool of stirred
water.
So practically speaking, a mean diameter D , is circumference pi x D which when multiplied by H gives the are of sides As= pi x D x H. Then for the end(s) it is Ae = pi x (D/2)^2. So he maybe has total area A= As + 2 x Ae = pi x ( D x H + 2 x (D/2)^2) Then he needs to break out a caliper and measure thickness of the material, t. The total heat loss becomes (k/t) x A x (Th-Ta), h= heated volume, a= ambient. This will not require a whole of power for a 12oF= 7oC differential. All measurements metric. Simplest control is to PWM the heaters down to the few watts needed.
Practically speaking, it wouldn\'t make much difference if the bucket
was made of silver instead of plastic. Well, it would cost more.
The heated exterior of the bucket will create a draft, however slight, but the heat loss calculated through the conductance stays unchanged. The only effect of the air and other material inside the bucket is to contribute to thermal capacity of the whole, which may be considerable, but won\'t change anything once he brings it up to temperature. Most of these home projects with heaters and light bulbs end up overheating the plants.
Just did a quick measurement on a 5-gal HDPE: D=12\", H=14\" t=1/16\"
A=pi x ( 12x14 + 2x(12/2)^2)= 754 sq in., so heat loss= 0.5 x (1/16) x 754 x 7oC/ 39.36= 4 W. Maybe only 2 W for the 2-1/2 gal bucket. The 39.36 factor is inches/meter. And that\'s just the bucket, there may be leakage. If he strings the two 12V 12W in series and applies 19V, he gets 2 x (9.5/12)^2 x 12W=15 W , so maybe a 15% duty on the heater just for the bucket. If the pulse is safely 100ms, then the frequency would be about 700ms, 1.4Hz. That\'s just a little op amp/ logic circuit.
The thermal resistance from the interior of the bucket (assume it\'s
isothermal inside) to the world depends very little on the thermal
conductivity of the bucket material. The first inch of air outside of
the bucket has a huge thermal resistance. Convection or wind will
reduce the thermal resistance of the air but it\'s still big.
The solid bucket walls, plastic or silver, will have very low
temperature drop. Air wins.
I have ordered four 12V 12W Flexible Polyimide Adhesive Hot Foil Heating Film. If I wire two in series and then put that in parallel with two more in series and run it on 19V, (solving my 19V problem) I will have about 30 Watts of heating. I\'m heating the air of a 2 gallon plastic bucket ~0.25cuft. I\'m thinking (guessing) this will be plenty of heat to keep the bucket at 80*F with a minimum ambient temperature of 68*F.
I do have a temperature controller to shut off the heater when the desired temperature has been reached. It does have a small insulating air gap between the two buckets.
Your thoughts?
On 4/14/2023 11:17 AM, Lamont Cranston wrote:
I have ordered four 12V 12W Flexible Polyimide Adhesive Hot Foil Heating
Film. If I wire two in series and then put that in parallel with two more in
series and run it on 19V, (solving my 19V problem)Â I will have about 30
Watts of heating. I\'m heating the air of a 2 gallon plastic bucket ~0.25cuft.
I\'m thinking (guessing) this will be plenty of heat to keep the bucket at
80*F with a minimum ambient temperature of 68*F.
 I do have a temperature controller to shut off the heater when the desired
temperature has been reached. It does have a small insulating air gap between
the two buckets.
  Your thoughts?
Fill the gap with water and use an (tropical) aquarium heater to keep the water
jacket at the desired temperature.
The interior cavity will (eventually) stabilize at the temperature of
that water jacket (unless you are continually opening the space)
I have ordered four 12V 12W Flexible Polyimide Adhesive Hot Foil Heating Film. If I wire two in series and then put that in parallel with two more in series and run it on 19V, (solving my 19V problem) I will have about 30 Watts of heating. I\'m heating the air of a 2 gallon plastic bucket ~0.25cuft. I\'m thinking (guessing) this will be plenty of heat to keep the bucket at 80*F with a minimum ambient temperature of 68*F.
I do have a temperature controller to shut off the heater when the desired temperature has been reached. It does have a small insulating air gap between the two buckets.
Your thoughts?
Mikek
As added info, I\'m building a small bean sprout grower with adjustable watering schedule and temperature control. it is all set inside a two gallon bucket, that is slipped into a second bucket.