Cold air sampling - heat exchanger design ideas sought please

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Nick McCloud

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Jun 9, 2025, 5:28:12 PMJun 9
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I've a student launch in a couple of weeks time which is unlikely to have any science module so I'm thinking of flying the new Sensation SEN66 Particle, Temp, Humidity, VOC, CO2 and kitchen sink sensor.

It's rated 0 to 60+ deg C so I will need to use the internal heat generated by the PitS tracker to warm up the incoming air. The SEN66 has a fan on it, so moving the air is solved, it's just making sure it's adequately warm to not kill the unit.

I can either just use a coil of copper tubing or a computer water cooling block or combo thereof with some Aluminium heatsink fins added for good measure.

However as I get older I find I come up with all the good ideas about a week after I needed them, so I'm running this past y'all just in case someone has any suggestions.

I've got about 100g in hand, so please, no suggestions of an oil filled mains radiator with diesel generator set!

Mika Köching

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Jun 9, 2025, 5:32:18 PMJun 9
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I would probably just put it into a box and pass the air through some thin PTFE tubing. Should be fairly light and pretty much the simplest solution. That way the tracker would heat up the air in the box which would then heat up the tube the air from outside is flowing through.

Nick McCloud

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Jun 10, 2025, 6:32:33 AMJun 10
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On Monday, 9 June 2025 at 22:32:18 UTC+1 Mika Köching wrote:
I would probably just put it into a box and pass the air through some thin PTFE tubing. 

Definitely simplest but the trouble is that a common application of PTFE is as a thermal insulator ...

One observation offline of this group was to try stuff out with the freezer - only half as cold but should give some good indications of what may or may not suffice.

John Laidler

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Jun 10, 2025, 7:20:43 AMJun 10
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With 100g to spare one of the chemical hand warmers you can get might work. Something like in the link below, but how they might react at low pressure is of course something of an unknown!


John

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Nick McCloud

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Jun 10, 2025, 7:38:12 AMJun 10
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On Tue, 10 Jun 2025 at 12:20, John Laidler <rjohnl...@gmail.com> wrote:
With 100g to spare one of the chemical hand warmers you can get might work. Something like in the link below, but how they might react at low pressure is of course something of an unknown!


Ah, good idea, but I'm expecting the box to be warm enough inside - if I go mad with the gaffer tape I've seen +40degC. But if I put one on the SEN66 itself it should keep it warm. Or use one to ensure the pipes are warm enough.

 What I've got to figure out is the most reasonable weight/efficiency of warming the air before it hits the sensor. And preferably without being too Heath Robinson. At present I'm going to buy various copper tubes from Amz and see what I can build with them.

Nick


Steve

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Jun 10, 2025, 7:50:18 AMJun 10
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Warm the sensor itself with a small resistor - use the sensors own temperature measurement in a feedback loop to turn heating on/off at setpoint temperature inside the sensors working temperature range.

Ive seen this technique used on semi-commercial HAB equipment - where a pressure sensor was warmed with a resistor on the other side of the (thin) PCB.  Very little power needed to get into working range.

I think I have looked at payload warming before and concluded electrical heating was the most efficient power to weight wise.

    Steve

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Mika Köching

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Jun 10, 2025, 7:55:04 AMJun 10
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Electric heating is definitely the way to go. 
Those chemical handwarmers need oxygen of which there is not very much at altitude so I'd guess they won't work well.
You're probably fine just putting the sensor in the box then.
Humidity sensors for radiosondes are usually heated only a bit so they don't gather any condensation, I think. As far as I know the one from the RS41 is kept 5°C above ambient temperature.

Steve

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Jun 10, 2025, 8:20:16 AMJun 10
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Yeah - forget hand warmers

    * "tea bag" hand warmers only work when there is air (its an oxidation process) and release humidity at low pressures.

    * fuel hand warmers likewise don't work when there is no air.

    * Gel crystallization hand warmers - Sodium Acetate latent heat of fusion is about 275 kJ/kg

Energizer Ultimate Lithium Iron Disulphide AA battery = 5Wh for 15g = about 1000 kJ/kg (i.e 3.5x better power to weight than gel hand warmers) - these batteries capacity is reduced at low temperatures with heavy currents - but for heating a sensor should be no problem.

    Steve G8KHW

Steve

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Jun 10, 2025, 8:26:01 AMJun 10
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Oh and I should add the obvious: electrical heating is far more controllable.

    Steve G8KHW

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