temperature mystery - evaporative cooling perhaps?

[Hippie Dickie]

Over the last several months i have been wondering, off and on, about an observed temperature effect while vaporizing that i can't explain.

Here's the situation ... as the 7.5 minute Bud Toaster session progresses, the temperature gradually increases. Starting at 385F, by the time 6 minutes has passed the temperature is showing 390F. All throughout the session, the green LED is slowing flashing on and off (2 or 3 flashes per second), as the temperature bounces above and below the SETPOINT temperature. Given the jitter in the temperature reading from the MAX6675 of 0.5C, i think this is the ideal behavior.

So, as far as the processor knows, the actual temperature matches SETPOINT (at least as well as the PID implementation allows).

However, when i look at the thermometer display, it is doing this slow drift up. i think the k-type thermocouple has much faster response than the probe used in the thermometer, so the thermometer is showing an average temperature. This is quite convenient -- automatic damping of a rapidly changing value.

Now, it is true that for every toke, the temp is "pulled" up by about 0.5F, and then settles back to SETPOINT -- and the green LED stays lit during this above temp excursion and then goes back to the slow flashing. i think the derivative term in the PID algorithm is overcompensating -- but i really like this action.

i like to open the current operational Bud Toaster every month or so to check for wear and tear, and upload the contents of eePROM to see how the algorithm is cooking. So this time i checked the relative position of the thermocouple and the probe.

Here is the disassembly:

and here is a close-up of the two probes:

The k-type thermocouple is one polyimide tube thickness away from the wire of the heater coil. The thermometer probe is touching the glass oven tube. It just seems to me this configuration is close enough for both probes to see the same temperature.

So, i'm confused: the algorithm thinks it is at SETPOINT, but the thermometer shows a gradual increase in temperature. Not a bad thing, per se, but confusing.

The only explanation i have come up with is that there is evaporative cooling as more vapor is created earlier in the session, and then, as the trichomes empty, the actual temperature increases. That is, the heater is actually at 390F, but the glass is cooled and can't reach thermal equilibrium with the heater until the end of the session when the trichomes are spent.

more proof that i would have been a lousy physicist.

 

[tdavie]

I took thermodynamics was in electrical engineering, eh it was 30 years ago, and I did drop out after 3 years, but still.... I think I understand what you're saying.

In my opinion, for the trichomes to have this sort of effect, they would either have to have a kargerlvolume, or you're also looking at evaporative cooling from water removal from the much larger mass of the non trichome portion or
component of the weed.

A lot of people talk about heat capacities of the vaporizers and the ability to maintain a stable(ish) temperature because of mass.

Sorry if I should like an idiot (it has been 30 years and I am a biochemist now), but have you noticed this same effect on bone dry weed as well? Or just on wet weed?

Tom

 

[lepstadder]

I dunno what your talking about, derp

But could condensation cause it?

 

[Hippie Dickie]

have you noticed this same effect on bone dry weed as well

Good thought, but when the bud has any moisture at all, it condenses inside the bottom of the draw tube -- like 1" above the top of the vial. i find that annoying, so all the bud i vape is properly dried so there is no condensation of water from the green.

However, the trichomes also have a waxy outer cellular coating that also gets melted -- to release the vapor. Maybe that has a cooling effect too.

i also wonder if this should be a user selectable feature -- to allow the user to specify how much and how soon the temperature will rise toward the end of the session.

These days i only use the heat setting controls (+5F or -4F) based on the strain and density of the bud material. But, really, the standard 385F setting is pretty efficient.

Also, i think the interior of the cube is getting heat saturated -- of course, there is never any transfer of heat to the outer surface of the cube, except right around where the oven tube pokes up throught the top of the cube. This will be fixed in the future units i make, using a centering guide -- this should also prevent checking (cracking) of the wood around the top hole.