Now that I've got my roaster giving me meaningful temperature readouts I asked H-B member Dregs (Barry) offline about his recommendation for using the TJ-067 gas roaster (propane or natural gas) with different charge sizes. This was after osanco (Steve) mentioned Barry had been experimenting with small loads. Barry's response was so helpful that I got his okay to post it here, verbatim.
Dregs wrote:While most of my batches are sized for the "sweet spot" on this roaster, 20 to 28 oz, I have done batches varying in size from 4 to 44 oz. The roaster is still easy to control and very responsive in that broad range. In fact, I recently picked up some Panama Perci N2 Level 39 over at GCBC ($58/lb, just crazy) and did 3 batches of 4 oz each before roasting a larger batch. The roaster performed flawlessly. My taste buds, on the other hand, lack the sophistication to appreciate that level of bean. Anyway, I have the confidence in the roaster to do a 4 oz batch, even with that bean.
My general approach for any size batch (after letting the roaster heat to about 450F and cool to charge temp) is:
-Preheat to a specific temp on the BT thermocouple. This temp varies from 255F for 4 oz batches to 325F for 44 oz batches. I determined that temperature range based on results of many batches. I keep the exhaust fan low (my fan stops at 10, so 12 is my minimum during a roast).
-Gas is off for the 1st minute after charge with the fan on 12. I usually hit the turning point at around 1 minute. At 1 minute, I turn the gas on. The level is about 40% of max for 4 oz batches, up to 80% of max for 44 oz batches. I also turn up the fan a bit, to about 20. I adjust mostly with the fan to hit my target End of Dry (300F) time.
-At EoD, I raise fan speed a few points, but don't change the gas level for 1:30 to 2 minutes (although large batches may need max gas at this point). Then I continuously lower gas and raise fan speed throughout the ramp to hit my target time for 1Cs. I use the projection lines and HUD readout in Artisan to help do that. I try to get to a gas pressure of 30 - 60% of max and a fan speed of about 40 at least 1 minute before I expect to hit 1Cs. Obviously, larger batch sizes require more gas than smaller.
-If I have gotten into 1Cs at an appropriate gas and fan level, I make no adjustment for about a minute. I've been trying to do roasts as Scott Rao recommends. Since "exothermic flash" will cause the ROR to drop as the beans enter 1Cs, you want to reduce heat input before, not at that point. As 1C is ending, I drop gas pressure and increase fan further. I am usually at 20-30% of max gas and fan setting of 40 - 70 to coast to drop. I rarely roast darker than Full City.
The one problem with smaller batch sizes is that the BT thermocouple does not read accurately. With anything less than about 8 oz, it reads low. With a 4 oz batch, first crack will likely read 10 - 12F lower than a 16 oz batch of the same bean. I really don't find that to be a problem. I know it will happen, so I just compensate in setting my time/temp targets. I do reduce the drum speed for smaller batch sizes:
<16 oz 40RPM
16-28 oz 60RMP
>28 oz 80RPM
The slower drum speed helps to keep the BT thermocouple in the bean mass.