As mentioned in my previous post I recently performed a deep clean and am now getting +/- 15* difference between IBTS and BT probes. What’s crazier is the impact a dirty IBTS seems to make
Fired up my Bullet for the first time since the cleaning and went to roast a new recipe for an Anaerobic Nicaraguan I have for a client. I was absolutely shocked with my previous RoR was entirely different than this roast. Pre-cleaning the recipe was PH 265* and about 5-6 power and fan adjustments with a steady 8-10*C RoR throughout the roast with the 775g batch ending around 10:30-11min.
After 6 back to back roasts of this bean I am FIGHTING to keep the RoR above 6-7C. I’ve tried stripping the recipe down to D9-F2 and a couple of Power decreases and that was unsuccessful. To get it back to what I had originally created I bumped up the PH to 275 and am only making 2 power and 2 fan adjustments to try and get this back under control.
I understand that a dirty IBTS would impact readings and therefore roast adjustments, curves, etc but not to this degree. I’m definitely hoping these roasts turn out ok because I’m on a quick turnaround for them and can’t spend two weeks burning through beans to re-develop the recipe.
You’re right; a dirty IBTS Lens can heavily affect the IBTS Temp readings, and consequently, roasting performance. This stems from the IBTS Lens reading lower than it should, causing the actual temperature to overshoot during preheating.
When this happens, you’re essentially roasting at a higher preheat temp. Once you’ve cleaned the IBTS Lens, with the Bullet preheated to the accurate temperature (which is noticeably lower than before), you might find it hard to carry the heat over to the development phase. If you’re seeing this, you might want to consider preheating to a higher PH temp.
I wonder if you could look back at your pre-cleaning roasts and check your BT at charge. Whatever IBTS PH you need to get a similar BT temp at start might get you much closer to where you were.
I find the BT to be a more reproducible metric for the preheat state of the roaster. IBTS will tell you the roaster’s ready just 10 minutes (or maybe less) from a cold start, and it clearly doesn’t have as much stored energy as it will after the first roast finishes.
Thanks for the reply @da_kevin and the explanation. After about 3 roasts I bumped up the PH temp by 10*C and that seemed to smooth things out and carry me through development okay.
One thing I am still confused by is how much I needed to increase PH temp to achieve this. I’ve never had a recipe at my normal batch size of 775g preheat above 275C. Based on your feedback and the data with this roast that would mean I would need to adjust all my current recipes to accommodate this 10 increase?
It might take a little bit of experimentation to find out what you’ll need for the other roasts, as the required energy varies among different beans and batch sizes. Now that your IBTS Temp readings are back to normal, we would encourage you to test out different preheat temperatures and see which settings works for you.
