Fluctuating temperatures in corretto. Advice sought.

[LordL]

So I'm having some difficulty in fine tuning my roasts.
I use a corretto with a Bosch PHG 630 DCE.
About a month ago, something changed and the roasts appear harsher.
What has changed during the roasting process is when I reach about 130 deg., the ROR is hard to control.
In the attached jpeg, from 130 deg. right up to first crack, the gun temperature was constant. the ROR fluctuated from about 5 to 9 deg, and I could not get it above that. (I think I had the gun set at a constant 400) As you can see from the blue line, it's a regular fluctuation. I have studied my old roast profiles and the ROR line is generally smooth.
Raising the temperature on the gun doesn't seem to help.
So I'm wondering if I have a problem with the gun. Is it the thermostat that is malfunctioning?
Has anyone else had a similar experience?

[doobs]

Hi LordL, I have a similar problem. For the last half dozen roasts or so I’ve been getting temp fluctuations on my ROR. It looks like the multimeter is not reporting as consistently as it used to rather than an issue with anything else. I suppose it could be the gun temp fluctuating but my probe is buried in the bean mass so I doubt it.
It makes it difficult to manage the ROR because it fluctuates between 9-15 with a semi-constant of my targeted 12 ( I hope that makes sense). It used to vary by only a degree either way. I’ve tried a new thermocouple, check batteries in the multimeter, run my bread maker off a different circuit because I started getting temp reading spikes from plugging in my cooler.
I was wondering if it was the communication from the thermocouple to the PC USB. I’ll have to monitor the thermo reading against the reported reading in the PC and see if they have the same pauses.

[Dimal]

G'day "LordL"...

Even though there is a wobble going on with the RoR trace, the overall profile still doesn't look to bad, apart from the droop after 1st-Crack. If it's down to anything, given the short cycle nature of the reported temperature excursions, I'd reckon it's more likely to be something going on with HG temperature control than anything else. How old is the HG by the way? May be able to claim a replacement under warranty...

If you have a Bead t/couple, you could try positioning it in the HG exhaust stream while varying the output of the Gun, and watching the trace in the Roast Monitor software and the DMM indicated temperature at the same time...

Might be able to determine where the origin lies that way...

Mal.