Re: PID

jggall01 wrote:
Jacob -

I started down that road a little while ago, using an adhesive mounted
t/c placed nearly at the base. I had taken some measurements, so I
knew I needed a much lower set point for a probe at that location. I
chose 186 because that was the steady state temp I measured
corresponding with steady state 228 on top.

I think you may have placed the t/c too low where there is litle if any
water. 186F reflects the temperature of the water almost as poorly as
228F. The main body of water is in the top 2/3 or so of the boiler (of
course with the exception of the absolute top where the vapor is).

Placing the t/c on the middle of the boiler side should give the best
reading of the water temperature.

The problem I ran into, probably solvable in hindsight, was the boiler
heating too much for my comfort. Because of the long lag in the lower
boiler coming up to operating temperature, the top surface temperatures
were hitting around 300 during the initial warmup/heatup phase. I
wasn't comfortable with that, not wanting to risk blowing the
protective thermal "fuse."

This may actually be because you were even further away (thermally) from
the water than the top plate. I see no such overshoot.

All this just reflects that placing a t/c on the outside of the boiler
will always be an indirect measurement, and readings will vary widely
depending on the exact placement.

If settling times and overshoot is ok the t/c is placed ok. If not,
moving it can change the behaviour more than is possible by adjusting
PID parameters. Just remember that this requires the PID to be
recalibrated and the target temperature to be adjusted.

/Jacob
.

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