Boiler heater element life PID vs Pressurestat?

[Kristi]

Here's an off the wall question.

A pressurestat would cycle less than fifty times for a single brew for me (power up, wait a half hour, brew, shut down).

A PID would cycle probably close to a thousand times for the same thing.

Both are simple on/off switches as far as the heater element is concerned.

Isn't a PID tearing up the element?

(yes Dan, I have searched and googled.)(I think I'll put that in my sig.) ;o)

Any way to soften the blow of an on/off cycle?

tia
Kristi

[snoboy]

Kristi-

I know I have read the opposite in fact, that higher frequency PID control is easier on the heating element in the long term. Can't remember the reference right now, but I know that it was somebody who I thought sounded legit, a heating engineer or something like that.

Oh, here we go, here's a quick find via Google:

The most effective way to minimize heater element temperature cycling, and the most expensive solution, is to use solid state relays (SSRs) and SCR power controllers coupled to PID temperature controllers. This combination provides the best performance for both your thermal system as well as for the heater itself. Solid state switching devices cycle power to the heater very rapidly (from one second with a SSR, down to milliseconds with phase-angle fired SCRs). This fast-power cycling dramatically reduces heater element wire temperature excursions and substantially extends heater life.

[bold mine]

From a Watlow.com page...

[Kristi]

Thanks.

From that same page, right in front of it:
Tip 6: Prevent excessive heater cycling
Excessive temperature cycling is very detrimental to the life of a heater. The most detrimental is the cycle rate that allows full expansion and contraction of the heater resistance wire at a high rate (30 to 60 seconds' power ON and power OFF). This causes severe stress and oxidation of the resistance wires inside a heater. A bad temperature cycle is typically found when thermostats are used. Thermostats respond slowly to temperature changes and have large switch ON/OFF temperature differentials. An improvement, but a somewhat more expensive solution, is to use ON/OFF or PID controllers with mechanical relays. It is crucial to not switch the frequency or cycle time too rapidly (somewhere between 3 to 10 seconds), because the relay contacts can wear out quickly.

With a PID, it cycles on and off for less than a second, every few seconds, modulated by the parameters and the distance from the target temp. In other words, the attempt to have little variance of the boiler temp results in much on/off switching.

confused!
Kristi

[snoboy]

I think the bit you quoted is talking about the need to keep the switching frequency down for the sake of the mechanical relay. In our case, most of us are using SSR in combo with the PID.

A thermostat allows the heater to fully heat and cool in each cycle, which stresses it the most.

[Kristi]

Yeah, and it seems to stress that a long 30-60sec heating on time or off time is bad because it allows big changes in the element, whereas the little changes due to a PID are probably negligible.

Thanks.

[timo888]

I have searched and googled

Should I, after 'spresso shots and ices
Have forced the element to crisis?
But though I have searched and googled, searched and prayed,
Though I have seen my crema (grown slightly flat) grow every day much flatter,
I am no prophet--and here's no great matter;
I have seen the light upon the p-stat flicker
And I have seen the Eternal Schulman hold my cup, and snicker,
And in short, I was afraid.
--
T.S. Eliot, Love Song of J. Alfred Pidrock

[AndyS]

Yeah, and it seems to stress that a long 30-60sec heating on time or off time is bad because it allows big changes in the element, whereas the little changes due to a PID are probably negligible.

Thanks.

A watlow guy once told me that a one second cycle time was almost as good as pure analog control when it came to extending heater life.

[Kristi]

A watlow guy once told me that a one second cycle time was almost as good as pure analog control when it came to extending heater life.

Interesting. So I guess one would use a PID to extend heater life...

I hear pressurestats give out after a year or 2. I wonder how long before an element controlled by a pressurestat would give out. Course if the pressurestat gives out then you have a good excuse to get a Sirai........
Kristi