PID or Relay for Circulating Shower

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Discussion Overview

The discussion revolves around the choice of control methods for heating water in a circulating shower system. Participants explore the feasibility of using a PID controller versus a relay-based approach, considering factors such as temperature fluctuations due to water circulation and heat loss.

Discussion Character

  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • One participant considers using a PID controller with PWM and a MOSFET for heating but questions its effectiveness due to potential temperature variations from water circulation and heat loss.
  • Another participant suggests that the effectiveness of the control method may depend on the reservoir's time constant and proposes maintaining a fixed thermal mass in contact with the water.
  • Some participants express a preference for avoiding relays due to wear but acknowledge that relay circuits are simpler to design, indicating a separation between the choice of switching device and control method.
  • A participant mentions that a simple Arduino setup might be beneficial for future modifications and suggests considering an ATtiny85 for the project.
  • One participant argues that a simple temperature switch and relay would be sufficient for the application, expressing skepticism about the need for a PID controller.
  • A participant recalls that their circulating water bath does not utilize a PID temperature controller, implying that simpler methods may be adequate.

Areas of Agreement / Disagreement

Participants express differing opinions on the necessity and effectiveness of using a PID controller versus a relay. There is no consensus on the best approach, with multiple competing views remaining in the discussion.

Contextual Notes

Participants highlight potential issues such as the impact of water pollution on heating elements and the varying thermal dynamics due to water circulation, which may affect the choice of control method.

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TL;DR
I am making a closed loop circulating shower for my campervan, and I am trying to decide between PID and Relay.
I am considering using PID (autopid arduino lib), in order to control the heat, with PWM and a 50amp mosfet heating a 40amp 24v heating element in a 3litter tank, but I am thinking, that since the water is going to be circulating, and sometimes losing heat to a person, sometimes falling straight into the drain, it will be varying anyways between +/- a few degrees, does it even make sense to use PID? or should I just use a 50amp relay with a basic range cycle?
 
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It will depend on the reservoir time constant. You might do better with a fixed thermal mass in contact with the water, maintained at the required temperature.
 
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I think you could do it either way. My preference would be to avoid relays, they wear out. However, relay circuits are easier to design. The switching device selection is, in general, a separate issue from the control method.
 
A simple arduino set up may be best - even if just to make edits in the future. Look at an ATtiny85 - not "native" in Arduino IDE but can be done.

On the Recirculate point - if you are recirculating water from the show back to the heater - be wary, as the pollution ( oils, soap, organics, etc) can really make a mess of a heater.
 
Seems like overkill to me. A simple temperature switch and relay is probably sufficient. I would not use a pid controller.
 
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