Switching a 3Kw Heater

[JohnW2]

Top sensor used to take priority 1 reading, then middle used and compared to top. Bottom used to oversee and has a high limit set to shut things down if it goes crazy it gets stuck on. Basically at draw off is most critical temperature and it uses a high accuracy probe to grab that data to react fast.

I can understand the bottom one - although, as you said previously, that 'safety' function could be performed by the immersion's own thermostat. No harm in 'belt and braces', though!

However, what is the result of the 'comparison' of the middle and top temps? Will that sometimes result in the temp at the top (at draw-off point being raised to a little more than you actually want (for the draw-ff temp), because the temp in the middle is a little 'low'?

Apologies for all the questions, but I'm just trying to 'understand'!

Kind Regards, John

 

[oldbutnotdead]

Go for either. The Sonoff stuff used relays on the output so would switch an SCR quite happily. Me I'd go for a 20A relay (got one on the output of a Sonoff quad card as it happens), long as you're not switching every minute it'll be fine and you do get a nice clunk when the thing changes state!

 

[wcavanagh]

Go for either. The Sonoff stuff used relays on the output so would switch an SCR quite happily. Me I'd go for a 20A relay (got one on the output of a Sonoff quad card as it happens), long as you're not switching every minute it'll be fine and you do get a nice clunk when the thing changes state![/QUOTE

What relay are you using? I’m struggling to find any 20A 240v coil

 

[oldbutnotdead]

Seek and ye shall find ...https://smile.amazon.co.uk/gp/aw/d/B07JRFY3KX?psc=1&ref=ppx_pop_mob_b_asin_title

 

[wcavanagh]

Seek and ye shall find ...https://smile.amazon.co.uk/gp/aw/d/B07JRFY3KX?psc=1&ref=ppx_pop_mob_b_asin_title

Never thought to look at Amazon lol, was on RS and Farnell etc.

Thank you, that’s perfect. I have ordered

 

[wcavanagh]

I can understand the bottom one - although, as you said previously, that 'safety' function could be performed by the immersion's own thermostat. No harm in 'belt and braces', though!

However, what is the result of the 'comparison' of the middle and top temps? Will that sometimes result in the temp at the top (at draw-off point being raised to a little more than you actually want (for the draw-ff temp), because the temp in the middle is a little 'low'?

Apologies for all the questions, but I'm just trying to 'understand'!

Kind Regards, John

no worries, life is all about questions. So the middle one comes into play when I need extra precision control of final temperature. When I switch mode, I use the middle probe to grab temperature and then I can predict the time it will take to heat the water using the input water temperature and current top temperature. Then I know to turn off the heater before it gets too hot and this avoids overrun. Only in one type of run do I need this level of control. But having this setup gives me enough need for the brewing process.

 

[Harry Bloomfield]

It’s a project for a brewing system. The switching differential on the stat with the heater is awful.

I brew and ferment wine, neither of which I find are that critical of temperature control. I used an Inkbird type controller, remote sensor taped to the side of my 5gallon FV, which sits on the heating panel. Around the outside of the FV, I wrap insulating foam. Hysteresis of the Inkbird is adjustable which I have set at 1.5C. Separately and in the brew I have a floating temperature sensor, which feeds its data to wifi and is logged. Temperature of the actual brew never varies more than 0.5C I find.

 

[wcavanagh]

Very nice, but I’m making something a little different where temperature is required to be spot on for final product.

can’t give too many details just now.

but it’s been years in development and critical temperature control is required

 

[Harry Bloomfield]

but it’s been years in development and critical temperature control is required

Then you need the precision of proportional control.

 

[wcavanagh]

Then you need the precision of proportional control.

What I got is working perfect, now I have the relay, we are sorted.

Thank you everyone for your help

 

[JohnW2]

no worries, life is all about questions. So the middle one comes into play when I need extra precision control of final temperature. When I switch mode, I use the middle probe to grab temperature and then I can predict the time it will take to heat the water using the input water temperature and current top temperature.

Fair enough, and thanks for explaining - but that presumably requires you (the software) to also know the flow rate - are we perhaps talking about a 'circulating loop' with a fairly constant rate of flow?

Kind Regards, John

 

[wcavanagh]

Yep, the loop is constant for sure, it goes into a coil used for indirect heating

 

[JohnW2]

Yep, the loop is constant for sure, it goes into a coil used for indirect heating

If temperature control is as crucial as you have indicated, would not the ideal be to have the primary temperature sensor in whatever is being heated by the coil, rather than anywhere in the vessel providing the heated water flowing through the coil?

Kind Regards, John

 

[wcavanagh]

Have tried that before, and it overshoots, found much better control with the other side. I got my relay, and it is working sweet as, just now

 

[JohnW2]

Have tried that before, and it overshoots...

Fair enough, but if it were overshooting, that presumably would be down to the settings/behaviour of the control logic, wouldn't it?

Maybe I'm missing something, but I would have said that if one wants precise temperature control of 'something', it is always desirable/preferable to do so on the basis of sensing the temperature of the 'something' - any more indirect approach will invoke additional factors which may be unknown and/or varying.

To mention an extreme example, imagine trying o control the temperature of the air within a room by sensing the temperature of the hot water entering the radiator!

... found much better control with the other side. I got my relay, and it is working sweet as, just now

I'm a great believer in pragmaticism - so, if it's working to your satisfaction, then that's great, and all that matters, despite my confusing things with theoretical discussion!

Kind Regards, John