Recommendations on sensing water temperature using a BS2(solar hot water application)

tentoes1

Hello all:

I am new to the world of microcontrollers and I am in the initial stages of designing an active solar hot water heater that uses a BS2 as the brains. I know that there are very simple designs but I wanted this one to be more robust and that can handle situations like if no hot water is used (doesn't overheat), etc. Basically sweat-soldered copper pipe as the collector, with a pump that switches based on water temperature. The solar-heated water then feeds a separate heat-exchange tank before the water enters a regular hot water heater.

What recommendations do you have as the temperature sensor/method? I envision just using a thermistor and measuring the RC time constant using the stamp. What problems/issues do you see in this?

72sonett

I would use an ADC like an MCP3204 to measure temperature in the solar collector and the temp in the boiler. Obviously if the temp in the collector (outside) is lower than the temp in the boiler, then you do not need to switch the pump on.
As you mentioned, if the temp in the boiler is at or above a high limit you do not want to heat it up any more, so the pump stays off.
Also, you may want to measure the water level in the boiler, that should not be below a certain level.
I would also add an LCD to show status and temperatures etc.

These limit conditions pretty much determine your program, in pseudo code;

DO
     measure boiler temp
     measure collector temp
     measure water level
     message "boiler temp", "collector temp"
     IF boiler temp > high limit THEN pump off, message "boiler temp high"
     ELSE IF water level low THEN pump off, message "water level low"
                       ELSE IF collector temp > boiler temp THEN pump on, message "pump on"
LOOP forever

Duane Degn

An ADC chip is one option for reading a thermistor but I think you're original idea of using a RC time measurement is also a viable option.

Genetix

Take a look at this which uses an LM34 and an ADC0831 with the BS2 for temperature control.
http://www.parallax.com/sites/default/files/downloads/122-28176-Process-Control-Text-v1.0.pdf

LA6WNA

Hi.
Interesting project. I love solar heaters and stuff like that. Thought you might could use my BS2 program that runs my pool system as a beginning, so I`ve attached that file. This is basicly two solar heater circuits, one for the pool heaters and the other for the shower tank. I use LM35CZ as sensors and the signal is feeded to the BS2 via ADC0834. (four channels for my four sensors.) The resolution of temperature in my system, is whole celsius degrees. The tenths variables is not used. (used in an earlier version)
This system also have an radiomodem that sends data to my Spinneret, but you can easily delete all the things you won`t use. Just take a look at the file and see if you can get any ideas from it. Hope it helps.

PS. The debug text in this file is in Norwegian languege. You`ll surely undertand...



EDIT: I first started out with NTC`s and RC-measurement, but that readout would newer be right over a large temperature span. (ex 5-90 degrees) I tried many formulas and different ways to calculate the RC-readings, but it always got a deviation from the right temperature. That`s why I changed to the LM35CZ sensors. Thought I would let you know...

dre

Hi,
Does this link help?

http://emesystems.com/OL2heat.htm#TF_LM34

cheers, David

tentoes1

Thank you for the responses everyone........I have my homework in front of me. I really want this to be as simple as possible and I am not too concerned with hyper-accurate temperature measurements. Thank you guys for the reference material.....it's all fantastic, really!

tentoes1

Hey thanks again everyone for the great responses! My wife (aka "the admiral") won't let me begin a new project (this solar water heater) until I finish the one I am on now, which is just a small, LED-illuminated sign which is powered by rechargeable batteries that are charged using a small solar panel. (This is just to illuminate our address sign at night) Like I said, I am new to the hobby but I really appreciate the help.

I will post again when I can devote more time to it.

72sonett

Aw... boring... buy one ready made, plenty of those around for less than you can make it for. A BS1/2 controlled solar boiler is much more interesting :-)