well the hot tub controller has been up and running now for around 2 months, no problems at all, however I have not had time to fully integrate the SMS modem.
the problem is the SMS board (sim900) , seems to give different messages back on start up , depending if it was a warm start or cold start, the timing of these messages is a bit random too, I think its due to some of the responses only being sent when there is a signal lock , its playing havoc with my start up routine.
I am just thinking of ditching the SMS, it seemed a good idea at the time, and I thought when I started this project it would be simpler in implement than a web interface.
now of course the Ethernet shield comes with a SD card slot , and as my programming skills have improved, I think I may go down the web interface route.....
I have fine tuned the software a bit, the 'filtration pump time' routine, now runs the pumps for a minute, before sampling the temperature, this is due to the temp sensor being in the pipework not in the main tub, most tubs are set up like this as the sensor measures the water flowing out of the heater , so now it runs for a while before deciding the next step, if the water is up to temp, it just runs the pump in filtration mode, if it needs the temp raising , the filtration mode gets heat and the time is extended until the water temp is correct, the filtration mode runs every hour.
I still have not implemented a watch dog timer, must get around to that...