24 hr temperature data logger

Discussion in 'The Projects Forum' started by ak52, May 29, 2015.

  1. ak52

    Thread Starter Member

    Oct 15, 2014
    Hello Everyone,

    I am going to make a temperature logger and i need a few suggestions from you guys.
    Firstly here is the setup which i cannot change.
    There are 8 heaters in our factory and we would like to log data on an hourly basis for each oven individually.
    Each oven has a K-type thermocouple as the temperature sensor.

    Coming to the design part ,
    I am planning to make a unit (3inch x 3 inch,approx),where in all my embedded hardware would fit in to.
    I would like help with the following:
    Choosing a micro-controller that operates at very low power,(from microchip,if possible).
    The opamp/instrumentation amplifier used for sensing and amplifying the voltage from the thermocouple.
    The memory: an EEPROM where i would store all the collected data.
    The RTC for getting the actual time for logging the data.

    For the power supply,there may be a dc source in the heater control panel ,but as a backup i would like to run the unit with a 9v battery.

    Thanks in advance
  2. kubeek

    AAC Fanatic!

    Sep 20, 2005
    How much of that thing do you want to design by yourself? It might be easier to get for example an arduino with a thermocouple shield and just write a little piece of software.
    What is your temperature range and required accruracy?
    Why very low power? Is it going to be battery powered?
  3. ak52

    Thread Starter Member

    Oct 15, 2014
    I would like to design the whole thing myself.,it would be a good learning curve for me if i do it completely myself :)
    I have just now confirmed that there is no DC supply in the heater control panel.
    I will have to convert AC to DC - I can do that using a miniature meanwell power supplies ,which will convert 230c ac to 12v dc,as current would be in milli-amps the power supplies would be very cheap. as a backup option i would also like to power the unit using a 9v battery.I am not sure if a 9v battery would provide sufficient current to operate continuously for a day at least??

    Temperature range from room temperature(30-32) to 100 degrees Celsius
  4. kubeek

    AAC Fanatic!

    Sep 20, 2005
    Well if you really need to do just 24 mesurments in a day, than I think the 9V battery could power it even for a few weeks.
    What accuracy is needed, i.e. +/- 0.5°C?
  5. AnalogKid

    Distinguished Member

    Aug 1, 2013
    Agree. Microchip has some very low power devices that can do the RTC function, interrupt itself to wake up once each hour for a few seconds, then go back to sleep. You could get months out of a battery. How are you collecting the data?

  6. ak52

    Thread Starter Member

    Oct 15, 2014
    yes,0.5 degrees is more than enough.By 9v battery i do not mean those big lead acid batteries,the rectangular shaped batteries from duracel,i am not sure about their current ratings as they generally do not mention them now a days.

    @AnalogKid ,could you mention those devices for me,The data would be taken out on a weekly basis.
    Collection of data depends on the micro-controller,most probably probably via RS232.
    I cannot use an removable external storage unit,like a usb stick or an SD card.(at the risk of it being stolen). My best option is to use an EEPROM, which would communicate via SPI or I2C.
  7. DickCappels


    Aug 21, 2008
  8. ak52

    Thread Starter Member

    Oct 15, 2014
    Hey guys,
    After a lot of searching ,i think i found the ideal IC which would do everything for me at the sensor side of things.
    It has a built-in 12 Bit ADC and an inbuilt temperature sensing diode,for cold junction compensation.
    If i use this i would not have to manually do cold junction compensation.It would already give me a compensated hot junction temperature value.
    Regarding the MCU to be used,i am going to use a PIC18F series MCU
    Any other suggestions and recommendations would be welcome,i will post the full schematic in a few days.

    Thanks in advance,
    ebeowulf17 and ErnieM like this.
  9. ErnieM

    AAC Fanatic!

    Apr 24, 2011
    Using a purpose built chip such as the MAX6675 is the way I would go.

    Just BUY some smart guy's solution and be done.
    ebeowulf17 likes this.
  10. shteii01

    AAC Fanatic!

    Feb 19, 2010
    I have used this board with k-type thermocouple: https://www.adafruit.com/products/269 . The communication hardware/protocol is SPI. The interesting thing about SPI is that all the devices can share the Clock and Data lines, only Select line must be unique for each device.

    In my case we had 3 thermocouples. That was 2 lines (for data and clock) plus 3 lines to select individual thermocouple.

    You have 8 heaters/thermocouples. 2 lines (for data and clock) plus 8 lines to select individual heater/thermocouple, total of 10 lines. The Select lines just need to be digital On/Off, nothing special, any digital i/o pin that can be output pin can be used for it.

    The only thing I want to caution you about in this case is that I don't know the distance that SPI can be used reliably. In my case the SPI signal traveled 2-3 meters/yards, but I never looked into its maximum range. There might be extender chips that can be used to extend the range of SPI signal.

    Forgot to add. in my case we hooked up thermocouples to Arduino, Arduino was connected to PC using USB and used PLX-DAQ to get the data into Excel. The interesting thing about this setup is that you don't need RTC. You use PC time to log the data. You might be able to use PLX-DAQ with a PIC, but you need to do a bit more research on that. If you have an old pc/laptop sitting around, this might be a good option to log, graph, analyze the data.
    Last edited: Jun 2, 2015