Ravelli RV80 - Problem: Air Flow Meter

BrianJ Posted By BrianJ, Dec 30, 2015 at 3:18 PM

  1. Joe into Physics

    Joe into Physics
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    Nov 21, 2017
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    Thanks, Ssyko, for morale boost, and thanks especially, bob bare, for your voice of experience. Being "into physics" in my case (and for many) can mean forgetting to think about things like plain ol' dirt. Had the scientist in me prevailed, I would have attempted separate experiments to separate the variables affecting the problem:
    1) Since the nominal-10-KOhm thermistor on my unheated reference side was 11% too high, solder a nominal-100-K thermistor in parallel with the 10K to match the sensors that I presume ought to be matched and were not; and after trying that and (ugh!) removing the corrective 100K component for separately evaluating the following step ...
    2) Clean off the sensor probe, in particular the downstream heated green "tongue" (which looked clean to begin with, but the unaided eye can deceive), so that the air flow would remove heat from the dust-free sensor surface with the original efficiency of the new stove.
    The frustrated-with-bad-stove engineer in me dispensed with the scientific controls and did both corrections at once, and I cheerfully report that the flow sensor error warning is gone and -- thank you for your wisdom on this, bob -- the flame looks like it's getting the proper proportion of air: not short like a blowtorch and not lazy orange tongues reaching high into the combustion chamber. In the months leading up to the warning beeps and error messages, the flames were getting shorter and more blow-torch-like, a seeming indication that the flow sensor was under-reporting the air flow and the blower control was responding by blowing too much air. The fact that the problem was progressive would point to cumulative sensor dirt buildup, while a system with properly matched temperature sensors would probably take longer to develop significant problems.

    So for others frustrated with this problem, refer to my first entry in this thread, find that red rubber body sensor whose photo Ssyko provided, take it out, clean the two green circuit board probes that normally extend across the middle of the air inlet tube, and put the sensor back like you found it. For removal, you'll have to cut the plastic tie wraps securing it (if the factory did yours like they did mine), so obviously you'll need new tie wraps to put it back securely. I cleaned the sensor with a cotton swab dipped in acetone, but 90% isopropyl alcohol (prefer to 70% because the 90% dries better) would probably do just fine. No guarantees, but if the stove worked when new and then developed this sensor problem, there's a good chance the sensor cleaning will fix it (if I'm not putting too much stock in my statistical sample-of-one :) ).

    I was unable to find any advice or instructions like this in the Ravelli manual, and I could not identify the flow sensor in the system schematic on the next-to-last page. To those who service and clean these stoves, I'd guess that pulling out and cleaning the flow sensor should become a regular part of maintenance. And, Ravelli should get the message.
     
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  2. Ssyko

    Ssyko
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    Food for thought. I use an ultrasonic cleaner with 90% isopropyl alcohol to clean circuit boards. Works great for some that have been in service for years. This would work also on these “debimeters”
     
  3. bob bare

    bob bare
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  4. Ssyko

    Ssyko
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    Oh come on Bob, to easy lmao;)
     
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  5. Monica in France

    Monica in France
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    Jan 22, 2014
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    Its amusing to note that cleaning was once again the solution to the problem.
    and I for one would not have thought that unseen dirt could have caused all this .
    We live and we learn.
    But I am perplexed : Joe states that the sensor was over-reporting ; his flame ( or at least his stove's flame) was needing more air. This is the opposite to what I would have expected. This layer of unseen dirt is surely stopping the second heated sensor from being cooled , so the resistor current needed to maintain the small temperature differential between the two sensors would be less and not more . I am expecting it to under-report.
    Can someone explain where I'm going wrong ?

    Bob Bare :
    Did your Integras MAF have its own ( or BMW) electrics ? From what I understand the support components needed for the Ravelli 'MAF' make the two thermistors and a resistor look silly.

    Happy Thanksgiving.
     
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  6. bob bare

    bob bare
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    Hi nice to talk to you again.I will be honest,I only scanned Joes ramblings(good stuff,just overcomplicated for the normal pellet stove owner),dirt/coating would underreport,same as early car MAF sensors.He may have got it wrong,but the theory is proper.The early integra maf was built(devices and programming) on BMW format,and built by BMW supplier.As BMW was one of the first to use the maf on vehicles,and make it work,makes sense.Maf sensors work by having an ambient air sensor,and a sensor in the airflow,which is provided with a heater,to keep at a constant temperature,the loss of the voltage to keep the sensor at the same temp is what calculates the differences,lots of computer programming.Early car sensors had the ambiant sensor back in the housing,as sensors got better,they put them both in the airstream,I do not know how that works,I am just a mechanic.
     
  7. Joe into Physics

    Joe into Physics
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    Nov 21, 2017
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    To Monica in France and bob bare, I sometimes enjoy writing more than people are inclined to read, but that's OK, that's my style. In answer to Monica, I was saying "the flow sensor was under-reporting the air flow and the blower control was responding by blowing too much air" ... which was perhaps confusing. There are actually two ways that the circuitry might work, but either method comes to nearly the same thing:

    First Way: A constant voltage is applied to the heater resistor, which puts out a constant wattage. The temperature sensor next to the resistor measures how much that heated circuit board tongue heats up, and that is compared to the reference temperature of the unheated tongue. If the heated sensor is dirty, then for the same air flow, the sensor has to get hotter to get the same amount of heat out through the film of dirt and into the air flow, which is AS IF there were less air flow. So I say that the sensor is under-reporting the air flow, i.e. saying that there is less air flow than there really is. The blower has to blow harder to compensate and get the sensor to "think" that it's seeing the proper air flow.

    Second Way (as described by Monica): The big controller circuit that this little sensor circuit board is plugged into it. The big circuit adjusts the voltage or current going to the heater resistor in the little sensor circuit, in such a way that there is a constant temperature difference between the reference thermistor and the thermistor next to the heater resistor. If there is a thin insulating "blanket" of dirt over the sensor, then less power to the heater is required to maintain the same temperature difference -- it's easier to keep something warm under a blanket. The big controller circuit then "thinks" that the lower heater power requirement (to maintain the same temperature difference) indicates less air cooling, therefore less air flow, so it responds by making the blower run faster.

    I once built a hot wire anemometer using light bulb filaments, which can heat and measure temperature at the same time, since the resistance of the filament increases with increasing temperature caused by heating in the filament whose resistance is being computed. I found that the "Second Way" worked better because I could measure very high winds and the extra power to the heated filament would make it more sensitive -- but if the wind went away, doing the sensing the "First Way" caused the filament to glow red hot and burn out in the air, while the "Second Way" reduced the power and kept the filament from overheating. Also the response was much quicker doing it the "Second Way." So I thought I had invented a new and improved method, when in fact there are probably many people and manufacturers that do it the "Second Way."
     
  8. Monica in France

    Monica in France
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    Jan 22, 2014
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    Thanks Joe , That explains my confusion. Not that your prose was at fault but my reading of it.
    My description was based on my memory of the Ravelli installation manual which makes ( made ? ) a big thing of those two thermistors and a resiststor.
    But your experience disillusions me somewhat.
    My stove has what you could euphemistically describe as first generation firmware . With the introduction of a third generation technique
    like a MAF , I had hoped that Ravelli had developed some serious code.
    I would like to hear some ideas on how one could pre-empt your problem long before the <MAF not working> message with a < MAF needs cleaning > message.
    And any ideas on what actually prompted the final message ?
    My suggestion would be that on initialization the exhaust fan is run from low to high and a check made that the MAF output follows this changing flow with appropriate figures.
    But how to tell the difference between a dirty MAF and a dirty stove ? ( I think I can answer that one )

    PS Joe : you should have mentioned that you had to break the glass on that bulb.
     
  9. Joe into Physics

    Joe into Physics
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    Hello Monica ... You could say that the sensitivity of my light-bulb-filament anemometer would suffer if you don't break the glass bulb first. ;)

    As for Ravelli software, that's up to them, and it seems that something better than an "MAF sensor error" message is overdue. As for a firmware check of the thermistor, I would expect that unless the stove is extremely dirty or there is a significant obstruction in the air passageways entering and leaving the stove, then the fan output should be much less sensitive to accumulating dirt than the MAF sensor. If that's true, then your suggestion of a self-calibration, running the fan from low to high and checking the sensor output, ought to work. Also, to a limited degree the firmware could adjust for changing sensitivity of the MAF sensor, until the level of needed adjustment is high enough to suggest significant problems with the system. It's always good to have a system like that tell you SPECIFICALLY what is going bad, along with a severity code, for example: Code 1: Have the technician check this at your next regular service. Code 5: Don't operate the stove, you might burn down the house.
     
  10. Monica in France

    Monica in France
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    I like your idea of using the results of the start-up test to calibrate the session.
    Setting the motor with ( lets say ) three different voltage/currents and comparing the fan speed and airflow that gives. The fan speed tells you when the stove needs cleaning , the correlation of fan speed and airflow tell you when the MAF needs cleaning. And have a ' squeaky clean' calibration in the menu to set the reference figures after installation?
    As an engineer I like your 1 - 5 level error messages but I can not see the sales people agreeing .
    < Warning – limp mode – Stop > sounds easier to grasp.
    Could the the next generation have a CAN interface for the technician ?
     
  11. Joe into Physics

    Joe into Physics
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    A CAN interface, so you can actually query the system and get a record of what's been going on, and maybe even make adjustments and updates??? In other words, turn this into a 21st century appliance.:cool:
     
  12. bob bare

    bob bare
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    Tails1 and Ssyko like this.
  13. Ssyko

    Ssyko
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    Damn I want one!
     
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  14. bob bare

    bob bare
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    There are several more,but are on other computer,could find them later if you want the info.
     
  15. Ssyko

    Ssyko
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    If and when you get a chance Ill save them on my pc instead of this phone
     
  16. Monica in France

    Monica in France
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    Jan 22, 2014
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    That is really interesting , a 'MegaSquirt' for pellet stoves. Bookmarked.
    Every bell and whistle but no MAF control ! ( no CAN either , just RS232 )
    I had a good look because that could be just what I want to bring my stove into the 20th century.
    The Ravelli hardware is good but their firmware does leave a lot to be desired.
     
  17. Ssyko

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    BC53FEA8-C97B-40B9-B3C7-93E5069CFC67.png Now that is prices without the import and broker fees but that is on my Christmas list
     
  18. Ssyko

    Ssyko
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    :) thanks for the links Bob
     
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  19. bob bare

    bob bare
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    No problem. almost bought one of the ebay ones,to play with ,but money is tight right now.
     
  20. Ssyko

    Ssyko
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    I here ya. i need to get my extra stoves done so i can make some $$$ back.
     
  21. Tails1

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