Oxygen Depletion Sensor (ODS) of Unvented Gas Heater

aruzinsky

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Dec 17, 2016, 3:59:26 PM12/17/16

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See post 24 and diagram here:

http://www.expeditionportal.com/forum/threads/38090-Mr-Heater-Buddy-Heaters-on-Sale-at-Lowes/page3

What is the mechanism by which the ODS turns off the pilot flame? The author of the post seems to suggest that the air inlet hole of the pilot gets smaller as the oxygen content of the air decreases.

Frank

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Dec 17, 2016, 6:52:57 PM12/17/16

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Interesting question. I looked and found this:

https://www.youtube.com/watch?v=6rKE-R_6DqI

Several years ago friends hunting camp had these put it in because gas
well drilling on the property gave him free gas. Scared the hell out of
me sleeping with them on but we had no problem.

There is a concern that it only operates when oxygen is low but you can
still fill the room with pollutants from a malfunctioning heater before
low oxygen shuts it off.

My friends may have had a CO detector as an additional failsafe.

Poutnik

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Dec 18, 2016, 3:29:31 AM12/18/16

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Dne 17/12/2016 v 21:59 aruzinsky napsal(a):

> See post 24 and diagram here:
>
> http://www.expeditionportal.com/forum/threads/38090-Mr-Heater-Buddy-Heaters-on-Sale-at-Lowes/page3
>
> What is the mechanism by which the ODS turns off the pilot flame? The author of the post seems to suggest that the air inlet hole of the pilot gets smaller as the oxygen content of the air decreases.
>

It seems to me it reacts on a flame change,
based on changed burning kinetics at lower oxygen.
Probably based on high temperature bimetal.
It may be similar to those sensors in natural gas ovens,
that turn off the gas if the flame extinguishes accidentally.

The picture does not look like it is based on more direct oxygen
concentration evaluation, even if that is available. For the interest,
see some methods from the paper below. Perhaps a thermal conductivity
sensor used in leakage detectors could be used as well.

Oxygen sensors: Materials, methods, designs
and applications

http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.500.4609&rep=rep1&type=pdf

--
Poutnik ( The Pilgrim, Der Wanderer )

A wise man guards words he says,
as they say about him more,
than he says about the subject.

aruz...@general-cathexis.com

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Dec 18, 2016, 10:29:01 AM12/18/16

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On Sunday, December 18, 2016 at 2:29:31 AM UTC-6, Poutnik wrote:
> Dne 17/12/2016 v 21:59 aruzinsky napsal(a):
> > See post 24 and diagram here:
> >
> > http://www.expeditionportal.com/forum/threads/38090-Mr-Heater-Buddy-Heaters-on-Sale-at-Lowes/page3
> >
> > What is the mechanism by which the ODS turns off the pilot flame? The author of the post seems to suggest that the air inlet hole of the pilot gets smaller as the oxygen content of the air decreases.
> >
> It seems to me it reacts on a flame change,
> based on changed burning kinetics at lower oxygen.
> Probably based on high temperature bimetal.
> It may be similar to those sensors in natural gas ovens,
> that turn off the gas if the flame extinguishes accidentally.
>

I doubt it because the ODS is the cause of the flame change of the pilot. If the ODS is both cause and effect of flame change, that would imply feedback which would likely lead to unwanted oscillations.

I have a personal problem with an ODS that I believe is too sensitive and I want to either lower its sensitivity or bypass it, otherwise, the heater is useless to me.

More specifically, I believe the ODS in my Mr. Heater MHVFB10NG is too sensitive because I tried operating it along side another unvented heater, ProCom MD200TBA. The ProCom works fine but the Mr. Heater turns off after about 2 hours of operation. The heaters are in a 10' x 18' aluminum frame greenhouse covered with double walled polycarbonate panels. Judging from the amounts of rain that leak into my greenhouse, it is far from tightly sealed. The heater flames never turned yellow therefore I believe that Mr. Heater ODS is at fault rather than the one in the ProCom heater. As it is, the Mr. Heater is useless to me because I can't risk the cold killing my plants.

In my opinion, like all mechanical products, pilot ODSs are manufactured to tolerances and the larger the tolerances, the cheaper the cost to manufacture. However, heater ODS manufacturers err more on the side of too sensitive than on the side of not sensitive enough because they are much more afraid of lawsuits over wrongful deaths than useless heaters and other consequential damages. That is the mechanism by which I am stuck with a useless $107 Mr. Heater. And, Mr. Heater customer support is terrible. They don't even offer a replacement part ODS on their website. The part number, 80057, doesn't even come up on their website search.

Meanwhile, I am worried about subzero outdoor temperatures harming plants inside my greenhouse. But, you may find it intellectually interesting to know that the temperature inside my greenhouse is very nonlinear with the outdoor temperature. When the Procom heater is continuously on, the inside temperature is slightly higher at 0 than at 20 deg. F. outdoor temperature. I suspect this is because frost seals air leaks. Anyway, the Mr. Heater fails regardless of outdoor temperature and this is all the more evidence that the Mr. Heater ODS is too sensitive.

Poutnik

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Dec 18, 2016, 4:08:06 PM12/18/16

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Dne 18/12/2016 v 16:28 aruz...@general-cathexis.com napsal(a):

>
> I doubt it because the ODS is the cause of the flame change of the pilot. If the ODS is both cause and effect of flame change, that would imply feedback which would likely lead to unwanted oscillations.
>

I have just guessed from the picture.
It does look like there is a flame feedback sensor.
But it may be the same safety switch as is in the ovens.

aruzinsky

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Dec 19, 2016, 10:06:17 AM12/19/16

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On Sunday, December 18, 2016 at 3:08:06 PM UTC-6, Poutnik wrote:
> Dne 18/12/2016 v 16:28 aruz...@general-cathexis.com napsal(a):
>
> >
> > I doubt it because the ODS is the cause of the flame change of the pilot. If the ODS is both cause and effect of flame change, that would imply feedback which would likely lead to unwanted oscillations.
> >
> I have just guessed from the picture.
> It does look like there is a flame feedback sensor.
> But it may be the same safety switch as is in the ovens.
>

If you mean the thermocouple, yes, that generates electricity that holds a solenoid valve open that supplies gas to both the pilot and the burner, but, that is not part of the ODS. The ODS either bends the pilot flame so that it misses the thermocouple or completely extinguishes the pilot flame. In either case, the thermocouple becomes cold thereby turning off gas to both pilot and burner.

Yesterday, by trial and error, I fixed my Mr. Heater MHVFB10NG by covering about 75% of the air inlet hole on the pilot tube with aluminum tape. I also repositioned the thermocouple about 1/8" more into the pilot flame, but, I doubt that that was necessary. I am still uncertain of the ODS mechanics, but, I suspect that it may be as simple as follows: The air inlet hole on the pilot tube is of maximal size such that, with normal air, the air/fuel ratio sustains a pilot flame (if the air hole were slightly larger, there would be no flame). If the oxygen content in the air drops enough, the air/fuel ratio remains constant but the pilot flame goes out. And, if I my theory is correct, the incremental cost to the manufacturer, over making a conventional pilot, is zero.

Anyway, I fixed it in the nick of time because the outdoor temperature dropped to -10 deg. F. last night.