Dwyer Model 25 or Dwyer Magnehelic 2000-00?

 
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McGiever
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Post by McGiever » Wed. Oct. 15, 2014 9:54 pm



 
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windyhill4.2
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Post by windyhill4.2 » Wed. Oct. 15, 2014 9:58 pm

No tube ????? how does it get its readings ? Magic ?? :) I see it mounted on the wall, how does it then read stack pressure/vacuum ?? wires from a sensor ???

 
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Post by McGiever » Wed. Oct. 15, 2014 10:00 pm

yes, air tube to stove pipe...no tube w/ red oil. :)

 
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Post by windyhill4.2 » Wed. Oct. 15, 2014 10:08 pm

Well, that sounds better than a fluid drive gauge .

 
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Post by lsayre » Thu. Oct. 16, 2014 5:57 am

My only beef with the magnehelic is that as for all gauges it must be kept in proper calibration. Every time you level and Zero a Model 25 you have just fairly reliably re-calibrated it. How can you be certain over time that the magnehelics calibration has not drifted off?

 
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Post by McGiever » Thu. Oct. 16, 2014 7:52 am

Re-calibration of magnehelic, although seldom required, is easily accomplished by removing hose(s) and turning a screw to position the needle to dead zero. :)

 
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Post by windyhill4.2 » Thu. Oct. 16, 2014 8:50 am

That sounds like something that even I can do. :)


 
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Post by lsayre » Thu. Oct. 16, 2014 10:39 am

windyhill4.2 wrote:That sounds like something that even I can do. :)
Yes, but what would you use as your benchmark calibration "control"? A conventional Model 25 Mark II perhaps?

 
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Post by titleist1 » Thu. Oct. 16, 2014 11:38 am

Isn't the calibration issue the same for the Model 25? Or any gauge?

For our purposes no matter which one you like to look at, just take the hoses off the inlets and turn the set screw or turn the knob until the needle or fluid level is at 0.

Do it a couple times a winter just to re-cal it for fun.
After all we have lots of time since we aren't heating with wood!! :P

 
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Post by MudFlapLip » Thu. Oct. 16, 2014 11:33 pm

titleist1 wrote:Isn't the calibration issue the same for the Model 25? Or any gauge?

For our purposes no matter which one you like to look at, just take the hoses off the inlets and turn the set screw or turn the knob until the needle or fluid level is at 0.

Do it a couple times a winter just to re-cal it for fun.
After all we have lots of time since we aren't heating with wood!! :P
That pretty much sums it up Titleist. When I was on the phone with Dwyer that's what they said in terms of calibrations. There were no differences between the two and the method of calibration was the same.

 
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Post by lsayre » Fri. Oct. 17, 2014 5:53 am

Perhaps I'm getting overly picky here, but In my experience an instrument should always be calibrated to read most accurately in the range you most likely will be running at, and not at zero (where you hope to never be running at). If you want to target the highest level of accuracy for a nominal draw of 0.04" then you need to have it attached to a calibration instrument for which the confidence is high that it is delivering 0.04". Then you dial in the device you will be using to match the control instrument and also read 0.04". Neither a single Dwyer Model 25 or a single Dwyer Magnehelic 2000-00 can be properly calibrated to this level of accuracy, since none of us have the certified control instrument with which to do so. The best we can do is set then to read zero when detached from the chimney. But the room itself will always be other than zero to some greater or lesser degree....

 
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Post by titleist1 » Fri. Oct. 17, 2014 7:47 am

lsayre wrote:Perhaps I'm getting overly picky here, but In my experience an instrument should always be calibrated to read most accurately in the range you most likely will be running at, and not at zero
I agree our 'calibration' is not exact by any means. Best case we would be doing a multi point calibration over the span of the gauge and also the expected range of operation with an instrument that has been recently certified by a metrology lab. I used to sell Beamex calibration instruments for a short time and truly only understood about 1/2 of what they tried to train me on. :oops:

'Repeatability' and 'accuracy' are different and for my purposes on the stokers the manometer 'repeatability' is probably all I am worried about. Since I realize my 'zero' cal is not exact, I want it to be at that same inexact point every time - or 'repeatable'. The same with my 0.04 reading where I try to run the stoker at full fire. I realize it will be off of that .04 by a little compared to a lab calibrated instrument reading .04, but if it is off by that same amount each time then I have a reading I can compare from day to day to see if conditions changed in my draft.

I think that is one of the benefits of having a manometer (or temp gauge) connected all the time. You get used to seeing a draft number at a certain level and when something different shows up you start looking for the cause. If you only pull it off the shelf to connect it at certain times of the year to check draft my feeling is the 'accuracy' comes into play more than the 'repeatability' of the gauge.

 
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Post by McGiever » Fri. Oct. 17, 2014 7:54 am

Magnehelics come w/ the person that calibrated it signiture on the scale face.
Also, I have never needed to re-zero mine ever. :)

 
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Post by lsayre » Fri. Oct. 17, 2014 9:53 am

titleist1 wrote:I agree our 'calibration' is not exact by any means. Best case we would be doing a multi point calibration over the span of the gauge and also the expected range of operation with an instrument that has been recently certified by a metrology lab. I used to sell Beamex calibration instruments for a short time and truly only understood about 1/2 of what they tried to train me on. :oops:

'Repeatability' and 'accuracy' are different and for my purposes on the stokers the manometer 'repeatability' is probably all I am worried about. Since I realize my 'zero' cal is not exact, I want it to be at that same inexact point every time - or 'repeatable'. The same with my 0.04 reading where I try to run the stoker at full fire. I realize it will be off of that .04 by a little compared to a lab calibrated instrument reading .04, but if it is off by that same amount each time then I have a reading I can compare from day to day to see if conditions changed in my draft.

I think that is one of the benefits of having a manometer (or temp gauge) connected all the time. You get used to seeing a draft number at a certain level and when something different shows up you start looking for the cause. If you only pull it off the shelf to connect it at certain times of the year to check draft my feeling is the 'accuracy' comes into play more than the 'repeatability' of the gauge.
Well stated! Repeatability is all we can ask for. I like the tidiness of the magnehelics and the fact that they don't contain a liquid which evaporates over time, so some day I just may get around to replacing my Mark II with a 2000-00.

 
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Post by Lightning » Fri. Oct. 17, 2014 6:09 pm

titleist1 wrote:Since I realize my 'zero' cal is not exact,
uhhh, why wouldn't it be exact? If there is no pressure on it (by pulling the tubes off) then zeroing it, I don't see why it wouldn't be.. I mean, there would be zero pressure since its disconnected and the gauge would also say zero..

Also, I'm quite confident in its accuracy since it's reading is generated by balancing liquid in the unit. It's manually calibrated to balance at zero when no pressure is acting on it. A gentle push or pull from differences in air pressure upsets this balance and that's what we see on the gauge. Usually the simplest mechanism is the most accurate and most reliable.. Just my opinion :)

I think both are good at what they do, I just prefer the model 25 because of its simplicity.


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