This is the first Dwyer gauge I played around with last year (2012). Hooked it up to the exhaust, outlet side of the drum air fan. Pretty much a failure, it looked like a pitot tube would be necessary to read positive air velocity and the meter went on a shelf.




A fellow Mini500 hobbyist (chang00) has talked about airflow on and off for years and I was lucky enough to hear Willem Boot/Boot Coffee mention the air flow inside a shop roaster would read @ -0.7. I decided to give it a go again and try snagging a Dwyer Magnehelic gauge on Ebay settling on a 0-0.5 range (2000-0). What is the air pressure inside the drum and how is it affected by the standard damper? The damper is located on the inlet side of the drum fan so it made sense to measure at the drum faceplate or close by.





I ended up removing the exhaust air dial thermometer mounted on top of the drum and luckily found a fitting from an air hose blower that fit the 1/8 pipe threads on the gauge and the thermometer hole.



Success!!!!! The fitting is connected to the low pressure side with a plug on the second low pressure port. Using this temporary method I was able to get a maximum reading -0.30 and minimum -0.02. This was without the exhaust venting connected and I'll check that later to see what affect it has.



Here is the air valve on the roaster with the fan bolted to the left hand side. The air control is a simple flapper and the effect is not linear. My initial calculations are:

1=7% air, 1.5=10% air, 2=13% air, 2.5=26% air, 3=53% air, 3.5=66% air, 4=83% air, 4.5=93% air, and 5=100% air.

There is quite a difference going from 2-3 in the percentage of air pressure inside the drum on my machine. I can see fitting the gauge in a rubber plug that would snug into the tryer opening on just about any roaster. At least for temporary use the dwyer magnehelic could be useful for checking declining performance or airflow valve settings.

Will the Dwyer Magnehelic give the same readings using the tryer opening vs the exhaust channel bolted to the face plate? Time for more experimentation.

This is very cool! Interesting to see a doubling in air pressure just from 2 to 2.5. And I think you'd need to have the exhaust tubing in place to properly measure the resulting pressure accurately. When I calibrate our mechanical ventilators in our ICU, the CPU is capable of measuring the tubing compliance and resistance so that our values are accurate. In your case, there shouldn't be any significant change in compliance due to the ridged walled exhaust, but it certainly will affect restriction to flow, and thus, the back pressure of the system.

I currently try to measure airflow by just recording the position of the butterfly valve. I don't remember anything from my fluid dynamics class in college, but I did study some curves of airflow produced by various butterfly valves and the numbers you record seem to be good. I wonder exactly what happens to the airflow as the butterfly valve transitions between 2 and 3 on your machine. I suspect the flow may be transitioning from laminar to turbulent, or some sort of oscillating pattern occurs, or something. On my own machine, I notice how quickly the ET can slow and/or drop with just a 5 degree movement of the damper. it's almost like there's a "critical zone" where, as i've said, something fishy must be happening to the airflow. It would be interesting to see some wind tunnel footage of a damper opening.

I use a simple air speed instrument to measure the changes in the damper(placed at the end of the exaust tubing). Biggest changes happen in the middle of the middle of the gauge.
1 - 1.4mph
2 - 2.1mph
3 - 4mph
4 - 6.3mph
5 - 8.3mph
6 - 10.3mph
7 - 11.3mph
8 - 11.7mph
9 - 11.7mph

Its good to be aware that changing the damper can make either big, or negligible changes in airflow.

That's kind of where I started, with an anemometer. It's measuring a positive pressure but inside the roasting drum is negative pressure. I've found a rubber stopper that fits the tryer opening, next step is to rig up a fitting by drilling out the rubber and mounting the gauge.

The damper/flapper is not very precise, at least on the mini500. There is a "precision air valve" option that cost 10,000nt as I recall. It comes standard on all the other size machines and could work like a camera aperture. It would be great if an owner could verify the operation.

Do you not feel like there is going to be a correlation between the air speed exiting the roaster and the negative pressure inside the drum? Obviously they arnt exactly the same thing, but getting an idea of how much of a change was what i was after.

Or is it just one of those things you want to know, because you just want to know it?

One is a result of the other, so they have to be correlated. You're just measuring it a different way (flow vs pressure). We used to use anemometers in the hospital rooms that we need to verify negative pressure to ensure airborne pathogens like tuberculosis don't exit the room. Special vents, much like those in our roasters are turned on in the room which move a great deal of air out of the room creating a slight negative pressure. That way, even if the door to the ante-room is cracked open, air will only flow in, not out (and potentially spread an airborne pathogen).

As long as you can trend it consistently, it's just as applicable to your profiling.

Do you not feel like there is going to be a correlation between the air speed exiting the roaster and the negative pressure inside the drum?

I'll have to check this; first time was with the anemometer on top of the 3" chaff collector. Probably for calibration purposes the air speed at the end of the exhaust vent is adequate. Dwyer magnehelic gauges have been retro-fitted to shop roasters; they also make a 160s series pitot tube for smoke stacks that might work as well. In a commercial roasting facility keeping track of air flow sounds like a good idea for quality/safety.

dustin360 wrote:Or is it just one of those things you want to know, because you just want to know it?

Just an experiment.

TomC wrote:As long as you can trend it consistently

Exactly!

Is there a way to hook up an air flow meter to the 800n and dial it in like the gas pressure gauge? The Giesen line has self-adjusting airflow; assuming a variable speed air fan connected to a gauge/pid device. The magnehelic gauges are pretty easy to pick-up on ebay, mine ran $25.

If someone is in the market for a Mini500/800n, is the precision air control valve worth the 10,000nt upgrade price?