Air Pressure Gauge for TJ-067

Discuss roast levels and profiles for espresso, equipment for roasting coffee.

#1: Post by Dregs »

SJM started a thread here, Phidget and Artisan and Gas and Air Pressures, that has lots of helpful information about how to use air pressure gauges in roasting applications. MakoMo showed the mechanics of how to connect a gauge; hankua provided information regarding analog gauges. I used that information to install an analog gauge (quick, simple and cheap) on my North TJ-067. Thank you, folks.

Why install a pressure gauge measuring air flow through the roaster? I am hoping it will help me better understand some variables in roaster performance. I start every roasting session by setting the gas to a specific pressure (5" WC) and the exhaust fan speed to a specific value (10 on a 1-100 dial) to let the roaster warm up. I observe the time the roaster takes to hit a specific temperature (360F). That time can vary between 5.5 and 9 minutes on different days. Some of the difference is explained by changes in the ambient temperature, but the roaster is in my basement and the ambient temperature doesn't vary that much. I suspect that much of the time difference is due to how air flow through the roaster is helped or hindered by the difference in air pressure inside vs outside my house acting through the exhaust piping. Actual roasting time for first batch of the day can vary as well, but the effect usually dissipates by the second batch. I expect the pressure gauge readings to help evaluate and compensate for the unexpected air flow variation. Of course, I could just let the roaster run 30 minutes before starting the first batch and skip the meter, but what fun would that be?

I used the Dwyer Magnehelic model #2000-0 Differential Pressure Gauge and brass fittings similar to those suggested in the earlier thread. The photos below show how the parts went together for my particular roaster. The brass parts (1/4" MPT x 1/4" Compression Fitting, 1/4" FPT x 1/4" Compression Fitting, 1/8" MPT x 1/4" Compression needle valve, 1/4" OD brass tubing) are available from any hardware store. The white hose is high-temperature silicone from Amazon. The gauge came used from ebay ($25). Its measurement range of 0 to .50" WC is a perfect match for my roaster. I include the brass needle valve in case the pressure range in my roaster was greater than the range of the gauge. Turns out I didn't need it. The gauge is mounted on a piece of aluminum sheet.

The Dwyer differential pressure gauge has 2 ports, one "High" and one "Low". The high pressure port is just left open to measure ambient air pressure. The low pressure port is connected via the fittings/tubing through a hole drilled in the flex pipe that channels the exhaust air flow from the roaster to the chaff collector. The L-shaped piece of brass tubing that sits in the exhaust air flow points toward the exit so it doesn't fill with chaff. The exhaust air flow creates low pressure compared to ambient, and sucks air through the gauge. The pressure varies from .06" WC at a fan setting of 10 (on the 0-100 dial), up to .38" WC at full fan. Actually, the pressure hits its maximum value when the fan is set to 60. Turning the dial above 60 does not increase the air flow as measured by the gauge, even though the fan gets noisier.

When I get some roasting experience with the gauge installed, I will report back on any observations regarding how actual air flow varies from dial settings and how that impacts the roast.

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#2: Post by millcityroasters »

+1 Dregs.

I've had this on my "to do" list for several months planning to use the Phidget package, but I hadn't thought to use my Dwyer gauge. Pretty neat.

I just got back from a visit to North Coffee. Misters Tim and Wang (aka Tim's boss) say "Thanks" and hope to exhibit at SCAA next year.

The pic is a snap of the 3 kg roaster with a touch screen automatic profiling system w/ automated air flow control.
He's running it in the shop into an exhaust scrubber. "Look, Ma! No venting!" Pretty impressive and they'll be showing at the upcoming big Shanghai show. They had a 6 kg wrapped up for shipment with the same control.

They set up the control in English and it toggles between a GUI and a real time roast profile graph. Sweet.

These guys have some really cool double top secret stuff in the pipeline. The shop may look medieval, but it reminds me of my father's old blacksmithing shop. To me, it looked like home. - but with way better coffee.

Between you, Marko, and Mr. Wang, I've run into smart talented people across 3 different continents this month. It's almost enough to make me rethink my default despair of the human race. :wink:


#3: Post by SJM »

Oh Dregs, this post is so welcome right now.

My Phidget air sensor (1036) arrived and I've been trying to get my mind around the assembly of the parts to put it into operation in the HUKY, and your pictures are just perfect.
The port on the Phidget requires a tubing with 3/32 ID, so my next step is to source copper tubing I can insert into that....

Thank you for the detailed pictures.
And, can you tell me what you used to bend the pipe?


Dregs (original poster)

#4: Post by Dregs (original poster) »

millcityroasters wrote:
The pic is a snap of the 3 kg roaster with a touch screen automatic profiling system w/ automated air flow control.
Exciting stuff coming down the pike, Steve. Please put my order in for the automated profiling retrofit kit for a TJ-067!



Dregs (original poster)

#5: Post by Dregs (original poster) »

SJM wrote:
And, can you tell me what you used to bend the pipe?

With 3/32" ID, OD should be large enough to fit in this: ... 94571.html

I would be most interested in hearing about your experience with the 1136 when you are able.



#6: Post by SJM »

Well, of course it should, thank you.
I was getting caught up in that small number....duh.... :oops:

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#7: Post by hankua »

Great job Barry!

Now you can calibrate the fan controller or mark the Dwyer gauge with a sharpie. The TJ-067 fan is on the chaff collector, no damper? I think it's better for the pitot tube to be located before a damper and air fan for reading negative pressure. If the exhaust air exits the TJ through a plenum/fixed pipe; Steve could have the factory install a removable plug for air pressure measurements. :D

Also a good idea for commercial applications, to moniter exhaust systems.