Which reading is more reliable on a Minneapolis Duct Blaster?

I have recently switched to using a Minneapolis Duct Blaster and have noticed that switching the rings has been resulting in varying results. My question is which reading is more accurate?


I know that each ring is used depending on the amount of leakage in the duct system, however it doesn’t seem clear which ring I should be using. Yesterday I test a 1.5 ton system in a condo and I used the #2 ring. I got a result of 240 CFM then I put in the #3 ring (smallest one) and got 180 CFM. Which one is accurate? 

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A good rule of thumb when you get readings that stretch credibility - or just don't make sense - Break down your (DG) set-up and redo it.  That will often make a glitch , like not switching the gauge to match the ring, stand out.  If that fails, there are two, relatively inexpensive bits which I think should be in all DB kits. 

TEC makes a calibrated plate to field-test your DB for accuracy.  They also have recently released a "Ring 4", which you'll probably need if you have any small units with tight systems.  The smallest ring which will give you a reading s most accurate because the flow across the pitot ring is most accelerated.

The flex connector can develop wear pinholes which you may not find otherwise unless you hold it against a strong light and look down the duct for dots of light.

I hope this helps.


I often change rings to qualify my findings. With the numbers you stated, my experience leads me to think you forgot to change the configuration setting. Using leakage to outside clearly gives more information to use in the analysis of duct tightness testing. In my area 30% would be typical, overall ranging up to 40% and greater. Homeowners are still confused when a 15% result is considered above average.

Some of the responses provide useful information but others ignore the fact that his readings on the same range should give the same result. The Principle is, if what you are blowing into, a duct or flex with a hole in it, is at the same pressure then the flow must read the same regardless of the Range used.


I suggest checking your gauge and Duct Blaster on a regular basis with this simple set up. You’ll learn a lot and have fun too.  Here’s how you check your Blaster.  You should do this at least monthly.


Attach your Duct Blaster to a field calibration plate or just use a box taped to the end of your flex with a 3 x 6 inch hole in a thin cardboard sheet taped to the end of it. Square hole is ok. Your test pressure goes to channel A.  Flow tubes to Channel B per normal.  Take the pressure up to 25 Pa. The flow should read about 94 CFM on Ring 2 and 3. If not one of them is wrong. Test pressurize and depressurize but use your flow conditioner and two tubes for fan pressure.   Using @ 25 Pa will help.  


After you prove your Duct Blaster is ok, see if you can get the flow up to 180 CFM on Ring 3.  I doubt you can, even in free air. 


Your 240 CFM on Ring 2 would require 230 Pa fan pressure that translates into only 96 CFM when you switch ranges to Ring 3.  Another dead end.


There are several plausible explanations as to why Ring 2 and 3 would NOT give the same readings on the same duct. So, what can mess up on the Duct Blaster end?


If depressurizing the duct the flow conditioner and connection to the second port of the flow gauge would give a higher than normally possible flow reading. In order to read 180 CFM, the fan pressure would have to be around 788 Pa on Ring 3.   Possible?


As suggested, the Range on the gauge may not have been changed when the Range Ring was changed but I doubt it in this case.


The backpressure was really high, obscuring the real result and since @25 Pa was used, the result looked real.  Is this possible?


Gauge was set to say @75 Pa instead of 25 Pa.


Or, Ring 2 and 3 actually read incorrectly which you’ll have checked out with the above test.  This is how they get checked out at the factory; same principle but more complex apparatus. 

If your getting a good reading with the 2 ring on, using a DG 700 and it is not flashing LOW then that is the proper flow range. Sure you could put on a 3 ring and get a different number but why? The 2 ring matches the flow range. The 3 ring has a flow range of 10 - 125 CFM. A reading of 180 CFM is clearly wrong since it doesn't match the rated flow range of the the #3 ring. The leakage for that system is +/- 240 CFM.

Michael, how do you know the tester would not start out with Ring 3, take a reading and assume it was ok?

What this repeated test is pointing to is that there is a problem here, so what is it? Is Ring 3 out of calibration for some reason, was the flow conditioner not used? His comparison is similar to the gauge check where you connect channel A to Channel B and let's say one reads 100 Pa and the other reads 150 Pa. One is wrong for sure! It is not good enough to hope you use the range without the error. Same, here. It is simple to find out which range is wrong and it could be both which is why I suggested the Field Calibration check which is now a recognized procedure to give us all confidence that both the gauge, the flow Device (Duct Blaster in this case) and the operator are all doing the right thing. Simple and easy to do and we all need to know how and practice it on a regular basis. 

I sat on the RESNET committee where they were going to require regular calibrations of your flow devices where you'd have to send your Blaster or Blower Door in regularly, pay for the cost of shipping and calibration to show it was working. I argued that a perfectly calibrated fan is useless if the problem exists elsewhere in the tubes or connections that could leak or be plugged or in the method. I suggested the Field Calibration Check as I outlined in the previous message. 

Here is a fun oversize you can try that proves my point about application.  A Duct Tester was used in a commercial test at 500 Pa where the user swears he read 850 CFM on his DG700. This is like saying my Chevrolet went 550 MPH; totally impossible. So I tried to duplicate what he did and you can too. Take your Duct Blaster on Open, lay it on the floor face up with no flex. Turn it up to full speed. You'll read 850 CFM but the flow is zero because it's lying on the floor.  Nothing wrong with the Blaster, just the technique. You'll be generating about 500 Pa pressure which is well above their spec which is 100 Pa maximum. The user failed to follow that part of their manual and got a false reading.  No amount of calibration will substitute for not following the instructions.

These tools are easy to understand if you experiment to see where their limits are which will cause you to use them as intended and thereby get good results. 

Try it. 


"take a reading and assume it was ok?"

Assumptions, Ah yes good point. Well taken. 

Sorry for chiming in so late.  Sean’s comment early on in the discussion was the closest to the point.  160 CFM is off the calibration charts on the high end.  Your Duct Blaster® Ring 3 may be able to top out at 160 CFM with the fan sitting on the floor with no back pressure, but you would not be able to get this duct system to 25 Pa.  I did a test using out TEC Trainer and adjusted the tightness to about 240 CFM25, the reading Stephen had with Ring 2.  When I switched to Ring 3, I was only able to get the Trainer up to 11 Pa.  If I have the gauge set to the CFM@25 mode, I will get a reading, but this is not the best, most accurate option. 

You should keep these things in mind when deciding which ring to use:

  • Can you get the system up to pressure?  If not, install a Ring with a larger hole.
  • Is the DG-700 alternatively flashing between LO and a number?  If so, install a Ring with a smaller hole.
  • If you can get the system up to pressure and the gauge is not flashing LO, then your readings are within the calibration range of the system and there is not a reason to change rings.


Now I am going to stray a bit into tech geek mode.  In the example that Steven used, the DG-700 is doing a Can’t Reach Pressure extrapolation using formula to convert a flow at a pressure near 10 Pa to 25 Pa.  The DG-700 is assuming a flow exponent of 0.60.  The range of error in this extrapolation could give you a range between about 9% higher to 15 % lower than the actual reading.  Steven’s reading was about 25% lower, leading me to believe that something additional is going on.  One explanation would be a depressurization test without a fan reference tube connected between the Channel B reference on the DG-700 and the tap on the flex duct.


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