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I think I have it figured out. But please correct me if I am wrong. My fieldpiece STA2 hot wire anemometer will measure the correct velocity (and calculated CFM) at any altitude. Also the blower will produce the expected CFM of air at any altitude, based on the manufacturers static pressure chart, or direct measurement of the air flow. That has been my experience, the static pressure chart and my measured airflow match up pretty good. But the air density changes at higher altitudes, the density gets lower as the altitude increases. And since you cool pounds of air, and not CFM of air, you will need more air across the coil to get the rated capacity of the system. Here are a few articles that explain it: http://www.trane.com/cps/uploads/use...039en_1210.pdf The constant of 4.5 * CFM * Delta-enthalpy for total BTU is based on standard air density of .075lb per cubic ft of air. 4.5 = 0.075 lb/ft3 × 60 min/hr At 4500 ft elevation, the air density is .063 lb/ft3, so the constant is .063 * 60 = 3.78 instead of 4.5. That means that the airflow at 4500ft altitude needs to be 4.5/3.8 = 1.19 times higher to get the same lbs of air across the coil. I found the air density and conversion constant on page 20 here: http://rockymtnashrae.com/downloads/...itude_hvac.pdf So instead of 400cfm/ton, it will need 400 * 1.19 = 476 cfm/ton. If I have a 4 ton condenser (which I do have), I will need 4 * 476 = 1904cfm of air. One condenser is on a 5 ton drive, If I can get the static pressure down to .5 inches I should be able to get near the correct air flow. As it is now, I get about 1650cfm with the 2000cfm motor tap and .75 inches static pressure. So the TEET shows that the airflow is still too low, and the measured delta-T across the coil is too high, it needs more air. My other 4 ton condenser has a 4 ton drive, currently providing 1450cfm with TSP of .75 inches. So there is no way that I will get the correct airflow on that unit even if I get the TSP down to .5 inches. I will need to modify the furnace blower to increase the CFM, or replace the furnace with a 5 ton drive furnace. Or perhaps replace the 4 ton condenser with a 3 ton unit, its oversized anyway and the ducts would work fine for 3 tons of air, that's what I am getting out of it now. Some more links I found on air density and altitude: http://contractingbusiness.com/archi...ct-air-density http://www.digikey.com/Web%20Export/...udeCooling.pdf http://www.achrnews.com/articles/sel...altitude-homes
Thanks for the reply Jim! Along this line of thought, if I measure static pressure across the blower, say it is .7 inches, then look up the airflow in the manufacturers docs to get the airflow for that motor speed (3,4,or 5 speed PSC motor taps) and TSP, and say it is on high speed with a 5 ton drive, and it says 1850 CFM with that amount of static pressure, is it really the correct CFM at 4500' elevation? Also the STA2 gives a similar reading. So I think that the airflow is probably correct, but like you said the density is lower so you need 20% more air for the same density. If so, then I still want to increase the required airflow by 20% when calculating the airflow per ton, is that right? I just want to know how to think about it correctly in my head. So maybe instead of shooting for the nominal 400 CFM/ton air across the coil, I start out with the 20% increase because of the lower air density, and therefore shoot for 480CFM/ton, or 1920 CFM for a 4 ton condenser. Then measure the airflow with the STA2 or check it on the manufacture chart with static pressure to see what I have, but the goal would be to get at least 480 CFM/ton, and let the instruments and blower chart report the CFM without adjustments. So in order to get the correct airflow at altitude, then I would really need a 5 ton drive and low TSP of .5 or so just to have a shot at getting there. And if I have a 5 ton condenser and a 5 ton drive, then it is going to be at least 20% short on air over the coil. Unless the drive has a setting for 450 CFM/ton (some do, like an ECM), and then I could get a little closer.
Originally Posted by kdb4 I work at typically 4200 to 4600 ft elevation. I showed my STA2 to my buddy who is a test and balance engineer, he said that I need to multiply the readings from the STA2 by 1.09 at 4500ft because it will be low by about 10%. He showed me other hot wire units that have a setting in their setup to compensate for pressure and/or altitude. I've been using this for over a year and never heard of it. At least there should be something in the operation manual that talks about this and an elevation table and the adjustment factors to get a correct reading. Not everyone lives on the coast. Does this sound correct, and can you direct me to some information on how to use the STA2 at different elevations? I completely understand your frustration with this as it has been a pet peeve of mine industry wide with all instrumentation. Users assume that the tool provides an accurate measurement, particularly when the tool has a digital display. It is almost like the non True RMS meter issues when working on a variable speed drive. This is a hard one to fault instrument manufacturers with as it really comes down to the HVAC industry as a whole not properly training technicians how to use test instrumentation and the limitation of each tool that they use. Most of us, me included that went to a trade school never measured airflow. You friend who is a test balance engineer has spent years learning how to use the tools of his craft. The Pitot tube which is a staple tool in his trade does not come with detailed directions on its use, air density correction, traversing nor does it state it limitations. Here is a good place to start. It is an air density calculator. I am thinking that you will want to keep the temperature at 70 degrees in the calculator as the STA2 corrects for air temperature. Enter the temperature at 70 and elevation and that will give you the current density. Using 70 degrees, 4500' and 30% Rh the density is .063lbs/ft3 .075 standard air / .063 = 1.19. so you need about 20% more airflow at that elevation to keep the mass flow rate the same. http://www.denysschen.com/catalogue/density.aspx As far as the hood versus the hot wire, they will both read the same if properly used and properly corrected. But they are two completely different tools with completely different applications. A hood is designed to measure volume, a hot wire really to only measure velocity. A lot of hot wires have a volume calculation, but it is only accurate if the area can be accurately measured and the velocity accurately averaged. A hot wire will provide a decent volume measurement in a duct, but it would take a lot of skill and a little luck to get good measurements with a hot wire at a register.
Id like to see a comparison to a flow hood.
Originally Posted by jim bergmann Where additional correction might be required is above 3000-4000', and at humidity levels over 80%. If air density is not corrected, when densities are far outside of the calibration parameters in regard to RH or barometric pressure, a reading that is low by up to 10% could very easily happen. Provided the point of measure is carefully selected and hot-wire is used correctly, it will provide a very accurate airflow measurement. Taking that into account, I have no objections to recommending the Fieldpiece STA2 for airflow measurement. I work at typically 4200 to 4600 ft elevation. I showed my STA2 to my buddy who is a test and balance engineer, he said that I need to multiply the readings from the STA2 by 1.09 at 4500ft because it will be low by about 10%. He showed me other hot wire units that have a setting in their setup to compensate for pressure and/or altitude. I've been using this for over a year and never heard of it. At least there should be something in the operation manual that talks about this and an elevation table and the adjustment factors to get a correct reading. Not everyone lives on the coast. Does this sound correct, and can you direct me to some information on how to use the STA2 at different elevations?
Originally Posted by JTCHVAC Is there a certain time frame that this instrument stays calibrated for before having to send it in to be calibrated, or does it stay pretty well calibrated for its life span? at least once a year under normal usage. Or if you suspect that the readings are not proper or accurate is another reason to send it in for evaluation.contact Fieldpiece if you want more information.
Originally Posted by jim bergmann Did a video this afternoon on how the Fieldpiece STA2 works, how to layout a traverse and how to make a correct airflow measurement in a residential system. http://youtu.be/6mWOnU9rT_g Any feedback would be appreciated. Jim Bergmann Is there a certain time frame that this instrument stays calibrated for before having to send it in to be calibrated, or does it stay pretty well calibrated for its life span?
The AAT3 only measures velocity and it does not calculate CFM like the STA2. On the surface it looks like the perfect accessory, but it leaves all of the calculations and the averaging to the technician. The STA2 does it all for you. Thanks for the feedback.
Jim, How does the sta2 compare to the aat3, I wonder why they dropped the web bulb reading on the Sta2. Thanks for all your dedication to the trade and quality testing.
Dean, Thanks for watching. You can subscribe to my you-tube channel, see all of the older ones, and get the new ones whenever they are added. Jim
Originally Posted by billg How do the results compare with Testo's 416? A vane will tend to read a higher airflow than a hot-wire or a Pitot if the location is in favor of the hot-wire or Pitot tube, non turbulent air. This is because a vane measures airflow independent of air density. As air density decreases, (relative humidity goes up, temperature goes up, or barometric pressure goes down) the hot-wire assumes that there is less air going over it. Temperature, relative and barometric pressure all affect the air density. The Fieldpiece STA2 compensates for only air temperature which is the largest contributor to density changes. But if the RH is high or the barometric pressure low (typically do to elevation) the hot wire or vane will indicate a lower airflow then actually exists. To get everything apples to apples, you really have to convert everything to pounds mass flow, and make corrections for density. Although a mini vane is my first choice for airflow simply because of the work that I do and the desire for very precise airflow measurement, it is cost prohibitive for a lot of technicians. With the basic density correction in the Fieldpiece STA2 and a relatively low cost it is a perfect airflow instrument for the work that 95% of the technicians will encounter. Aside from initial cost, the STA2 has the advantage of a smaller head which means smaller holes in the duct work, automatic calculation of areas, and a fairly intuitive operation. Where additional correction might be required is above 3000-4000', and at humidity levels over 80%. If air density is not corrected, when densities are far outside of the calibration parameters in regard to RH or barometric pressure, a reading that is low by up to 10% could very easily happen. Provided the point of measure is carefully selected and hot-wire is used correctly, it will provide a very accurate airflow measurement. Taking that into account, I have no objections to recommending the Fieldpiece STA2 for airflow measurement.
Originally Posted by Timber Jim, did you get your airflow corrected ? What was it , dip switches incorrect ? Timber, dip switch settings, I had messed with it earlier and set it for 4tons of cooling instead of 3 tons. In addition I have the airflow set to 110% for the additional capacity and efficiency on second stage hence the 1740 CFM reading. I play with my system once in awhile and must have either set it wrong, or forgot to set it back.
Jim, Thank you for your very informative videos. I scour Youtube and the True Tech Tools sites for your older ones and I look forward to each new one.
Jim, did you get your airflow corrected ? What was it , dip switches incorrect ?
Originally Posted by itsiceman Great video. Always love the tips and tricks Jim. Just one thing that I saw though is the magnet is either too weak or the plastic is too slippery to hold the units weigh. It started slipping just as you set it without the weight of the wand. It might be hard to work the unit hands free anyway but just an observation JP You have a 510 don't you? have you considered using a pilot tube for air flow? http://www.trutechtools.com/Dwyer-Pi...-OD_p_628.html Yes. The problem with a pitot for me is that I work on equipment from 5 to 100 tons with the varying sizes of ductwork AND varying ease of access to said ductwork. Plus, I'm thinking that a pitot would require special care and handling in MY van whereas an anemometer already has a nice case making it a bit more rugged instrument.
How do the results compare with Testo's 416?
Thanks for the information.
Originally Posted by bigtime That was great! Exactly what I was wanting to see. I would love to see how that traverse compares to one 30" or so closer to the furnace, which to me would be pretty close to the "ideal" spot. Also compared to the one in your earlier video, and the test holes in your drop around the air cleaner. Maybe one right before the big drop to the air cleaner. Maybe one right at the top of the drop. Just a few less than ideal spots. I appreciate you taking the time to make and post the above videos. Id be happy just to see the numbers without a video on the other spots. I aint trying to over-work you, just curious. Closer to the furnace should yield exactly the same results, I wanted to have a straight run before and after to get the most laminar flow, you are correct however that the downstream location is ideal also. The initial spot I traversed was about 10% higher due to the turbulence from the turn. The velocity was very high at the back of the duct. I also tested a spot right after the EAC, it was almost exactly the same as the spot I did in the second video. I did notice that the probe position is less sensitive than a Pitot tube, and a little more sensitive than a small vane, but if you are careful with the probe position it is easy to repeat the result +/- 10-12 CFM every time.
That was great! Exactly what I was wanting to see. I would love to see how that traverse compares to one 30" or so closer to the furnace, which to me would be pretty close to the "ideal" spot. Also compared to the one in your earlier video, and the test holes in your drop around the air cleaner. Maybe one right before the big drop to the air cleaner. Maybe one right at the top of the drop. Just a few less than ideal spots. I appreciate you taking the time to make and post the above videos. Id be happy just to see the numbers without a video on the other spots. I aint trying to over-work you, just curious.
I've owned the STA2 for almost a year now and have had no problems. Very well built and solid. Thanks for the reassurance Jim!
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