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.
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.
Originally Posted by 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?
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 JTCHVAC
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.
Originally Posted by jim bergmann
Does this sound correct, and can you direct me to some information on how to use the STA2 at different elevations?
Id like to see a comparison to a flow hood.
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.
Originally Posted by kdb4
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.
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.
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.
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:
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:
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: