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BTMETER · BTMETER BT-100APP Bluetooth Anemometer, Handheld Wind Speed Meter with App | Air Flow Meter for HVAC CFM Calculation; App Data Logging/Export, Air Velocity Gauge w/Wind Chill, Temp, MAX/MIN/AVG Readings
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A vane anemometer with a Bluetooth link to a phone application, reading wind speed from 0.67 to 67.1 mph at plus or minus 5 percent of reading, air temperature from minus 10 to 45 C at plus or minus 2 C, and a derived wind chill. What marks the listing out sits in the accuracy bullet, where instead of claiming airflow in cubic feet per minute it prints the equation and tells you to apply it yourself. That is an admission that this is a velocity instrument, and it is the correct admission.
The listing prints the airflow equation instead of claiming the answer
The second bullet ends with an instruction: calculate airflow in cubic feet per minute manually using this formula, airflow equals average air velocity in feet per minute multiplied by duct cross sectional area in square feet.
Spend any time reading anemometer listings and you will notice how rare that is. Volume flow is the number a ventilation technician actually wants, and plenty of instruments in this class imply they produce it.
They cannot, and the reason is geometric. A vane measures how fast air moves past it. Volume flow is velocity multiplied by the area the air moves through. The instrument has no way of knowing the area, so either you type the duct dimensions into it or you do the multiplication yourself. There is no third option, and nothing in these bullets describes an area entry field.
Printing the equation is therefore honest twice over. It says what the instrument does not do, and it says what you have to supply. It also quietly flags the harder half of the job, because the formula asks for average velocity rather than a velocity. A reading taken in the middle of a duct is not the average, since flow is faster at the centre than at the walls. Getting the average means a traverse of several points across the opening, which our guide on measuring air velocity in a vent works through.
A floor of 0.67 mph, and what sits below it
The range is given as 0.67 to 67.1 mph. The bottom of it is the number to think hardest about, because a vane anemometer does not read inaccurately below its stated minimum, it reads nothing: the rotor has not enough energy to overcome its own friction.
Converted into the units the airflow equation uses, one mile per hour is 88 feet per minute, so the floor is about 59 feet per minute and the ceiling is about 5,900.
Against real work:
- A supply diffuser or register running at a few hundred feet per minute sits comfortably in range.
- A duct traverse on a residential branch is in range.
- A large return grille, a trickle vent or a low velocity plenum can sit under 59 feet per minute, and there the instrument has nothing to report.
- Checking whether extract ventilation is doing anything at the grille face often falls in the same dead region.
So the claim that it reads very low air flows is true relative to its own class and not true in absolute terms. A floor of 0.67 mph is a sensible figure for a vane, and a hot wire sensor is the right tool for anything below it, a distinction we set out in vane against hot wire anemometers.
Metric specification, imperial headline
Something worth noticing about how these figures were written: the range is quoted in miles per hour and the resolution is quoted as 0.1 m/s. Those are not the same system, and the mismatch tells you which one is original.
A range of 0.67 to 67.1 mph is 0.3 to 30.0 m/s to within a rounding step. Round metric end points converted into untidy imperial ones is the signature of a metric specification translated for a market, and the resolution figure was simply left in its native units.
The practical value of spotting that is knowing what the display will do. A resolution of 0.1 m/s is about 0.22 mph, so the reading moves in steps of roughly a fifth of a mile per hour and the bottom of the range is three steps above zero. It also tells you that the temperature resolution of 0.2 C is the real figure and anything you read in Fahrenheit is derived from it.
Wind chill is derived, so it carries two tolerances
Wind chill appears in the first bullet as one of four things on the display, alongside wind speed, temperature and battery state. It is worth separating from the other three.
Wind chill is not measured. It is a published index computed from air temperature and wind speed, intended to express how quickly exposed skin loses heat. No instrument contains a wind chill sensor.
That matters for how much weight to put on the number. The two inputs arrive with tolerances of plus or minus 2 C and plus or minus 5 percent of reading, and the index combines them, so the result has an uncertainty larger than either input. The listing states no tolerance for it, because there is not really one to state.
Used as intended, as a guide to how cold it will feel on a hillside or a firing point, that is entirely adequate. Quoted as a measurement it is not. The honest reading of the first bullet is three measurements and one calculation.
The published set, and the logging details that are absent
| Specification | In the listing bullets |
|---|---|
| Wind speed range | 0.67 to 67.1 mph |
| Wind speed accuracy | Plus or minus 5 percent of readings |
| Wind speed resolution | 0.1 m/s, about 0.22 mph |
| Speed units | m/s, km/h, ft/min, knots, mph |
| Temperature range | Minus 10 to 45 C, 14 to 113 F |
| Temperature accuracy | Plus or minus 2 C |
| Temperature resolution | 0.2 C |
| Sensing arrangement | Two temperature sensors, eight vanes |
| Wireless | Bluetooth, automatic pairing with phone or tablet |
| Export formats | Spreadsheet and text |
| Mounting | Quarter inch socket in the base |
| Power management | Auto power off with adjustable delay, manual and automatic backlight, data hold |
| Logging interval and memory depth | Not stated in the listing bullets |
| Supported application versions | Not stated in the listing bullets |
| Battery type and runtime | Not stated in the listing bullets |
| Humidity, pressure or dew point | Not measured, and not claimed |
| Ingress protection rating | Not stated in the listing bullets |
The logging gaps are the ones that bite. A tripod socket, a data logger and an adjustable auto power off delay together describe an unattended recording session, which is a genuinely useful thing to be able to set up. How long that session can run depends on the memory depth and the sample interval, and neither is published. The adjustable delay is at least the right companion to the socket, since a fixed timeout would end the session for you.
No ingress rating on an instrument whose own bullets suggest sailing, kite flying and cycling is a notable absence rather than a trivial one.
Who this suits, and the duct it will not read
This suits somebody who needs velocity readings they can keep. A logger, Bluetooth export to a spreadsheet and a tripod socket together turn readings into a record, and for anyone documenting airflow at a set of grilles or logging conditions at a shooting position that is the entire point. The eight vane rotor and two temperature sensors are a reasonable build for the task, and plus or minus 5 percent of reading is the better of the two ways to state an accuracy.
It does not suit low velocity work. Below about 59 feet per minute at the grille face you need a hot wire instrument, and no amount of application support changes that. It also does not suit anyone expecting the instrument to hand them a volume flow figure, although in that case the listing has already told you so to its credit. Our anemometer roundup covers which instruments take duct dimensions and do the arithmetic for you.
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Frequently Asked Questions
Does the BT-100APP calculate CFM?
No, and the listing says so in the clearest possible way by printing the equation for you. Airflow in cubic feet per minute equals the average air velocity in feet per minute multiplied by the duct cross sectional area in square feet. The meter supplies the velocity. You supply the area, and you supply the averaging. That is honest in a category where volume flow is routinely implied by instruments that cannot know the duct dimensions.
What does plus or minus 5 percent of readings mean in practice?
It means the allowance scales with the number on the display rather than with the top of the range. At 40 mph the allowance is 2 mph. At 2 mph it is 0.1 mph. That is the better of the two ways to state an accuracy, because the error stays proportionate to what you are measuring instead of swamping the low end. The limit at the bottom of the scale is therefore not the accuracy figure but the stated starting point of 0.67 mph.
Why does the resolution appear in metric when the range is in mph?
Because the specification was almost certainly written in metric and converted. A range of 0.67 to 67.1 mph is 0.3 to 30.0 m/s to within a rounding step, and the published resolution of 0.1 m/s belongs to that metric scale. Converted, the resolution is about 0.22 mph, so the reading moves in steps of roughly a fifth of a mile per hour and the floor of the range sits only three steps above zero.
Is it any use for balancing a duct system?
For supply grilles, yes, within its limits. One mile per hour is 88 feet per minute, so the floor is about 59 feet per minute and the top of the scale is about 5,900. Supply diffusers usually run well inside that. Large return grilles, trickle vents and low velocity plenums can sit under 59 feet per minute, and there the instrument has nothing to report at all. A vane also needs a traverse of several points to produce the average the equation asks for, not one reading in the middle.
What does the app add that the meter alone does not?
The listing describes automatic pairing with a phone or tablet over Bluetooth and the ability to log data and export it to spreadsheet or text format. That turns a reading you write down into a record somebody else can check, which is the difference between a measurement and a note. The tripod socket in the base belongs to the same idea, because an instrument clamped in one place and logging over time answers questions a handheld reading cannot. What the bullets do not state is the logging interval, the memory depth or which application versions are supported.
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