A New Way of Testing Fan Airflow

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There was no question that the airflow box solved at least one of our problems: The swirling vortex of fan-generated airflow and the effect of its spin direction on measured LFM. Was there some way that we could measure airflow using the box without running into the higher pressure problem with bigger fans?

One solution would be an anenometer with a much larger impeller. Perhaps there was one with 120mm diameter blades?

An extensive search through test equipment manufacturers' catalogs ensued. In short, the anwser was no. There does not appear to be any anenometer on the market with a 120mm diameter impeller.

However, the search did turn up a different type of anemometer we had not considered before: Hot Wire Anemometers. This type of tool was ignored in our initial market search for anemometers back in 2003; they were simply too costly to justify at the time. Today, with SPCR well established, the investment in an important tool is more easily justified.

"The hot-wire anemometer, principally used in gas flow measurement, consists of an electrically heated, fine platinum wire which is immersed into the flow. As the fluid velocity increases, the rate of heat flow from the heated wire to the flow stream increases. Thus, a cooling effect on the wire electrode occurs, causing its electrical resistance to change. In a constant-current anemometer, the fluid velocity is determined from a measurement of the resulting change in wire resistance. In a constant-resistance anemometer, fluid velocity is determined from the current needed to maintain a constant wire temperature and, thus, the resistance constant." (Cited from The Engineer's Edge web site.)

The most salient aspect of a hot wire anenometer is that its sensor is very small, and mounted at the end of a thin wand. This means that it can be placed in the airflow without creating any significant resistance.

Close market research turned up the Extech Model 407123 as the least expensive, full featured hot wire thermo-anemometer on the market. An order was duly placed, and a week later, the item arrived at the lab.

Extech Model 407123 hot wire thermo-anemometer

The cable from the telescoping antenna-like wand plugs into the top of the handheld digital readout meter. (The telescoping wand is useful for HVAC personnel when assessing airflow in large building ducts.) The black colored head of the wand is where the sensors are located. A close up photo is shown below.

Sensors at the end of the wand.

So how were we going to use this new test instrument? Well, we wanted to stay with the fan test box, for sure. It's a low impedance setup, but has some impedance, and it's more representative of actual use conditions than free air. We also wanted to stay with fixed position measurements, as opposed to hand held muddling.


After a few days of experimentation, we came up with a setup and methodology that appears consistent, repeatable and reliable. Here it is in a photographic nutshell:

Fan airflow test box set up with hot wire anemometer.

The exhaust vent is the cutout for the power supply on the back panel of this acrylic ATX computer case. This is where the airflow is measured. The exhaust vent is about 10% larger than the impeller area of the typical 120mm fan. This means the exhaust vent should not constrict the airflow in any significant way, except perhaps in extremely high airflow models — but we wouldn't be interested in such fans anyway because they'd be far too noisy from the air turbulence alone.

Experimentation with sensor wand placement indicated that despite the ~1.5' distance from the fan and the baffle to minimize direct airflow between fan and exhaust vent, the airflow rates are not the same across the exhaust opening. To be specific, there is always a peak of up to 10% on the left side of the exhaust opening, compared to the center. Further to the right, the flow tended to dip about 5%.

There are all kinds of possible reasons for this positional variance in airflow — the airflow path is not perfectly symetrical, and neither is the vent opening, and the swirling vortex effect of the fan exhaust may still be apparent at the exhaust. The reasons are not really that important. What's more important is whether we can get rid of the variance, and if not, how we can deal with the variance.

In the end, we decided to accept the positional variance, and to deal with it by taking the high and low readings from the three positions, then use the average of all six readings. This averaged LFM figure would be multiplied by the area of the exhaust opening to give us the CFM value. We set up a jig with grooves to secure the sensor into the same positions every time; this is the white closed-cell foam block that's holding the sensor wand in the photos above and below.

Sensor wand locked in the right position.

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