On 4/15/2011 4:23 AM, Dr. David Kirkby wrote:
> On 04/13/11 07:24 PM, Jim Barber wrote:
>> I wonder if a hand-held anemometer would have significant back pressure?
> I would not think it be very significant myself.
>> It's fairly easy to convert flow rate
Flow rate is so many CFM in either english or metric units and should
not require conversion.
Velocity OTOH would require conversion.
>> to volume if the duct size is known.
>> Inexpensive units go for 20-30 USD on eBay.
It depends on how accurate you want your measurements. Like most any
instrument you get what you pay for but if what you pay for and get
suits you needs, who cares.
All of the methods mentioned so far have some problems. The most
accurate for pressure is a "properly placed manometer but it also
depends on design of the compartment and the airflow. Turbulence at the
socket or exit of the pressure chamber can have some strong effects on
air flow and localized turbulence determines how well the seals are
cooled even at the proper air flow.
Which means even with the proper air flow volume and pressure you are
not guaranteed proper cooling of the base seals.
> My ex-boss (Prof. D. T. Delpy) used a technique for measuring air *velocity*,
> which I don't think is particularly relevant here, but I'll describe it since
> it
> is quite interesting. Although it used platinum wires, I suspect you could use
> copper, though I do see platinum would have some advantages.
This sounds like a derivative of how gas chromatography or liquid and
gas chromatography detectors work although they do the temperature
sensing with a bridge. They pass a gas through a molecular sieve (clay)
in a small tube at a given pressure and temperature. The smaller
molecules go though faster than the large ones. So you can measure the
components of the gas and even the amounts to a fraction of a percent
when compared to a calibrated gas of a known percent.
That's the long way around but they are basically measuring the time
lapse between entry and exit.
> It was used for measuring the speed babies breathed out air, so had to have
> negligible restriction on the airflow. Using a plastic bag would not have been
> very good for this purpose!
>
> Going left to right there is:
>
> Source of air ---> heater ----> thermometer
>
> 1) The source of air. This was the babies mouth.
>
> 2) Heater. This is a thin platinum wire through which a short pulse of high
> current is passed. I've no idea of the temperature the wire reached, but it's
> not important to know the temperature.
Relatively speaking the higher the temp the more response out of the
sensor which as you say has relatively little effect on the accuracy as
long as it is hot enough to give a useful response. A higher temp will
minimize the effects of the thermal inertial of the measuring wire. The
*fine* platinum wire will have a relatively high resistance change with
very little thermal inertia. If memory serves (which it seems to do
less and less) the fine Platinum is relatively strong compared to a
copper wire the same size and it is not going to oxidize like the copper
will. On the fine copper wire oxidation will, or can have a profound
effect on the resistance and strength. Again the resistance is
relatively unimportant, but the change in thermal inertial due to the
oxide layer can be.
The construction of a device using this technique is quite easy, BUT
getting it properly calibrated might be beyond the realm of most. It
relies on the good old manometer and a properly designed air chamber
with smooth or laminar flow yet keeps the pressure side of the manometer
out of the direct high velocity air.
I will add I spend a good portion of my working life before switching to
computers, building and calibrating this type of equipment.
In the end for what we want a ball park figure *that errs on the low
side* of which all of these measurements do is more than *good enough*.
I should add that it is possible to position the inlet and outlet of the
manometer in such a manner that it will read high.
73
Roger (K8RI)
> 3) Thermometer. This is another thin platinum wire, forming part of a platinum
> resistance thermometer. This responds very rapidly to temperature changes.
>
> The time between heating the first platinum wire, and when the temperature
> increase was observed on the second is measured. Knowing the distance between
> the two wires and the time allows one to calculate the velocity.
>
>> The "Bagometer" method is straightforward as well; just curious about
>> alternatives.
> Well, above is a curious alternative, though not very useful in this
> application.
>
> I'd be surprised if this technique was not described in some medical journal
> by
> him, but a quick glance with Google Scholar was unable to unearth the paper.
>
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