On 4/15/2011 10:39 AM, George wrote:
> This method is called a Hot-wire manometer, more airflow equals less
> heat transfer between a heat source and a thermal sensor (Thermocouple,
> Thermistor..). A similar method is also commonly used in high vacuum
> systems to determine the absence of atmosphere called a Hot-wire anemometer.
We used those for up to 10^-1 torr, beyond that we used a pair of
electrified plates/electrodes, between two very strong magnets for up to
10^-7 torr (called Penning (sp?) tubes) IIRC The magnets caused the
electrons to move in spiral path between the electrodes effectively
increasing the path length substantially. Below 10^-1 the penning tubes
were useless as they would just ionize the gas between the plates.
73
Roger (K8RI)
> 73,
> K4GVT
>
> 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 to volume if the duct size is known.
>>> Inexpensive units go for 20-30 USD on eBay.
>> 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.
>>
>> 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.
>>
>> 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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