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- Xref: sparky sci.electronics:20841 sci.geo.meteorology:3521
- Path: sparky!uunet!hobbes!gorn!deeptht.armory.com!spcecdt
- From: spcecdt@deeptht.armory.com (John DuBois)
- Newsgroups: sci.electronics,sci.geo.meteorology
- Subject: Cheap surplus radiosondes (was: Re: How to measure humidity)
- Message-ID: <1992Dec16.101254.22093@deeptht.armory.com>
- Date: 16 Dec 92 10:12:54 GMT
- References: <1992Dec10.150003.16312@kong.gsfc.nasa.gov> <Bz2Kr7.2KI@vcd.hp.com> <1992Dec11.122331.2992@phx.mcd.mot.com>
- Organization: The Armory
- Lines: 57
-
-
- All Electronics (818-904-0524) is selling surplus radiosondes for
- $4.75 each. They are intended to be lifted to high altitude by a balloon
- and transmit back atmospheric information. They were built by Viz Mfg.
- Co. Since I'm working on constructing various weather instruments, I
- bought a couple of them and disassembled one.
- It contained two circuit boards. One is a very simple one-transistor
- radio transmitter, tuned by means of a disk of metal on a screw thread
- which along with two copper areas on the PC board forms a variable
- capacitor (now there's cheap :-) The other is a data encoding and
- multiplexing board (4066, etc.)
- A water-activated battery is included, sealed in a can with
- activation instructions.
- There are three instruments. An aneroid barometer bellows is
- connected to a wiper arm which moves across 180 traces connected in a
- complex pattern to three outputs. I would guess it's intended to allow
- the pressure to be transmitted using a small number of channels (3) by
- counting pulses, while still being able to to recover from missed pulses.
- Folded up in a baggie in the radiosonde was a "Baroswitch Pressure
- Calibration Chart". It gives the pressure it what I'd guess are millibar
- for each contact value from 1 to 179. Also in the baggie was a paper
- tape, perhaps giving a machine readable version of the pressure chart, or
- perhaps describing one of the other instruments. I don't have a paper
- tape reader handy...
- I'd like to use the barometer, but it seems to be mainly intended as
- an altimeter; the pressure on the chart goes from 0 to 1061.4, with about
- 10 (millibar?) between contacts at the high end. I suppose I could put a
- different sensor on the bellows and hope that a bellows intended for large
- pressure changes will move a useful amount, but then I lose the convenient
- machine-readable sensor it already has. Oh well... maybe I'll send it up
- in a rocket :)
- A thermistor is placed such that it will be outside of the radiosonde
- when it is deployed.
- Most interesting to me, a humidity sensing element came in a sealed
- can. I opened it (yes, with a can opener) and found a rigid strip about 2
- cm x 7 cm with conductive paint along the long edges and black coating
- between them. I put it in its bracket (spring wire which holds it &
- provides contacts) and connected it to an ohmmeter. It initially read
- about 24K. I breathed on it from a half meter away and it shot up to
- >100K, *very* fast. I was amazed, expecting a humidity sensor to be slow
- and insensitive and possibly not have a varying resistance at all. Since
- then I've seen it vary between 18K and 26K when people aren't playing with
- it. It does seem to move with the dial humidity sensor I placed it next
- to, though I haven't been keeping really close watch.
- I am curious about the sensor. Anyone know what effect it's based
- on? The fact that it was sealed in a can with a dissicant while awaiting
- use implies that it has a limited life. How long is it likely to last?
- Should I expect it to exhibit hysteresis? Would its output vary with
- relative or absolute humidity, or be a function of both?
- While we're at it, the dial humidity sensor is based on a bimaterial
- strip, metal and some-white-stuff which presumably expands and contracts
- with humidity. How reliable is this type?
- Has anyone made a condensing type humidity sensor?
-
- John
- --
- John DuBois spcecdt@armory.com KC6QKZ
-