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'An Interesting but Unscientific Test (was Re: Ligh'
1997\01\06@210014 by Martin McCormick

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       Around Christmas, I mounted a lighted star at the top of a radio
antenna tower.  The star was lit by a string of 35 2.5-V lamps in series
with 120 Volts across the string.  Just for fun and also to tell whether
anything seemed wrong with the lights, I read the resistance of the circuit
consisting of the bulbs and out-door extension cord that fed them.  It
occurred to me that one could read the outside temperature if one had a
very accurate bridge and calibrated the whole system.  With the lamps having
been off for several hours, the resistance was around 70 ohms when the
temperature was around 60 DG F.  We had one morning when it dropped to
11 or 12 DG and I seemed to recall that the resistance dropped to 67 Ohms.

       I don't have any figures on the coefficient of Tungsten or the other
parameters of the bulbs, but I wondered if I was seeing true temperature
variations or just a dirty socket on one of the bulbs.  When we got another
60 DG day, the resistance did go back up to around 69 or 70 Ohms.

       This whole idea might make a neat PIC project in that one could
run a small current through the string when it was supposed to be off and
sound an alarm if a bulb opened up.  Probably not worth it for Christmas lights,
but it might be for a safety light.

Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
OSU Center for Computing and Information Services Data Communications Group

1997\01\06@220648 by Reginald Neale

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Martin McCormick said:

>        This whole idea might make a neat PIC project in that one could
>run a small current through the string when it was supposed to be off and
>sound an alarm if a bulb opened up.  Probably not worth it for Christmas
>lights,
>but it might be for a safety light.
>

A company called MINCO makes a Temperature controller in which the
resistive heater also serves as the temperature transducer. The tempco
isn't large, but it's stable and obviously, the sensor is closely thermally
coupled to the source!

Probably could do this with a 16C71 and an opamp.

Reg Neale


"Politics is the art of preventing people from taking part in affairs which
properly concern them."    .......Paul ValŽry

1997\01\07@015935 by Shel Michaels

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Martin McCormick writes:

<<  I don't have any figures on the coefficient of Tungsten or the other
parameters of the bulbs, but I wondered if I was seeing true temperature
variations or just a dirty socket on one of the bulbs.  When we got another
60 DG day, the resistance did go back up to around 69 or 70 Ohms.
 >>

Tungsten:  at 20 degrees C, alpha = .0045; at 1000 degrees C, alpha = .0089
Copper, hard drawn: at 20 degrees C, alpha = .00382

These from Handbook of Chemistry and Physics, 39th edition, Chemical Rubber
Publishing Co.

I never realized tempcos were so high!  Means that you should have observed a
resistance change on the order of eight ohms or so.

Regards,
Shel Michaels
Massachusetts, USA

1997\01\07@171159 by peter

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Martin McCormick wrote:
>
>         Around Christmas, I mounted a lighted star at the top of a radio
> antenna tower.  The star was lit by a string of 35 2.5-V lamps in series
> with 120 Volts across the string.  Just for fun and also to tell whether
> anything seemed wrong with the lights, I read the resistance of the circuit
> consisting of the bulbs and out-door extension cord that fed them.  It
> occurred to me that one could read the outside temperature if one had a
> very accurate bridge and calibrated the whole system.  With the lamps having
> been off for several hours, the resistance was around 70 ohms when the
> temperature was around 60 DG F.  We had one morning when it dropped to
> 11 or 12 DG and I seemed to recall that the resistance dropped to 67 Ohms.
>
>         I don't have any figures on the coefficient of Tungsten or the other
> parameters of the bulbs, but I wondered if I was seeing true temperature
> variations or just a dirty socket on one of the bulbs.  When we got another
> 60 DG day, the resistance did go back up to around 69 or 70 Ohms.
>
>         This whole idea might make a neat PIC project in that one could
> run a small current through the string when it was supposed to be off and
> sound an alarm if a bulb opened up.  Probably not worth it for Christmas
lights,
> but it might be for a safety light.
>
> Martin McCormick WB5AGZ  Stillwater, OK 36.7N97.4W
> OSU Center for Computing and Information Services Data Communications Group

I have repaired a system like this used on a tug boat,
but don't know how common it is for boats to have this system
The system monitored the navigation lights
both when they were on and when they were off
--
Peter Cousens
email: spam_OUTpeterTakeThisOuTspamcousens.her.forthnet.gr
snailmail: Peter Cousens, karteros, Heraklion, Crete, 75100, Greece,
phone: + 3081 380534,    +3081 324450   voice/fax

After Bill Gates announced to the world that he was Microsoft,
his wife was asked to comment. She said that as his wife, she
had been the first to notice this problem

1997\01\07@200944 by Martin McCormick

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In message <.....970107012613_1990957654KILLspamspam@spam@emout01.mail.aol.com>, Shel Michaels writes
:
>I never realized tempcos were so high!  Means that you should have observed a
>resistance change on the order of eight ohms or so.

       Well, 3 to 4 Ohms is fairly close when one considers the unknowns.
I don't know how much ambient heat transfer there is between the lamp
filaments and the outside air.  I would think that since the bulb is mostly
evacuated, the filament is somewhat insulated from the world.

       Any slight change in the formulation of the filaments would probably
also make a big difference in the resistances one would read.

Martin McCormick

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