(Chuck Yerkes) monitoring temperatures

Tue Aug 12 20:13:51 PDT 2003

His info seemed valuable and he is a member of the list, it just didn't
spot the mismatched plus address.  Enjoy.

-* Heather Stern * Acting postmaster, BayLISA * http://www.baylisa.org/ *-

----- Forwarded message from owner-baylisa at baylisa.org -----
  Date: Fri, 8 Aug 2003 16:31:45 -0400
  From: Chuck Yerkes <chuck+sage at snew.com>
  To: Alan Factor <afactor at afactor.com>
  Cc: baylisa at baylisa.org
  Subject: Re: Monitoring Colo Cage Temperature
  In-Reply-To: <oprtlh6llkfy4d3d at mail.afactor.com>

Hmmm, kinda long, but fun I hope and will tempt others to
play with "real world" input.

Quoting Alan Factor (afactor at afactor.com):
> Can anybody recommend equipment/methods to monitor temperatures in a colo 
> cage? My client has several cages (10'x10' to 10'x20' with 5 to 15 racks) 
> in a colo facility and I need to monitor hot spots in the cages.

This has come up from time to time on this list.

It was also recently on the NYSA.org list where it was advised the
HUMIDITY detection is a good thing along with "is the ground wet?"
(for those not in colo's but with modest server rooms that have
smaller AC units in them :)

I don't have those messages any longer.  They may have archives.
Perry Metzger, of NetBSD note among other things, had some answers
as I recall.

I got back into computers after getting really bored of "I type
stuff in and stuff shows up on the screen" when a teach got me into
electronics on Apple //'s and Forth.  Game ports on those are handy
little A/Ds with several buttons.  My working Apple ][+ STILL has
a row of 1/8" jacks to plug sensors into.  I just wish I had
a serial card for it :)

That served me well handling motion control systems and other data
acquisition and monitoring things.  Now its mostly a hobby and fun.

In Unix land, I used to have "WizTemp" devices - DB25 plug that
had a temp probe attached to a 1/8" jack at the end.  Cost around
$75 in 1993 when we got 20 or so.  It's a little A/D in the DB25
plus a 232 thingy.  It was a no brainer to use, but was a little
too binary to talk to.  Their software was ok and I always stuck
with that.

If you're inclined and not afraid to do non-soldering electronics,
I might suggest "Weedtech.com" - they have a set of devices
based on a PIC chip -> serial that take ASCII commands.  I like them
as a quick way to do this.  I'm also waiting for THIS:
   http://www.basicx.com/lcd+/LCD+overview.htm
to arrive, but that's got A/D, LCD, and other stuff.  Its fun for me.

All the manuals are online in PDF.

Back to the weeder tech stuff:
I have a Digital I/O one - I speak to it in ASCII commands to tell
it ports 1-12 are inputs, and send a message when they change
states; 13-16 are outputs and I send on/off messages.  I can
query to find if it's on or off so *I* don't have to also record
the state.  I pin it to a "device ID" and talk to it as
"$DevId, Command, Parameters".  It talks to me the same.
This is because I can CHAIN THEM TOGETHER on one serial line.

My use of it at home is doing things like seeing if the sprinkler
is on, or if the garage door is open or if a zone on the alarm is
activated (motion in living space -> turn on the light low at 2AM
cause the light switch is hard to find in the dark) and one reading
the UPS.
Others will be on buttons that let my "always on" machine read
them.  Push a button and it will run an "ssh $host halt -p" - mostly
so I don't have to have a live computer with a head to shut off a
home machine.  Eg.  it's bed-time, and I'm done with the loud
Solaris box and just want it off.  No good for real servers, but
at home I have machines that are up for building stuff occasionally.
At home, there's certainly the desire for "push button to make it
go off" desire.  2 system admins and we don't like to just power
cycle :)  This lets us do it right.
Nothing dire, it came out of "Chuck's toy budget" on the home
expenses along with my Lego robot set :)

The A/D one is equally simple to use: 8 inputs taking 0-4VDC.  You
set a threshold and if it passes it, it spits out a note.  Or you
can just query it out of cron.  Taking to it in perl is just
excessively simple.

They go for around $60.  It fits inside an Altoids box.  I've put
the IO stuff into an row of RJ12s (6 pin each has 4 IO lines and
2 gnd) so I can plug stuff in quickly.  The RJ12 stuff makes it
not quite fit in the Altoid tin :(

To use:
The IO pins appear on a screw down header.

Thermistors are excessively cheap (on the order of less than $1 or
so).  Nat Semi makes some temp sensors that are calibrated to emit
outputs that are directly bound to degrees F/C or Kelvin at
10mV/degree.  The LM34(F) and LM35(degrees C) in TO-92 packages go
for a around $2.  Digikey, Jameco, Mouser and the usual suspects have them.

Basic electronics can let us stuff more range into 0-4DV - either
by raising the bottom read (differential A/D) or compressing it.
Freezing to too hot (say 40 degreesC) can fit into 4VDC directly.
If you KNOW it will never be colder than 20C, you can increase the
accuracy.  This can do 12bit A/D (4k steps) at 120 samples/second.
In reality, 1 degree resolution is MORE than enough.  And 1/sample
every minute is plenty in this application.  I've read sliders for
light boards using just 6bit A/D giving 64 steps.  You could measure
accurately enough, from -40 degrees to boiling just fine with this.

Hooking them up is simple enough but you want to calibrate them.
For the Wiztemps (and indeed, thermistors hooks to Apple // game
ports in the day) we put it in a bag (dry), put THAT into a glass
full of ice water.  What's the A/D read? That's around 0C.
Put it into a glass of warm water with a thermometer and use
THAT as the higher measure.

Any A/D stuff needs this - a 30 meter lead to a sensor will cause
some power loss over a sensor that's just a couple CM away.

Basically, if your software reads the mV, converts it to "degrees C"
and then multiplies it by the "sensor B" factor that you've calibrated
(say 1.05) to get the right temp.

If you don't calibrate you risk being off a couple degrees - it
will be consistent however.  You might just always be reading 95%
of the temp.

It's not a canned package, but with a radio shack "project box" (or
you could put it inside an unused drive bay), and a little wiring,
you can have a very flexible 8 probes.  I've played with measuring
temps, measure light levels, pressure, etc, etc.

It's a nice break from typing in code to make machines churn stuff
with no actual "REAL" output or input. Tripping a relay or sending
X10 when the light level in a room gets low can be kind of fun.

I've probably spent more time writing up what you could slap into place
and have running than it would take you.

Device, case, probes and phone wire, a perl script.   You're done and you
can have it look as fancy as you want, but it will be more flexible than
a pre-canned thing.

----- End forwarded message -----