--On 06/10/2005 02:03:36 PM -0700 Wright, Ryan P wrote:
Thanks Steve. I'm guessing you bought the pre-boxed DS18S20 ("TAI8520")
sensors from AAG for $20 a pop, then? That makes things a lot easier in
terms of cabling.=20
Jerry:=20
First some general comments: I would definitely use +5 power and=20
simultaneous reads. For me it's about making the controls a bit more=20
stable. Even at 50-100 sensors per bus, I just think it's easier this
way.
I tend to agree, and this is the route I was looking at. Do you happen
to know how much current the sensors draw, max? I'm going to have to
find this info so I know how big / how many power supplies I need.
Spec sheet on the 19S20 says 1-1.5ma with 5V Vdd, so a 1A supply would
drive the entire set.
If I was attacking this, I would start by designing a simple PC board
that=20
gets RJ45 connectors and places to mount the sensors. Then a tech
wires up=20
the boards.
I had a second crazy thought that will require some testing first. I
believe I can crimp a DS18S20 right into the back of an RJ45 connector.
I don't have a DS18S20 handy but I do have some transistors with the
same form factor and they fit beautifully. I can then use RJ45 splitters
to run the daisy chain of RJ45 cables & RJ45-fitted DS18S20s from rack
to rack. Only real concern here is airflow past the sensor.
I would maybe consider this for a few, but not in your application. 300 is
a large number, and the time you spend figuring out what's wrong could
really add up over time.
Failing that, creating (or outright purchasing) some PC boards would be
the next best way to go. However, I'm guessing that doubles the cost of
my installation - not including labor. What I'm doing here is more of a
research project / proof of concept thing so I'm trying to keep costs
low.
I get the cost issue, but being successful and producing useful data is the
payoff. IMO, data dropouts really stand out and can cause others to
question the value of all the data.
Since you're doing this, I would also pick a few racks and sensor the air
intake and exhaust for each machine. In particular, I would want to see if
the machines at the top of the rack are getting hotter intake air than at
the bottom, which could cause system life problems. There are all sorts of
subtle airflow dynamics, just keep it simple and consistent to start with.
As for reading variations, first check to make sure it's not the chips. I
do bulk calibrations as follows: Take a water bath, put a bunch of sensors
in a latex glove or two and plunk them into the water. Swish them around a
bit then take a simultaneous read of the set. If you have a precision temp
sensor like a platinum RTD, you can calibrate the sensors. If not, you can
do bulk averages and record the differences against average. My experience
with this was that the chips did not meet the accuracy that was listed.
Since you didn't answer about the required accuracy, I would assume that a
couple degrees C don't matter as long as any sensor is repeatable.
jerry
-___-----------------------------------------------------------------
| , | ___ ___ _ -- Ryan P. Wright
| _/__ __/ _ \ | | | -- MSCF Operations
| \ \/ / /_\ \| | | | -- [EMAIL PROTECTED]
| _|\_\ /_/---\_\_|___| -- (509) 376-3502
-----|_|-------------------------------------------------------------
-----Original Message-----
From: [EMAIL PROTECTED]
[mailto:[EMAIL PROTECTED] On=20
Behalf Of Steve Lancaster
Sent: Friday, June 10, 2005 11:54 AM
To: owfs-developers@lists.sourceforge.net
Subject: Re: [Owfs-developers] 1-wire network design & OWFS
=20
=20
=20
Our sensor network is two separate runs off of a=20
(http://www.aag.com.mx)=20
TAI8595 hub.
=20
One run goes around the ceiling and to the air handler=20
sensors. We have sensors in each quadrant of the room, and a=20
humidity/temp sensor in the center.
=20
We placed our sensors in the center of each quadrant of the=20
room, in order to stay away from the low flow areas near the=20
walls and from doors to the outside.
=20
Our sensors are DS18S20's and DS2438's (from AAG as well.)
=20
we ordered pre-made jumpers from
=20
http://www.connectworld.net/cgi-bin/iec/home.html
=20
We had to special order them as plenum rated.. and we=20
ordered blue colored cables and hoods to make sure that no=20
one mistook them for ethernet.
=20
(Don't forget that the AAG connectors are 6 pin, not 4 or 8=20
which are much more common.)
=20
We did some early tests of things like distance and=20
"accuracy"... to confirm that we could go as far as we needed to.
=20
We put a bunch of them on a loop in an icechest, with a=20
small electric fan inside.. The heat from the motor gave us=20
a "changing" temperature to measure and the fan kept the air=20
moving. We left the covers off of the sensors to get the=20
fastest settling time. Even with this attempt at an=20
isothermal environment, there was almost a 3 degree=20
difference between the lowest and highest readings. (Good=20
enough for our purposes.)
=20
We did punch some more holes in the covers of the sensors=20
because there wasn't enough flow past the sensor itself. The=20
time constant was too long otherwise.
=20
We then put sensors all over a rack to see if we could make=20
sense of the readings.. and the answer was "not really"....=20
There was some variability based on where we put the=20
sensors.. but not enough to say "we are having a problem=20
with the left most fan on the SunXXX...". Factors such as=20
cable placment, how "full" the back area of the rack was..=20
and whether or not the back door was open, ventilated, solid=20
had much more effect on the outcome.
=20
About the best overall look at what a rack was doing was=20
achieved by mounting the sensor with a tie wrap to the grill=20
on the underside of the fan in the top of the rack.=20
=20
Our racks are full of all sorts of things, from "salvaged"=20
Sun desktops to dual processor AMD 64's in 1u chassias.
=20
The fans in the units do a lot of air mixing, making the=20
change in a given reading hard to relate to what caused the=20
change.. and other factors just swamped the "small signal"=20
changes due to things like stalled fans.
=20
Most of what we see is temperature traces which march in=20
parallel fashion across the graph.. with overall shifts that=20
match what happens with the room temperature.. There will be=20
an occasional "baseline shift" which is usually due to=20
opening or closing a rack back door.
=20
The biggest factor in the change is "what's the room temp".=20
We do see about a 2 degree change when the tape robot lights=20
off for the nightly backup run in the ceiling sensor just=20
above the robot.
=20
Our ceiling temps vary across the room by about 5.5 degrees=20
from coolest to warmest..=20
=20
And the delta T across the air handlers runs from a low of=20
-2.5 degrees on a lightly loaded day to a high of 6.5=20
degrees under heavy load.
=20
Humidity goes from a low of 34 to a high of 62.. tracking=20
nothing I've been able to spot.. except that it's highest on=20
days of low delta T... which makes sense.. it's hard to=20
remove moisture from a room that's already cold enough. We=20
don't do reheat for humidity control. (We could.. but since=20
we don't have the "problem" we don't want to pay high energy=20
bills to solve the nonexistent problem. :-))
=20
=20
If I had lots and lots of racks to do, I'd probably run the=20
sensor cable across the top of the racks, tie wrapped to the=20
top grill of the fan.. you will get the fan motor heat added=20
to the heat coming from the machines in the rack... but it's=20
not all that significant.. and you can run short jumpers=20
from one rack to the other, instead of going down to the=20
floor and all the way back up again in the adjacent rack.
=20
Entering air temp matched the air handler exit temperature=20
to such a great extent that we didn't bother to put a sensor=20
at the bottom of the racks.
=20
Alarms are done by scanning the rrd database for high=20
readings.. we look for three high readings before we page..=20
either three high in one scan (the whole room is getting=20
hot).. or three high in three different scans.. "one rack is=20
getting hot" before we fire off a page. It cut our false=20
alarms to ZILCH.. we do have an an occasional "spike" in a=20
reading for reasons we have not been able to correlate with=20
anything in particular.
=20
Spikes went WAY down after we solved this problem:=20
http://www.wes.net/construction_remediation_services-zinc_whi
sker_detail.htm
(I'm not endorsing this company... just using their web site.. We used a
local contractor.. and replaced the entire raised floor in the computer=20
room. We did do surveys using a scanning electron microscope to tell
when the cleanup was adequately done.. and it was a MESS.. We had MANY
power supply failures while this mess was going on.. it's a real
threat... with real results. At one point we were loosing a couple of
power supplies a week.. Fortunately it was rare to loose both supplies
on a redundant supply box.)
Watch the airflow patterns of dissimilar machines.. some draw cool air
from the front and exhaust hot air out the back.. some draw from the
sides... you can end up with "closed loops" forming between adjacent
machines that are hotter than you expect. and mixing with cooler air by
the fan at the top can hide this result. Mixed racks may be good
candidates for "more than one sensor".=20
From just one sensor per rack.. and other "environmental" sensors
arrayed around the room we get pretty reasonable results, without
spending a fortune to "over sense" a chaotic world for no gain.
Steve
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Jerry Scharf
laguna way consulting
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a projector? How fast can you ride your desk chair down the office luge track?
If you want to score the big prize, get to know the little guy.
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