Temperature sensing technologies

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Suggestions for inexpensive, repeatable (no need for high uncalibrated
accuracy) temperature sensing *technologies* -- ~-20C to +80C with
time constants on the order of a few seconds to many minutes (i.e.,
I'm willing to integrate for higher precision)

I'm particularly interested in "loose" wiring/interconnect constraints
(i.e., thermocouples are almost definitely out of the question), a lack
of signal conditioning requirements in the "field" and "durable" devices
(platinum RTD's are also out).

Some of the "integrated" devices look nice but I'm cautious about
adopting as a "universal" solution (to a variety of sensing problems)

Re: Temperature sensing technologies
On 26/11/2016 17:35, Don Y wrote:
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You haven't said what kind of initial accuracy, but first pass:

1. Vbe or diode Vf
2. LM334
3. Thermistor

piglet


Re: Temperature sensing technologies
On 11/26/2016 11:03 AM, piglet wrote:
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Not an issue (reread my post)

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I don't see how this would work (well) "remoted" (long cable lengths)

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sb LM34?

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So far, best in terms of versatility of interface (trade time for
accuracy).  But, a bit of concern for operating temperature range.

Re: Temperature sensing technologies
On Saturday, November 26, 2016 at 10:11:46 AM UTC-8, Don Y wrote:
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The sense end is a low-power voltage regulator, resistor, and diode-connected
transistor.  The voltage regulator makes constant voltage on the transistor and
resistor in series, so the current INPUT to the voltage regulator depends on
the Vbe (temperature dependent) and Vreg (temperature-independent).  
You measure the current in the long wire, not the voltage.   Remember
to average over an integer number of AC cycles, there will be pickup.

Re: Temperature sensing technologies
On 11/26/2016 1:07 PM, whit3rd wrote:
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OK, but there are "integrated" solutions that will do this.
"No assembly required".

I'd like something that I would then not be constrained, mechanically,
in terms of how I could apply it (e.g., thermistors are ideal, sizewise,
in this regard).

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Not a problem.  I use FLL's often for integrating converters
for exactly that reason.


Re: Temperature sensing technologies
On Saturday, November 26, 2016 at 12:14:29 PM UTC-8, Don Y wrote:
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You wish.  The 'integrated' solutions still have to get the wire attached, and most
are intended for PCB mount.   With bit of heatshrink over it,  for strain relief, it's  
four parts.  "Some assembly required".


Re: Temperature sensing technologies
On 11/26/2016 1:33 PM, whit3rd wrote:
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And a little PCB (or "air wired circuit") has FEWER parts?

There's always "assembly required" -- you've got to put a connector on
the "near end", etc.  But, you're not stocking a variety of different
parts (nor are you tempted to tweak the assembly for specific applications).


Re: Temperature sensing technologies
On Sat, 26 Nov 2016 11:11:39 -0700, Don Y wrote:

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Then don't bother with a sensor at all -- just assume that the  
temperature is 23C, and move on.

Oh, wait -- you DO have accuracy requirements!  You're just not going to  
bother quantifying them!

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You haven't defined "well", or, for that matter, "long", so how can we  
help you with this?

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That depends on those accuracy requirements that you don't want to  
state.  Put a thermistor in series with a resistor, read it  
ratiometrically from the supply, and you've got something that's most  
accurate at the temperature where the thermistor resistance equals the  
load resistance, and tapers off outside of that range.  It'll _probably_  
be good to +/- 5C with little work over your entire range -- but you  
refuse to say whether that's good enough, so no engineering can be done  
on your problem.

--  
Tim Wescott
Control systems, embedded software and circuit design
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Re: Temperature sensing technologies
On 11/26/2016 2:05 PM, Tim Wescott wrote:
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(sigh)  More limited imagination.

A thermistor that always produces the same result for the same
temperature environment need have NO bearing on an actual temperature:
"this reading corresponds to *some* temperature.  When I see it again,
then I'm seeing THAT SAME TEMPERATURE (to the resolution of my digitizing
device).

The thermistor (in this case) doesn't limit the precision of the
measurement -- the "digitizing" circuit imposes those limits independent of
the "sensing element".

Say I want to design a thermostat that will keep my house at whatever
temperature it happens to be at *now*!  Should I find an NBS traceable
sensor and install that with a super high precision digitizer?

Or, should I purchase something inexpensive -- but repeatable -- and
wire it up, LOOK at the output *now* and say "this is my target".
Do you care if it's 73 degrees or 85 degrees in the house?  Will the
output NOT be monotonic?  If the sensed value is "low", htat suggests
I have to turn on the heat; if its high, that suggests I've got too
*much* heat.

All the information you need to control is present.  And, if you're smart,
you can optimize that control by just "taking copious notes" each time
you engage an actuator.

What's the stated accuracy on your COTS thermostat?  Or, the temperature
controller in your freezer?  Automobile water jacket?   Do you KNOW?
Do you *care*?

Our freezers have dials that say "cooler <-> warmer".  We tweek it
until its correct (20 years ago) and leave it -- confident that
it will repeatably maintain that temperature.

If we were interested in the most *efficient* temperature setting,
we'd be looking at specific temperatures (that can still be expressed
in REPEATABLE bogounits) and specific *power* measurements:  this
temperature uses this many power units; this other temperature (presumably
equally satisfying for the frozen goods contained within) uses more/less
power units.

The cars have interior temperature displays.  And, that temperature reading
probably reflects some point *inside* the dashboard -- not in the passenger
compartment (let alone the back seat!) where it is pertinent to the
occupants.  It could just as easily have been a *letter* as a *number*
("It's warm in here.  Turn the temperature control DOWN to F, from G")

Don't overdesign things and impose on the user ("Do you want your
car interior at 72 degrees?  Or, 73??")

There are no accuracy requirements *specified* because there are none
imposed by the problem domain!

Re: Temperature sensing technologies
On 26/11/16 22:28, Don Y wrote:
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Please no! For decennia, I was annoyed by fridge dials marked in
meaningless units. Now that we have dials marked in degrees Celsius,
I *don't* want to go back. Even if the accuracy is poor, it's
still much better than a meaningless 1-to-5 dial.

Jeroen Belleman

Re: Temperature sensing technologies
On 11/26/2016 2:52 PM, Jeroen Belleman wrote:
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How many people can really tell you what their refrigerator's temperature
is?  How many can truly tell you what the temperature in their *home*
is?

"1-to-5" isn't meaningless:  1 is colder (warmer?) than 2 which, in turn,
is colder (warmer?) than 3, etc.

People here get edgy when faced with "metrification".  I.e., temperatures
will be "less precise" (if reported in whole degrees).  Yet, I challenge
anyone to tell me the difference between 25C and 26C *reliably*.

Instead, people think in terms of "warmer" (than THIS) and "colder".

If you want to be "accurate", at the very least, you'd say:  "My thermostat
THINKS it is 72 degrees AT ITS SENSING ELEMENT.  The temperature elsewhere  
(even a few feet away) is subject to interpretation..."

Note the examples that I offered.  Set your refrigerator/freezer/water heater
to some particular temperature.  Use an NBS traceable sensing/measuring
element.  *Hope* that it maintains that temperature "dead-to-nuts".

If *my* observational goal is to detect when the freezer has failed
(or appears to BE in the process of failing), do I need that same
"high accuracy" sensor?  If i have a *repeatable* sensor and I
"notice" what it typically reports for the freezer during operation,
then any deviation from that typical is what I'm concerned with, not
the magnitude of the deviation:
     "Hmmm... normally, the temperature would start FALLING at this point
     (whatever *it* may be, in SI units, unknown to me).  But, I see it
     rising!  Something is behaving in an atypical manner.  Perhaps the
     compressor has failed?  Perhaps the thermostatic control inside the
     device has failed?  Perhaps *my* sensor has failed?  *Or*, maybe
     someone just opened the freezer and added a dozen quarts of Marinara
     sauce that were previously at room temperature and the interior of
     is EXPECTED to increase in temperature.  But, wait -- no one is home!
     Is it likely that a BURGLAR has done this??  Or, MORE likely that
     the freezer is failing?!"

[Think in terms of an expert system.  Would it really be concerned with
ACTUAL temperatures?  Or, *relative* temperature changes?]

Re: Temperature sensing technologies
On 26/11/16 23:06, Don Y wrote:
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OK then. To what should I set a 1-to-5 dial to keep the lettuce
from freezing? I don't want to iterate to get the right setting.
That what we have technology for!

As for your freezer goal, I apologize in advance, but I think you're
just a little crazy. I just want it to maintain a set temperature,
ignoring brief transients, but have some indicator lit if something
goes really wrong. Freezers aren't very good at detecting burglars.

Jeroen Belleman

Re: Temperature sensing technologies
On 11/26/2016 3:40 PM, Jeroen Belleman wrote:
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Honestly, have you NEVER altered the setting in your refrigerator -- despite
having it *calibrated* in pseudo-accurate units?  Did you iterate -- even JUST
ONCE??  Why?

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Does yours have such an indicator?  Have you already *paid* for
*similar* technology?  (Ah, I see... just buy new appliances and
select from only those with these features.  How smart is your
furnace?  Air conditioner?  Washer/dryer/water heater?  Or, are
those on your "soon to be replaced" list?)

I want to detect when the freezer is failing or likely to fail
(gee, its taking longer to cool than normal).

My goal is to enable folks to live independently for longer periods
of time despite decreasing abilities.

How does a blind man decide that his freezer is on the fritz?  FEEL
every item to see if any of them are "unusually soft/warm"?

How does a man confined to a wheelchair accomplish this?

How does an elderly person with cognitive impairment do it?

Ans:  someone else ASSISTS them with these responsibilities,
despite the fact that they can otherwise maintain their own
lifestyle independant of others (feed themselves, cloth
themselves, remember to bathe regularly, etc.)

For folks without nearby assistance (friends/family), that
means moving into a facility that caters to these "needs"
(responsibilities) for them.  At a greatly increased cost and sense
of lost independence -- even if they are allowed to inhabit
their own "quarters".

For folks WITH nearby friends/family, it puts them in a position
of having to either pester that care giver (possibly needlessly)
*or* preserve their pride and live "suboptimally".

I have several friends/neighbors that fall into these categories.
They are perfectly capable of wiping their own asses.  But, can't
handle even routine "problems" without calling for "professional
services".  At which time, they are often exploited:  "You need
a new furnace"  "Your roof needs to be replaced"  etc.

Twice a year I make the rounds converting from winter (heating)
to summer (cooling) for them -- despite the fact that this is
a no-brainer operation.  They don't even *remember* that this
must be done!

One neighbor came to me, confused, wondering why he had a $300 gas
bill -- in October (heat is barely needed).  I asked him if the
furnace had been running a lot lately -- if he'd had the thermostat
set too high...

Of course, he couldn't understand what "running a lot" should mean...

Realizing that his "faculties" were obviously in decline, I went
to his house and verified that he'd not installed the baffle
to block the air flow INTO the ductwork from the swamp cooler
located on his roof ("winterize").  This is a no-brainer operation:
slide a sheet of tin into a slot, turn off water supply to cooler,
switch from COOL to HEAT.  Maybe 3 minutes if you're lazy...

As a result, all of the heated air from the furnace was going up
into the ductwork -- and then out through the roofmounted cooler!
Of course, the thermostat saw nothing wrong with this:  house is
still not warm enough, continue to call for heat!

A *smarter* thermostat could have told him of this likely problem
*or* "phoned" some "responsible party" of the problem before he'd
incurred $300 worth of "heating The Great Outdoors".

[And, that thermostat wouldn't need to know what the actual temperature
of the house was during that period!  "Still not warm enough!"]

The *city* lost its natural gas supply one night a year or two ago.
Plumbers were inundated with calls for service.  They gladly scheduled
all of those house calls -- and invariably said "your gas supply is
out; there's nothing we can do about it.  That will be $200, please..."

Of course, I suspect they also "suggested" new HVAC systems to a
fair number of those folks!

*I* didn't bother calling a plumber because I could observe how the
furnace was reacting to calls for heat:  there's something wrong
with the gas supply; nothing I can do about it other than report
it to the gas company.

I then called other friends/neighbors to try to gauge the extent of
the problem -- is it my block?  street?  neighborhood?  (the idea
of it being citywide never occurred to me).

If one of these people had been an individual for whom I had
some *responsibility* (beyond "the kindness of my heart"), I could
have queried their furnace to see if it was exhibiting similar behavior.
Or, their "furnace" (thermostat == furnace controller) could have
reported the problem saving them the expense and anxiety/uncertainty
of the plumber's visit (many hours later!)

[*Our* furnace has a set of idiot lights -- INSIDE the blower compartment.
PERHAPS they would have told me what I could deduce by observing the overall
performance.  *If* I could locate the manual, etc.  Had I not been home,
I'm *sure* SWMBO would have called a plumber!]

*Hope* you retain your physical and mental capabilities.  *Or*,
plan on your kids (or your bank account) taking care of you when
that time comes that you can't perform these tasks that seem
almost laughably trivial, NOW!  The $100-150/day (FOR THE REST OF
YOUR LIFE) alternative is far too costly (and embarassing) for many!

Re: Temperature sensing technologies
On 27/11/16 00:16, Don Y wrote:
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The one with the meaningless numbers on the dial, yes. Because
it froze the lettuce. The one with the display in degrees was
just set to 5 degrees at that was it.

In this age of talk about internet connected fridges, we should
at least get the basics right before the fancies.

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Yes, my freezer has a green and a red status light. It's not
even a recent freezer.

Jeroen Belleman


Re: Temperature sensing technologies
On Sun, 27 Nov 2016 00:40:22 +0100, Jeroen Belleman

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That's still not adequate unless you check the lights hourly.

I had a freezer fail that I didn't know about until I opened the
door... whew!  What an odor!

An audible alarm or one that texts you would be nice.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Temperature sensing technologies
On 11/26/2016 4:58 PM, Jim Thompson wrote:
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In our case, we have a lot of money tied up in the freezer's contents.
We'd hate to lose that "investment" just because something that could
have been FIXED, failed without our knowing!

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Repeat for:
- washing machine hose rupture (happened to us)
- toilet fill hose rupture (while I was asleep; thankfully light sleeper!)
[both of these PROBABLY caused by excessive municipal water pressure;
subsequently installed PRV *and* pressure gauges -- to be replaced by
sensors -- on each side thereof]
- freezer failure (breaker; unnoticed because we didn't access it, then)
- condenser fan failure (caught because I noticed ACbrrr performing poorly
   AND HAD THE EXPERTISE TO KNOW WHAT TO CHECK; $9 for a run cap)
- natural gas supply failure (described elsewhere this thread)
- water heater failure (unnoticed because located in seldom visited store room)
- natural gas leak (noticed because of sensitive nose and verified with
   portable mass spectometer? -- some gizmo borrowed from Safety Officer at
   local hospital -- traced to faulty repair by plumber)

And, we're "on top of" things -- the house is almost always occupied.
I can recount horror stories of neighbors who've had pipes burst in
their attic, water heaters leak (located in interior rooms), etc. while
they were away at work...

All of these things can quickly be addressed -- and often inexpensively -- but
only *if* you know they are happening, WHEN they are happening!  Sure would be
nice if someone (something) was "on duty" 24/7/365 to watch for these things
AND SUGGEST POSSIBLE REMEDIES!

Re: Temperature sensing technologies
On Sat, 26 Nov 2016 17:19:20 -0700, Don Y

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Previous house had best solution for that... recessed floor for
washing machine, with floor drain.

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Garp, "Pre-disastered" ;-)
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Temperature sensing technologies
On 11/26/2016 6:36 PM, Jim Thompson wrote:
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In our case, the jet of water from the ruptured hose cut a hole through
the drywall in a matter of a few minutes -- the time it took me to
wonder "Hmmm, why does that water supply sound funny?" and walk into
the laundry to see the failure.

Hose was brand new.  Washer and dryer, as well.  I figured the "different
sound" was due to different fill characteristics/acoustics of the new washer!

"Shitty hose"? (return to vendor for replacement)

Some months later, guys from city water department in the neighborhood:
     "Hey, do you guys have a pressure gauge?"
     "Yeah, why?"
     "I'd like to know what my household water pressure is..."
     "OK, screw this onto that hose bibb out where the water enters
      your house.... hmmm, that's awfully HIGH!"

It was only then that I recalled the episode with the toilet fill hose
rupturing in the middle of the night.  "Ah, not *two* shitty hoses but,
rather, one excessive SUPPLY!"

I've cautioned most of the neighbors to this fact and few have done
anything about it.  One had a PRV installed -- but plumber neglected to
install an expansion tank (so, problem is -- at best -- no better and
likely considerably worse as PRV makes it a closed system).

Always good to know you can count on quality work from PROFESSIONALS!  :-/


Re: Temperature sensing technologies
On Sat, 26 Nov 2016 19:16:20 -0700, Don Y

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Yep.  Our three year old house has a pressure regulator at the side of
the house.
        
                                        ...Jim Thompson
--  
| James E.Thompson                                 |    mens     |
| Analog Innovations                               |     et      |
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Re: Temperature sensing technologies
On 11/26/2016 7:25 PM, Jim Thompson wrote:
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But, do you have an expansion tank plumbed DIRECTLY (no intervening valve) to
the cold water inlet for the water heater?  The PRV essentially is a diode
(some caveats below).  When all the faucets are closed in the house, the
water *inside* the house (within the pipes) fills a fixed volume.

This includes the volume available in the water heater.

If the water heater engages, the water in the heater expands.  But,
the volume that is allocated for it remains unchanged.  The pressure
increases until the pressure relief on the water heater actuates
(which it hopefully does).

An expansion tank provides an additional "dynamic" volume into which this
water can expand.  Usually on the order of a gallon or two (depends essentially
on the size/volume of your water heater).

The caveat is that some PRV's have a "bypass" mode whereby another "reverse"
diode allows water pressure EXCEEDING that of the municipal supply to
open the valve in the reverse direction, dumping pressure into the municipal
supply.

But, this essentially means the municipal supply sets the actual pressure
of your (closed) system -- the reverse bypass just acting like a clamp to that
supply.

Here, for example, our PRV is set at 50psi.  The pressure gauge downstream
of it confirms this (municipal supply is 120 psi or more -- most appliances
and fixtures are designed for 80 psi max).

If I disconnect the expansion tank (creating a fixed volume, closed system)
and do something that consumes a lot of hot water (e.g., a lengthy shower),
then when I shut off the water to the shower, the closed system is completely
full.  And, the water heater is full of "cold" (warm, not hot) water.

As the water heater does its job and heats this back to normal temperature,
it expands.  The pressure gauge QUICKLY climbs to ~100 psi as the increased
water volume has no place to go (expansion tank has been disabled).

If it ever attained a pressure at or exceeding that of the municipal supply,
the "reverse" valve in the PRV would open and vent that pressure into the
supply lines, effectively limiting our internal pressure to that level.

It's amusing how little extra volume is required to create this +50psi
pressure increase!  I.e., if you crack open a faucet for the briefest of
moment, enough water escapes to drop the pressure back to the 50 psi
PRV setpoint.

With the expansion tank in place, there is a bladder inside into which the
water can expand.  The bladder operates against a sealed AIR pressure
(there's a Schrader valve located on the "bottom" of the expansion tank
that you can use to set the optimal pressure, based on how you have
the PRV set).
to expand


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