Discussion:
Environmental sensor recommendations.
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John Green
2018-04-05 14:44:48 UTC
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Why has no one mentioned thermocouples?
I had some experience with thermistors a few years back designing thermal
attenuators and equalizers for CATV. NTC thermistors can have a large
change of resistance for a unit change in temperature. They aren't linear,
but there are formulas for computing resistance vs temp. PTC thermistors
have a much smaller change per unit change in temp., but are much more
linear. And, they are susceptible to self heating, which makes things
interesting. If I remember correctly, in my research something called an
RTD was supposed to be the king when it came to accuracy and repeatability.
As someone else has stated, the IC devices are supposed to be quite good,
but you have to interface with them.
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Dr. David Kirkby
2018-04-05 15:27:51 UTC
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Post by John Green
Why has no one mentioned thermocouples?
I had some experience with thermistors a few years back designing thermal
attenuators and equalizers for CATV. NTC thermistors can have a large
change of resistance for a unit change in temperature. They aren't linear,
but there are formulas for computing resistance vs temp. PTC thermistors
have a much smaller change per unit change in temp., but are much more
linear. And, they are susceptible to self heating, which makes things
interesting. If I remember correctly, in my research something called an
RTD was supposed to be the king when it came to accuracy and repeatability.
As someone else has stated, the IC devices are supposed to be quite good,
but you have to interface with them.
Keysights nano volt / micro ohm meter takes

https://www.keysight.com/en/pd-1000001296%3Aepsg%3Apro-pn-34420A/micro-ohm-meter?cc=ZA&lc=eng

can do direct SPRT, RTD, Thermistor, and Thermocouple measurements

SPRT = Standard Platinum Resistance Thermometer

I don't claim to know much about this, but the uncertainty quoted for the
SPRT probes with that meter is 0.003 deg C.

When I Google SPRT probes, I see they are incredibly expensive - many
thousands of USD each.

I'm a bit puzzled there seem to be a number of 3-wire platinum resistance
thermometers. I can understand using 4-wires for a Kelvin connection, but
can't understand the use of 3-wires.

Of course, for environmental monitor in a lab, one is most unlikely to need
very high accuracy.

Dave
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Neil
2018-04-05 20:06:52 UTC
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I've used a few Pt100 RTDs over the years for gas flow temperature
control in the 200-300C range.  At 0.385 ohms per K, the challenge of
managing the noise is a lot tougher than a cheapo NTC thermistor, but I
needed repeatability and stability.
http://www.farnell.com/datasheets/2278399.pdf

I use a 4-wire Kelvin connection, with 1 mA current drive. £14 at 10-off

Neil
Post by Dr. David Kirkby
Keysights nano volt / micro ohm meter takes
https://www.keysight.com/en/pd-1000001296%3Aepsg%3Apro-pn-34420A/micro-ohm-meter?cc=ZA&lc=eng
can do direct SPRT, RTD, Thermistor, and Thermocouple measurements
SPRT = Standard Platinum Resistance Thermometer
I don't claim to know much about this, but the uncertainty quoted for the
SPRT probes with that meter is 0.003 deg C.
When I Google SPRT probes, I see they are incredibly expensive - many
thousands of USD each.
I'm a bit puzzled there seem to be a number of 3-wire platinum resistance
thermometers. I can understand using 4-wires for a Kelvin connection, but
can't understand the use of 3-wires.
Of course, for environmental monitor in a lab, one is most unlikely to need
very high accuracy.
Dave
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Bob kb8tq
2018-04-05 16:25:47 UTC
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Hi

Thermocouples are *really* low output voltage devices at “normal” temperatures.
That gets you in to fairly fancy measurement setups ( no “just strap it to an ADC
input” stuff). They also require cold junction compensation. They measure offset
temperature to that junction. You do a lot of work to get just one sensor to operate.
That can be done with various chips from various outfits. It’s still not cheap. The
net result is no more accurate or higher resolution than the other stuff we have
discussed. The place thermocouples come in are for very high or very low temperatures.
If you want to know if it’s really 2,876C, a thermistor is not what you want to use.

RTD’s are indeed very stable long term. Properly handled they can do a real good
job. They are basically a strain gauge. Mount them wrong (yes, I have a few hundred
examples of this) or treat them wrong ….. they don’t do quite as well. If you really
want to get accuracy out of them, you run a triple point cell to calibrate them before
use. Yes, this is getting off into temperature nuts territory.

Heading back a bit to the original question:

What *does* impact your frequency or time standard?

1) Temperature
2) Pressure
3) Humidity
4) Load
5) Supply voltage
6) Acceleration (gravity)
7) Stress / strain (bending)
8) Time ( aging )
9) EMI ( injection locking )

I suppose that list could go on for quite a ways. The obvious one on the list that
has not been addressed yet is voltage.

Bob
Post by John Green
Why has no one mentioned thermocouples?
I had some experience with thermistors a few years back designing thermal
attenuators and equalizers for CATV. NTC thermistors can have a large
change of resistance for a unit change in temperature. They aren't linear,
but there are formulas for computing resistance vs temp. PTC thermistors
have a much smaller change per unit change in temp., but are much more
linear. And, they are susceptible to self heating, which makes things
interesting. If I remember correctly, in my research something called an
RTD was supposed to be the king when it came to accuracy and repeatability.
As someone else has stated, the IC devices are supposed to be quite good,
but you have to interface with them.
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Dan Kemppainen
2018-04-05 17:42:12 UTC
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Hi John,

Thermocouples are very robust, and have a very wide operating range.

However they require extremely accurate voltage measurements to get to
sub degree temperature accuracy. On top of that they require a local
temperature sensor to measure the 'reference' temperature (Or an actual
Icepoint bath). Once you have those that a lookup table or up to 14th or
so order polynomials (Depending on thermocouple, and range) is required
to convert the 'reference temperature' and millivolt reading to the
temperature measured.

Look at some of the NIST copies of the ITS-90 thermocouple tables and
coefficients:
https://srdata.nist.gov/its90/download/download.html
https://srdata.nist.gov/its90/download/allcoeff.tab

Overall, a lot of things going on there with errors that all stack up
(including silly things like not enough range in floating point number
routines for polynomial calculations). Don't get me wrong, they make
great sensors. Probably not the right sensor for a small home brew board.

IC Temp sensors, thermistors, or RTD's may all be reasonable options
here. IC Temp sensors for simplicity, RTD's for accuracy, and
thermistors (except where they are in a loop, holding the temp constant)
for cost.


Dan
Message: 11
Date: Thu, 5 Apr 2018 09:44:48 -0500
Subject: Re: [time-nuts] Environmental sensor recommendations.
Content-Type: text/plain; charset="UTF-8"
Why has no one mentioned thermocouples?
I had some experience with thermistors a few years back designing thermal
attenuators and equalizers for CATV. NTC thermistors can have a large
change of resistance for a unit change in temperature. They aren't linear,
but there are formulas for computing resistance vs temp. PTC thermistors
have a much smaller change per unit change in temp., but are much more
linear. And, they are susceptible to self heating, which makes things
interesting. If I remember correctly, in my research something called an
RTD was supposed to be the king when it came to accuracy and repeatability.
As someone else has stated, the IC devices are supposed to be quite good,
but you have to interface with them.
_______________________________________________
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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
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