Maker Pro
Maker Pro

Temp sensor "accuracy"?

J

Jon Slaughter

Jan 1, 1970
0
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading 98C
then I know I'm off by -2.0C and I can just compensate for that in all my
calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon
 
J

Jon Slaughter

Jan 1, 1970
0
Jon Slaughter said:
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading
98C then I know I'm off by -2.0C and I can just compensate for that in all
my calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon

Also does anyone know of any similar devices with better accuracy?
 
B

Bruce Varley

Jan 1, 1970
0
Jon Slaughter said:
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading
98C then I know I'm off by -2.0C and I can just compensate for that in all
my calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon
The spec doesn't talk about drift, it's not an easy aspect to specify, are
you talking over an hour, or a week, or 20 years? And do you expect the
devices to remain stable despite possible adverse conditions such as
transient temperature extremes or vibration?

In my fairly extended experience with industrial temperature measurement, if
you want to really hold accuracy around the 1C mark, you have to *work* at
it, whatever the spec sheets say. That includes calibration back to primary
sources regularly. So think about how much accuracy you really need, it'll
cost you one way or the other.
 
J

John Larkin

Jan 1, 1970
0
Also does anyone know of any similar devices with better accuracy?

Thinfilm platinum RTDs are available as little ceramic slabs with
leads, or as surface-mount things that look like resistors. They are
very accurate, typically around 0.1C maybe. But you have to measure
their resistance accurately.

Somebody makes digital (spi serial readout) temp sensors that are 0.5
C accurate, I think. National? ADI? Can't remember.

John
 
V

Vladimir Vassilevsky

Jan 1, 1970
0
Yes, you can compensate the error to some extent. However it would be very
naive to expect the long term accuracy of better then 1C from a
semiconductor sensor.

Drift, self heating, aging.
Also does anyone know of any similar devices with better accuracy?

If better accuracy is required, you need RTD.

VLV
 
R

Robbo

Jan 1, 1970
0
Jon Slaughter said:
Also does anyone know of any similar devices with better accuracy?

0.5C accuracy - but it's 1-Wire, not I2C. Do you neet to measure 100C or was
that just a nominal value?

http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2815

"The DS18S20 Digital thermometer provides 9-bit centigrade temperature
measurements and has an alarm function with nonvolatile user-programmable
upper and lower trigger points. The DS18S20 communicates over a 1-Wire® bus
that by definition requires only one data line (and ground) for
communication with a central microprocessor. It has an operating temperature
range of -55°C to +125°C and is accurate to ±0.5°C over the range of -10°C
to +85°C. In addition, the DS18S20 can derive power directly from the data
line ("parasite power"), eliminating the need for an external power supply."
 
R

Robert Baer

Jan 1, 1970
0
Jon said:
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading 98C
then I know I'm off by -2.0C and I can just compensate for that in all my
calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon
First, assume that compensation is not linear.
Second, assume that no two sensors are alike.
You can depend on those assumptions until precise measurements WRT
them indicate otherwise.
"Resolution" in this case is like having 5 digit data from a 3 digit
(repeatable? non-repeatable?) source.
Here, with that kind of resolution, it is fair to assume "repeatable"
within the "resoultion".
BUT, if you need something like that, then you must verify that from
a reasonable standard.
 
W

whit3rd

Jan 1, 1970
0
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Accuracy and resolution (sometimes called precision) have the normal
meanings here. The 'accuracy' is limited by high-speed test and
calibration conditions at the factory, while the resolution is limited
by expected
non-temperature environmental factors (stress, age), and noise.
Does this mean I can compensate for the error in the accuracy by using a
more accurate device?

No, not really; you can CALIBRATE it using a more accurate device,
though. Your calibration will take place when the first weeks of
device
aging have completed, so might be more effective than the production-
line
calibration was.

In my experience, calibration (of resistors and capacitors) lapses
when
soldering the little components down. So, you don't want to buy the
gizmo with good calibration, so much as have a plan to calibrate it
after it completes its stressful insertion/soldering phases.

Most semiconductor thermal sensors use logarithmic I/V characteristics
of a planar junction, and have a good fit to the 'ideal' curve,
precision about
.001 degree K, with only a single (scale) factor to be trimmed to
complete the calibration.
 
M

Martin Griffith

Jan 1, 1970
0
Thinfilm platinum RTDs are available as little ceramic slabs with
leads, or as surface-mount things that look like resistors. They are
very accurate, typically around 0.1C maybe. But you have to measure
their resistance accurately.

Somebody makes digital (spi serial readout) temp sensors that are 0.5
C accurate, I think. National? ADI? Can't remember.

John

http://www.analog.com/en/prod/0,,760_792_ADT7302,00.html
http://www.analog.com/en/subCat/0,2879,760%5F792%5F0%5F%5F0%5F,00.html



martin
 
Thinfilm platinum RTDs are available as little ceramic slabs with
leads, or as surface-mount things that look like resistors. They are
very accurate, typically around 0.1C maybe. But you have to measure
their resistance accurately.

Somebody makes digital (spi serial readout) temp sensors that are 0.5
C accurate, I think. National? ADI? Can't remember.

"Interchangeable" thermistors from Yellow Springs Instruments,
Betatherm, Thermometrics and so forth are available with accuracies up
to +/-0.05C - though a thermistor that accurate is going to cost
around $50. Parts with an accuracy of +/-0.2C are widely available at
a tenth of that price or less.

Thermistors aren't all that easy to use. The calibration curve is non-
linear - see the Steenhart-Hart fitting function - and, because they
are negative temperature coefficient devices, their resistance goes
unstable if you dissipate too much heat in the part. In my experience
10 uW is okay, 100uW can be risky. But their resistance decreases by
roughl 4% for every degree Celcius that the junction gets warmer,
which makes them gratifyingly sensitive.

Platinum resistance sensors are an order of magnitude less sensitive,
and semi-conductor sensors are appreciably noisier.
 
J

Jan Panteltje

Jan 1, 1970
0
Also does anyone know of any similar devices with better accuracy?

Its not digital, but the LM135 Kelvin sensors can be adjusted to within +/- 1°C
with a simple trimpot.
Also converts easy to Celcius:
Celcius = 25 + 100 x (voltage - 2.982)
Range is -55°C to +150°C
Rest depends on your ADC resolution and math.
 
C

Chuck

Jan 1, 1970
0
Jon said:
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading 98C
then I know I'm off by -2.0C and I can just compensate for that in all my
calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon

One issue not addressed is understanding
what you mean by the temperature of
something. Common objects are likely to
reveal different temperatures on their
surfaces and their interiors depending
on location. With crude instruments this
often isn't noticed. But with fractional
degree resolution, it may.

Imagine using a micrometer (albeit a
large one) to measure the length of a
piece of firewood. You have abundant
accuracy and precision, but misleading
results unless you map the entire end
surfaces, a process that could even
change the measurements.

Chuck
 
H

Henry Kiefer

Jan 1, 1970
0
John said:
Thinfilm platinum RTDs are available as little ceramic slabs with
leads, or as surface-mount things that look like resistors. They are
very accurate, typically around 0.1C maybe. But you have to measure
their resistance accurately.

Somebody makes digital (spi serial readout) temp sensors that are 0.5
C accurate, I think. National? ADI? Can't remember.

John

Of good performance is the SMT160 from www.smartec.nl
Can be PWM or analog out.

- Henry
 

neon

Oct 21, 2006
1,325
Joined
Oct 21, 2006
Messages
1,325
acuracy of +/- 2% means just that at 98c it could be 98 to 102 no guaranty of anything in between. can you adjust to it sure but then again buy one that is accurate to +/- to whatever you require. to adjust it to read 100c you must have another instrument that you can trust to read 100 c acurately resolution means that you will be able to tell the temperature to withing .0625 C at 98 it can read 98.0625c to 102.0625c
 
R

RRogers

Jan 1, 1970
0
"Interchangeable" thermistors from Yellow Springs Instruments,
Betatherm, Thermometrics and so forth are available with accuracies up
to +/-0.05C - though a thermistor that accurate is going to cost
around $50. Parts with an accuracy of +/-0.2C are widely available at
a tenth of that price or less.

Thermistors aren't all that easy to use. The calibration curve is non-
linear - see the Steenhart-Hart fitting function - and, because they
are negative temperature coefficient devices, their resistance goes
unstable if you dissipate too much heat in the part. In my experience
10 uW is okay, 100uW can be risky. But their resistance decreases by
roughl 4% for every degree Celcius that the junction gets warmer,
which makes them gratifyingly sensitive.

Platinum resistance sensors are an order of magnitude less sensitive,
and semi-conductor sensors are appreciably noisier.

We are getting medical thermistors with a claimed accuracy of .05degC
for about $2.00 at a quantity of 4,000. Unfortunately I am on
vacation and can't supply the name of the manufacturer. The reason I
said medical is that we only require accuracy at 37degC; I don't have
the information for other temperatures.

Ray
 
D

DeadCat

Jan 1, 1970
0
Jon Slaughter said:
What do they mean by accuracy and resolution here?

http://focus.ti.com/lit/ds/symlink/tmp100.pdf

It says +-2.0C accuracy then a resolution of 0.0625C.

Does this mean I can compensate for the error in the accuracy by using a
more accurate device? So say I'm measuring a 100C source and its reading 98C
then I know I'm off by -2.0C and I can just compensate for that in all my
calculations? Is there any issues with this(like maybe drift)?

Thanks,
Jon

For the fairly non-demanding task of measuring air temperature in my garden,
I've used some of these:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2737/t/al

They have a quoted accuracy of ±0.5°C and a resolution of 0.0625C. The
accuracy figure presumably only applies at the point it leaves the factory.
I don't know whether the difference between the reported and actual
temperatures varies long-term, but the difference seems to be fairly
constant on a timescale of minutes, because the graphs of temperature in my
garden (see below) vary fairly smoothly, rather than jumping about in a
±0.5°C range.

http://www.sgurr.co.uk/nailsea
http://www.sgurr.co.uk/lundycam/gallery.html
 
P

Phil Hobbs

Jan 1, 1970
0
DeadCat said:
For the fairly non-demanding task of measuring air temperature in my garden,
I've used some of these:
http://www.maxim-ic.com/quick_view2.cfm/qv_pk/2737/t/al

They have a quoted accuracy of ±0.5°C and a resolution of 0.0625C. The
accuracy figure presumably only applies at the point it leaves the factory.
I don't know whether the difference between the reported and actual
temperatures varies long-term, but the difference seems to be fairly
constant on a timescale of minutes, because the graphs of temperature in my
garden (see below) vary fairly smoothly, rather than jumping about in a
±0.5°C range.

http://www.sgurr.co.uk/nailsea
http://www.sgurr.co.uk/lundycam/gallery.html

The main problems with IC temperature sensors are the low thermal
conductivity of the package and the high thermal conductivity of the
leads.

Plastic has a thermal conductivity in the ballpark of 0.1 W/m/K, whereas
copper's is 400 W/m/K, so it's really really hard not to wind up
measuring the temperature of the circuit board rather than the air
temperature, and even if you manage that somehow, their thermal time
constant is many seconds.

Yellow Springs International (YSI) sells glass bead thermistors with
time constants in the 100 ms range and interchangeability at the 0.2 K
level. Glass bead thermistors are very sensitive, and also very
stable--a few ppm per year for the best ones, as long as you don't
stress the package.

Cheers,

Phil Hobbs

Cheers,

Phil Hobbs
 
H

Henry Kiefer

Jan 1, 1970
0
Phil said:
The main problems with IC temperature sensors are the low thermal
conductivity of the package and the high thermal conductivity of the leads.

Plastic has a thermal conductivity in the ballpark of 0.1 W/m/K, whereas
copper's is 400 W/m/K, so it's really really hard not to wind up
measuring the temperature of the circuit board rather than the air
temperature, and even if you manage that somehow, their thermal time
constant is many seconds.

Yellow Springs International (YSI) sells glass bead thermistors with
time constants in the 100 ms range and interchangeability at the 0.2 K
level. Glass bead thermistors are very sensitive, and also very
stable--a few ppm per year for the best ones, as long as you don't
stress the package.

You can set the pcb to equilibrium temperature if the copper space is
big enough on the pins.

The main problem is mechanical stress in the semiconductor package.
Other problem is the testing time the manufacturer allows having
financial benefit.

You can expect 0,1K drift in a couple of years for semiconductor
temperature sensors.


- Henry
 
P

Phil Hobbs

Jan 1, 1970
0
Henry said:
You can set the pcb to equilibrium temperature if the copper space is
big enough on the pins.

What do you mean by 'equilibrium temperature'? It sounds as though
you're trying to make the problem worse rather than better. Bigger pads
just increase the thermal forcing from the board.

Cheers,

Phil Hobbs
 
H

Henry Kiefer

Jan 1, 1970
0
Phil said:
What do you mean by 'equilibrium temperature'? It sounds as though
you're trying to make the problem worse rather than better. Bigger pads
just increase the thermal forcing from the board.

I'm not sure what you mean. If you want to measure board temperature
then increase pad area, if you want to measure the air temp then
increase the sensor surface with a heatsink for example.

I think this is logical for an engineer.


- Henry
 
Top