Temp sensor "accuracy"?

What do they mean by accuracy and resolution here?

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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

Reply to
Jon Slaughter
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Also does anyone know of any similar devices with better accuracy?

Reply to
Jon Slaughter

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.

Reply to
Bruce Varley

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

Reply to
John Larkin

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.

If better accuracy is required, you need RTD.

VLV

Reply to
Vladimir Vassilevsky

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

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"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."

Reply to
Robbo

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.

Reply to
Robert Baer

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.

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 =2E001 degree K, with only a single (scale) factor to be trimmed to complete the calibration.

Reply to
whit3rd

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martin

Reply to
Martin Griffith

a
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all

"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.

-- Bill Sloman, Nijmegen

Reply to
bill.sloman

On a sunny day (Fri, 28 Dec 2007 04:56:10 GMT) it happened "Jon Slaughter" wrote in :

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.

Reply to
Jan Panteltje

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

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Reply to
Chuck

John Larkin schrieb:

Of good performance is the SMT160 from

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Can be PWM or analog out.

- Henry

--
www.ehydra.dyndns.info
Reply to
Henry Kiefer

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

Reply to
RRogers

98C

For the fairly non-demanding task of measuring air temperature in my garden, I've used some of these:

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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.

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Reply to
DeadCat

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

Reply to
Phil Hobbs

Phil Hobbs schrieb:

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

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www.ehydra.dyndns.info
Reply to
Henry Kiefer

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

Reply to
Phil Hobbs

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

--
www.ehydra.dyndns.info
Reply to
Henry Kiefer

There are two problems with this. First, it takes _forever_ for a heatsink to come to equilibrium. Choosing a silly mounting method, and then trying to fix it with a ball-peen hammer like that is _not_ good engineering. Especially not without doing some error estimate.

Second, it doesn't fix the temperature error problem anyway. The first thing in any measurement system design is to figure out what you want to measure, and then measure that thing. Not something similar, not something hopefully nearby that's easier, but _that_very_thing_.

If you want to measure air temperature, then either (a) measure the temperature of the air itself, e.g. an acoustic thermometer that directly measures the speed of sound in air--which is a function only of temperature and the mean molecular weight of the gas, which is known very accurately; or (b) if you can't do that, at least make sure the transducer is at the same temperature as the air.

We're talking about using a thermistor, so we're in case (b)--there's one strike against us already.

I don't have time to go through a numerical example this afternoon--it involves looking up about a dozen numbers in different places--but here's a caution from the horse's mouth: National Semi's Temperature Sensor Handbook

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bottom of page 10):

"If the air temperature is much higher or lower than the surface temperature, the temperature of the sensor die will be at an inter-mediate temperature between the surface temperature and the air temperature. A sensor in a plastic package (a TO-92 or SOT-23, for example) will indicate a temperature very close to that of its leads (which will be very close to the circuit board?s temperature), with air temperature having a less significant effect."

Some air temperature sensor. Oh, and the settling time for the response is _minutes_, as you can see from the graphs, even with a heat sink. Small glass bead thermistors on really long, skinny, low-conductivity wires (e.g 40-gauge constantan, or a fine-line flex circuit) are a _huge_ improvement over any IC sensor, if you care about accuracy and speed of response. Platinum RTDs on alumina substrates are pretty good too, as long as you solder them on. Either one can give you thermal time constants of 100-200 ms, a good 2 orders of magnitude faster than your average IC, not even counting the enormous accuracy improvement.

On page 803 of my temperature control chapter (draft second edition version),

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there's a plot of temperature sensor accuracy vs sensitivity, which is also illuminating.

Cheers,

Phil Hobbs

Reply to
Phil Hobbs

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