any way to calibrate digital thermometer?

This is a bad idea on many levels. Check the operating temperature range. it's unlikely that it included

100C. Even if it survives... Most electronic temperature measurement methods rely on some form of linearization. Calibrating at twice the intended operating temperature range is unlikely to improve the readings at normal temps.

Do the math on boiling temperature. You've just converted your inability to measure temperature into an inability to measure atmospheric pressure.

Well, there are bags that are proof to 100C, folks boil their lunch in them...

However the point was raised about the survivability of the unit to

100C. I was actually only thinking of exposing the sensor to the hot water, not the module. Again, I need to be clearer in my descriptions.

Sigh.

John :-#)#

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Boiling Water in a Plastic Bag

Boiling water in a paper cup

As long as the water keeps the plastic bag and paper cup temperature below the plastic melt or paper ignition points, there's no problem.

Quiz: What's the difference in water temperature between a slow boil and a fast rolling boil? Answer: None. Moral: It doesn't pay to bring the tea kettle to a violent rolling boil because the tea will not get any hotter than 100C (at STP).

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Boiling Points of Water at Various Elevations

Roughly a 1C drop in the boiling point for a 1000ft gain in altitude. No need to measure atmospheric pressure. You can read your altitude from a topo map, compensate for the boiling point, and still be quite accurate at calibrating the thermometer. The effects of atmospheric pressure are negligible:

Unless there's a hurricane coming, the typical changes in atmospheric pressure might change the boiling point over a +/-1C range.

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But it can (depending on the local barometric pressure) be enough lower to mess up an otherwise good calibration run ...

Isaac

The friendly folks at the local airport will do that for you, and put the results on the internet ...

Isaac

Having done the boiling water thermometer calibration exercise, I can assure you that there are a wide variety of other screwups available.

For example, the temperature of the steam above the boiling water is at a higher temperature than 100C. With a stick type thermometer, that puts the bimetallic spring out of the water and into the steam. The result is a botched calibration. However, if you're a typical freshman fizzix student, that error can be eliminated by totally immersing the stick thermometer, at the expense of the thermometer, which doesn't work very well full of water.

Another is slow versus fast boil. While there's no difference in water temperature between a slow and fast boil, the fast boil does contain a much larger number of water vapor (gas) bubbles than the slow boil. This water vapor is at a higher temperature than the surrounding liquid phase water. If the stick thermometer was sufficiently covered or enclosed by these bubbles, the temperature will read higher than 100C.

While atmospheric pressure does have an effect, pollutants in the water also have an effect. Want to raise the boiling point? Just add salt.

The type of container and method of heating also has a slight effect.

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Well, We don't know where the OP is. I assumed he was concerned about being off by 3F. You admit to 3.6F variance due to pressure. I suggest that it's definitely a factor in increasing the absolute accuracy of his temp gauge...even if he's in C territory. If I hadn't mentioned it, I don't think anybody would have considered that as an error term. And you still don't.

These type of things use a "precision" thermistor that is supposed to be good to a degree or so against an internal fixed reference resistor. There is no individual calibration. You could try to trim the reference resistor, but personally I'd f'dget about it. 3 degrees (F?) could be the difference between being in the sun and not, for example.

What do you expect for < $10?

Best regards, Spehro Pefhany

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I got two different devices with fairly constant 3 degree difference. My one with wind gauge, I don't believe. I think it reads half. I'm going to take it for a ride some day. One transmitter failed after a few years. Bought replacement. I have other cheap units go bad, but my current models are half decent.

Greg

gregz wrote in news: snipped-for-privacy@news.eternal-september.o rg:

My WalMart $10 Acurite wireless remote thermometer also seems to read about that high. it's on my covered,screened patio,out of direct sun,and sheltered.

FYI,real cal labs use a platinum platinum-rhodium resistance thermometer with a correction chart(from NIST) for really precise temp calibrations. We had a temp-controlled circulating mineral oil bath in which to put the DUT and reference thermometer. I used to do that in the USAF.(PMEL)

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Oh, I remember PMEL. Had one on NASA site. I used to make do with what I had, and accuracy necessity, in a research design, and repIr shop.

Greg

You bought it at Wal-Mart and you're surprised that it is not accurate?? Duhhhhhhhhh!

3 degrees is not bad for something that sells for less than $10.00 stupid!!!

I have an Acurite with outdoor sensor that I got as a gift. The indoor is r ight on, but the outdoor temp is 6-7 degrees LOW based on comparisons with multiple sources including another wired thermometer to the outside, also i n the shade. That one always reads exact to my various other sources. It's just annoying and realize I should just replace it but Ive been looking for a weather station that gives me wind speed and direction and something of higher quality but not over 300 to 400$.

make sure your sensors are exactly in the same place and shielded from the wind/sun etc when you compare. It's noon. Front sensor reads 34F Side sensor reads 42F Back sensor reads 38F all are in the shade. If I put them all in the same place at night, they read within a degree of each other.

I have an Acurite remote thermometer. The clock loses about half a minute per day. Looks like we agree they're crap.

Make darn sure you have a place to mount your weather station. I got one on a whim at a garage sale cuz it was free. Got it home and started looking where I'd mount it. Turns that there's enough shadow from trees along the back to block a significant portion of the rain. And all the swirling from the trees made the wind speed and direction just random numbers. Glad I didn't pay $400 for it.

I use the cheapo radio shack indoor/outdoor thermometers and they drift as well. When new, the outside probe and inside probe matched, but not anymore. I think something happened with the insulation of the wire or the sensor which is potted in a little probe.

over 300 to 400$.

Put something over the sensor, maybe it has some self heating and the wind chill, as little as it maybe is cooling it.

Jamie

Anything that claims to be amazing, magic, miracle, or accurate in the name, probably isn't. Since you didn't bother providing a model number, I'll assume you've already given up and are now shopping for a $300+ weather station. I think you'll find that a decent weather station (Peet Bros, Davis, etc) are far more expensive. $300 is about the starting price for a bottom of the line Davis.

There's nothing wrong with the thermometer or sensor. The problem is location, location, location, and ummm... location. I "manage" several weather stations and have great difficulties getting consistent temperature readings. There are standards for locating sensors which must be followed. I need to run, so here are some links. Bug me (or post to a weather newsgroup) if lost:

Data quality:

Citizens weather observer corp, which monitors temp sensor consistency.

Radiation shielding:

Siting:

gone...

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Blundering onward...

This one is quite good on siteing.

Basically, what you're missing with your unspecified model Acurite weather station is a radiation shield for the temperature sensor. What you're trying to do is measure the air temperature, not the temperature of the plastic box, the nearby walls, exhaust vents, parking lot heat island, reflections from low-e glass, foliage transpiration cooling, and a zillion other sources of error. Here's a fun example: That's the local radio station's weather sensor array on the left, sited over the HVAC system.

More on badly located weather stations. This site is well worth skimming: Even the official weather stations are often badly located:

This is one of my early attempts at a radiation shield (pagoda): It was ok, but I missed something obvious. There's a gap near the top of the wall where the cable goes through. Inside the building are several high power transmitters, which produce lots of hot air, that greatly affects the indicated temperature. Here's the CWOP report on temperature and barometric accuracy. The station was removed and is being moved to a better location sometime in the next month.

Enough for now. Your problem will be:

1. Do some reading on proper weather station siteing and Stevenson Screen radiation shield construction.
2. Find a better weather station if you want accuracy.
3. Register with the various amateur weather web sites (Weather Underground, CWOP, etc and see if your numbers track those of the stations around you.

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