Author Topic: A few observations concerning temperature measurements - aspiration  (Read 2182 times)

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Offline Wtronics

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Hello again...

I few weeks ago I introduced myself to this group of weather people, and I mentioned my particular interest with weather temperature measurement.

I would like to share some of my observations and invite others to join and even to disagree if it seems appropriate. But at least we might share a few ideas and realities that I have learned in my study over the past 6 months.

My first observation relates to what I called "nano" climates" - or what I would describe as tiny, but measurable, changes in temperature in one place in a yard as an example or traveling as a biker mentioned in a very well written statement to me on this site.

The first lesson I learned related to nano climates is that the temperature changes we measure depends on the total mass of the "thermometer". So a small, well calibrated cooking thermometer, might see a changes in seconds where a large weather station with heavy radiation shielding for sunlight radiation might miss the nano burst completely.

So I have been experimenting with what is called "aspiration" in the weather world - an air flow fan blowing outside air over the temp sensor. This more directly measures real-time air temperature and not a closed station internal air temperature. I know aspiration is available commercially but I wanted to understand what it takes to make my own aspiration system and would it be possible to have a 24 hour a day system - how hard could it be? The answer is HARD - a lot more difficult than it sounds!

And does aspiration work? My answer to date is YES - most definitely - aspiration works!!!

I have a 1990s vintage Davis Vantage Pro in poor health and a new Vantage Vue which is my CWOP station. I have been able to repair my old Vantage Pro mostly by removing the "supercap" which was not shorted but leaking causing intermittent ISS transmissions from premature 3V battery discharge - especially in cold weather.

I recently installed a small (1 watt) 12VDC computer fan in the DAVIS Vantage Pro temp and humidity sensor area of the radiation shield. This required a small 12 VDC two conductor wire running to the station. The small fan helped but not enough. So last night I installed a 2.5 watt 12VDC fan and what a difference!!!

My Davis station now responds very quickly to temperature changes and does a much better job of catching highs and lows which can occur over a period of minutes as happened this morning with a low in my area.

One more point - each different type of temperature station will show a temp dependent on how fast the sensor changes temperature - obviously - but if the temp sensor is deep inside a almost airtight plastic case, or radiation shield structure, the sensor really gives the temp of the internal shield or case area - not outside air temp until the enclosure catches up with the outside air temp. That is why I (and many others) have become such an advocates of moving air through the enclosure. It also explains why placing 3 or 4 different types of thermometers in the same area will very likely give 3 or 4 different temperatures - it all depends on how long it takes to change each sensor and its case unless it happens to be a windy day. Sometimes my house looks like a weather equipment sales office.

As for the 2.5 watt fan...

To be wireless, the fan would need a 12 volt lead acid battery and solar charging unit of at least 3 watts and more likely 5 watts. I am trying to find a cheap solution to this issue but check out the 12V, 5 watt solar systems, including a regulator and you will see the problem even Davis faces with 24 hour aspiration. I am sure Davis has the smallest possible fan wattage to still give quick response times - they are very good at taking time to develop reliable equipment.

I have more discoveries, some of which many of you know already, but I thought I will submit this message to see what happens. I have a lot more information on a precision calibration thermometer I am working on and I will share if anyone is interested in my adventures with temperature sensors. All of my work so far has been with cabled systems because measurement resolution is in the 24 bit region and 24 bit wireless is to complex for me at this time. By the way, I just bought a high performance Honeywell humidity sensor (HIH series) - imagine how complex that will be to develop!

And my thanks for all of the very courteous and helpful messages sent to me on this project - you are all good people!!!

Steve


Offline DanS

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Re: A few observations concerning temperature measurements - aspiration
« Reply #1 on: December 25, 2011, 05:07:10 AM »
Hello again...

I few weeks ago I introduced myself to this group of weather people, and I mentioned my particular interest with weather temperature measurement.

I would like to share some of my observations and invite others to join and even to disagree if it seems appropriate. But at least we might share a few ideas and realities that I have learned in my study over the past 6 months.

My first observation relates to what I called "nano" climates" - or what I would describe as tiny, but measurable, changes in temperature in one place in a yard as an example or traveling as a biker mentioned in a very well written statement to me on this site.

The first lesson I learned related to nano climates is that the temperature changes we measure depends on the total mass of the "thermometer". So a small, well calibrated cooking thermometer, might see a changes in seconds where a large weather station with heavy radiation shielding for sunlight radiation might miss the nano burst completely.

So I have been experimenting with what is called "aspiration" in the weather world - an air flow fan blowing outside air over the temp sensor. This more directly measures real-time air temperature and not a closed station internal air temperature. I know aspiration is available commercially but I wanted to understand what it takes to make my own aspiration system and would it be possible to have a 24 hour a day system - how hard could it be? The answer is HARD - a lot more difficult than it sounds!

And does aspiration work? My answer to date is YES - most definitely - aspiration works!!!

I have a 1990s vintage Davis Vantage Pro in poor health and a new Vantage Vue which is my CWOP station. I have been able to repair my old Vantage Pro mostly by removing the "supercap" which was not shorted but leaking causing intermittent ISS transmissions from premature 3V battery discharge - especially in cold weather.

I recently installed a small (1 watt) 12VDC computer fan in the DAVIS Vantage Pro temp and humidity sensor area of the radiation shield. This required a small 12 VDC two conductor wire running to the station. The small fan helped but not enough. So last night I installed a 2.5 watt 12VDC fan and what a difference!!!

My Davis station now responds very quickly to temperature changes and does a much better job of catching highs and lows which can occur over a period of minutes as happened this morning with a low in my area.

One more point - each different type of temperature station will show a temp dependent on how fast the sensor changes temperature - obviously - but if the temp sensor is deep inside a almost airtight plastic case, or radiation shield structure, the sensor really gives the temp of the internal shield or case area - not outside air temp until the enclosure catches up with the outside air temp. That is why I (and many others) have become such an advocates of moving air through the enclosure. It also explains why placing 3 or 4 different types of thermometers in the same area will very likely give 3 or 4 different temperatures - it all depends on how long it takes to change each sensor and its case unless it happens to be a windy day. Sometimes my house looks like a weather equipment sales office.

As for the 2.5 watt fan...

To be wireless, the fan would need a 12 volt lead acid battery and solar charging unit of at least 3 watts and more likely 5 watts. I am trying to find a cheap solution to this issue but check out the 12V, 5 watt solar systems, including a regulator and you will see the problem even Davis faces with 24 hour aspiration. I am sure Davis has the smallest possible fan wattage to still give quick response times - they are very good at taking time to develop reliable equipment.

I have more discoveries, some of which many of you know already, but I thought I will submit this message to see what happens. I have a lot more information on a precision calibration thermometer I am working on and I will share if anyone is interested in my adventures with temperature sensors. All of my work so far has been with cabled systems because measurement resolution is in the 24 bit region and 24 bit wireless is to complex for me at this time. By the way, I just bought a high performance Honeywell humidity sensor (HIH series) - imagine how complex that will be to develop!

And my thanks for all of the very courteous and helpful messages sent to me on this project - you are all good people!!!

Steve



I believe with a weather station, the temperature probe/sensor sampling rate controlled by the system might be something to consider as well. Here is a thermistor that sees changes in seconds and my ws-2310 limits that speed although the thermistor could probably sample faster.

Happy holidays!

« Last Edit: December 25, 2011, 06:50:50 AM by DanS »

Offline WeatherHost

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Re: A few observations concerning temperature measurements - aspiration
« Reply #2 on: December 25, 2011, 09:37:16 AM »
Is there a way to turn the fan on/off?  Some kind of thermostat that will fit inside the housing?

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Offline Wtronics

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Re: A few observations concerning temperature measurements - aspiration
« Reply #3 on: December 25, 2011, 11:00:54 PM »
Hello to each of you who have read and/or replied...I wrote the original statement.

I would like to make one quick statement and then a few views on the replies...

First of all...what a great group of people to be in such a discussion with...and if I might make a simple compliment - very smart people.

Ok...

to "Old Tele man...you have an amazing understanding of the basic physics and mathematics of the problem - thank you!

If I might add to your statements - I am trying to say that each "thermometer" (temp sensor) must include its radiation shielding as a part of the sensor.  The shielding is critical because air temp is a shaded air temp and NOT a sun heated air temp. So the shielding becomes part of the temp measurement. BUT... a highly shielded temp sensor is really just reading the inside volume temperature of the shield cavity and is NOT reading the the true outside air temperature...real-time. So the radiation shielding slows the temp reading until the shield air temp catches up with the real air temp. This all causes a thermal delay or "thermal lag" as Old Tele man put it. For me this lag can be a significant problem. The lag, in my opinion could be as long as 30 minutes on a warm calm day.

But...the high for the day might only last for 10 minutes or less!!! So it is likely that high performance shields do an excellent job of giving real air temp but can be 30 minutes behind the real air temperature. Good, non-aspirated radiation shielding can even miss the high or low for the day if they happen quickly. The same is true for any critical temperature such as the freezing point, with which the vineyards in my area are so very concerned.

So how do we get a better real-time air temp reading? A simple fan placed in such a way that it draws in outside air and passes the outside air over the sensor. The real critical question "Old Tele man" asked is what air flow is needed? My answer to that, for me at this point in my knowledge of the problem, is the more airflow the better - without drawing in water vapor in a rain storm. But there is also the problem of battery power if the system is on batteries and a solar panel. With those limitations my answer, so far, is something between 1 watt and 2.5 watts of power to run the fan. The fan I am using presently is 2" x 2" (5cm x 5cm) @ 2.5 watts and is small enough to fit on its side, between radiation shield rings, near the temp sensor on a Davis Vantage Pro.. But I do not have information yet about heavy dew or rain and if it will be sucked in by the fan and onto the sensors. I will probably build a small baffle to protect the sensors. I guess if my temp and humidity go blank, I have to much fan power!

Actually any flow is better then none, it is a matter of reducing the temperature lag to a minimum. I would prefer to have a temp lag of less than 1 minute because I love to watch and understand "nano" climates, as I have come to call them. A few days ago, I saw a 4 degree F rise and fall, with no wind that I could feel or measure, all happening in less than 2 minutes. What was that?

As for "Nyquist", I am presently using an analog, cabled, temperature sensor measuring to 1/100th of a degree. The sensor and electronics are a project I  am working on. I am using two stages of precision instrumentation op amps, with extreme cable shielding. I can hold my hand 4 inches from the temp sensor and see the temp rise 5/100s and then fall again as I move my hand away. So for me, temp readings are real-time and cpu cycle times are not an issue yet...but you raise a very real potential problem.

For "WeatherHost" there would definately be a fan control switch and in a fancy system, the fan speed would change as the difference between outside air temp and sensor temp increased - to conserve battery power. If CPU fan speeds change in high-end PCs and laptops we can do it with weather stations.

For DanS - that trace is almost proof in itself that data is being missed!

Thank you for your comments!!!

Steve