Shop Products
Houzz Logo Print
dennis1983

Temperature measurements

dennis1983
9 years ago

Hi

I have seen some messages about temperatures here, some thinking how to get tempereature data. I have before wrote about how to get online data. If we think how to get your own temperature data, then we need some device for that. I have think it before and have read about this so i can tell something about this. It has been sometime when i looked this thing, so i don't remember all but i may look on internet some things so something might come back to mind. Because those are device they cost money, if you don't already have them.

Before we start let's think this little bit before going there. It might be possible to get your own temperature data. Do you want to know what your temperature is? Do you want to know your location is 1 celsius colder than nearby weather station in airport during day? Or 1 celsius colder in night time than nearby weather station in airport? If we use (daily maximum+daily minimum)/2 formula calculating growing degree days and if your daily high and low maximum are 1 celsius colder than nearby weather station in airport, you get 1 degree less per day in growing season compared to nearby weather station. If it happens all day during growing seasion. It might be that not all data you get is going to be good or make you happy. It might that you are warmer than nearby weather station in airport also. Question is are you ready for it? Does it make you feel bad if you are colder than nearby weather station in airport? You can measure other than outdoor temperature too, if you think about it. Also what would you do with your temperature data? Temperature data what would you got is just a number. Do you know what would you do with those temperature data, do you calculate growing season, want to see maximum temperature on some day or what? I think the temperature data should have some meaningful idea for you, some idea what you are going to use it for. Just looking numbers without doing any analysis on it might be fun first, but i think you need to do something with data to get some useful information from that data.

In U.S.A. weather station report 1 time in hour from airport, also some special reports if weather requires it for example thunderstorm. So i guess you want more oftren than 1 tempereature reading in 1 hour time. Another thing you might want to get is local temperature data. Now we can begin. There is 4 ways to get temperature, which i tell now : thermometer, weather station, temperature data logger and DAQ. Let's now look at those 4 using common sense.

Thermometer shows indoor and outdoor temperature. Usually 0,1 celsius resolution is showed in display. Thermometer might have maximum and minimum temperature recorded, but don't tell when maximum or minimum was reached. It has occured sometime after last reset button was pressed. If you want calculate average daily temperature using (daily high+daily low)/2 formula you need to reset it daily. This is relatively cheap, but you can't download data to computer and you need to write temperatures down yourself if you need them.

Weather station you probably know what this is. It is bigger in size than thermometer and have usually other sensors too than temperature sensor. You can have temperature data on computer. It cost more than thermometer. I think i don't write about this more here because i think people know this and can find information on internet about those.

Now is temperature data logger. I write here more about those because i have read about them and know something about them. Temperature data logger is small device which records temperature data. Temperature data logger have memory where they record their temperature data, they don't need computer to record data as they are stand alone devices. Most temperature data logger have battery, so they don't power line to work. So if you loss electricity you can get your temperature data. Temperature data logger are quite small in size usually, i have seen size roughly 4 cm each side size which is less than 2 inches for each side. So they are quite easilly removed to another location. Usually temperature data logger data is downloaded to computer using USB. Many temperature data logger allow data to exported to Excel for further analysis. There is 2 types temperature data logger how to download data to Computer one is USB memory type which you just put in USB port, another is digital camera type which you use USB cable for downloading data. There is 24 hours in a day, so if weather station gets 1 reading per hour, you might want more readings than that per day. I think it wouldn't be hard to get 1 000 readings or more temperature readings from temperature data logger per day. Let's look what to we would like to look at those devices.

What is size of memory? How many readings it can hold up before memory is full? I have seen some device can hold over 1 000 000 readings before memory is full, some hold some 32 000 readings or so. When memory is full you need to download the data to Computer. How fast can temperature readings taken? Many temperature data logger can record 1 temperature reading per 1 second. So you can have 1 000 temperature reading per day if you just take 1 reading per 1 minute, that is a lot more than weather station which got only hourly tempereature data. At that rate that 32 000 reading memory is not looking so big, that gets full in 1 month. But remember when you download the data, you can clear that memory or just let device over ride data. I think i should say something here, i think temperature data logger don't record maximum and minimum temperature separetely like weather stations do. So you need to have enough readings so that maximum temperature is recorded or about recorded. Think how much temperature change say in 1 minute usually? What is batter life? What is temperature resolution? That means if resolution is 0,1 celsius it shows temperature as for example 24,0 or 24,1 celsius. What is temperature accuracy? Is software provided free or does it cost extra? Does software have graphics of temperature, is statistics available? Does it calculate average, showing maximum temperature of desired time period? What is operating temperature range?

Does it have thermocouple, RTD or thermistor sensor? I think weather station has RTD usually, thermometer probably thermocouple. One important question is does it have only internal sensor, or does it have external sensor too? If it does how many? If external sensors can be installed, it greatly improves our temperature measurements possibility. If it is only internal sensor, we need to put temperature data logger outside to get temperature data. Cold weather may also drain battery more quickly than inside, also you need to get device back to download data. If it have external sensor, you can have temperature data logger inside and just put sensor out like in thermometer. You need to know what kind of sensors your temperature data logger uses. Let's take a look quick look fysics using common sense how those sensors work and what properties they have. This is general information for external sensor temperature data logger.

There is few words which we should perhaps look into what they mean. Accuracy means how accurate, how close true temperature we might get. It is not possible to get exactly right temperature but quite close to it depending conditions. Accuracy have many things to consider, stability means how well we got stable temperature when temperature is same, that means if temperature is same we get same temperature reading. Repeatability is conditions where sensor gives same temperature reading under repeated identical conditions. Response time is sensor ability to react in temperature change. Drift means how much sensor readings change from true temperature.

Thermocouples are temperature which read temperature using voltage. It has 2 metal pair, one is in temperature which is known and another is in what we measure temperature. When those 2 metal temperature starts to different each other their voltage increases. You get temperature when you know how much temperature change with some voltage change difference in those 2 metals. Thermocouples have high temperature range from about -200 celsius to 1 600 celsius, depending thermocouple type. Thermocouples might drift immediately from real temperature after they have taken to use. They are not as accurate as RTD and are cheaper than RTD. They have fast response time, which means they change temperature reading fast when temperature change. They may need cold junction compensation.

RTD is abbreviation and comes from words resistance temperature detectors. RTD's measures temperature by resistance. RTD needs current to measure temperature. Current cause self-heating RTD, which can lead to error to measuring temperature. RTD can be made from nickel, copper, tungsten and platinium. RTD can be used about -200 celsius to 650 celsius. RTD has slower response time than thermocouple, migh be many seconds. RTD is less Platinium has linear resistance and is good for measurement temperature. Resistance is measured in ohm. Weather station uses PT-100 sensors. PT is abbreviation and means platinium, 100 means it has 100 ohm resistance at 0 celsius. Linear means that platinium has linear temperature resistance relationship, so temperature change same amount when resistance change same amount. This linearity makes it easier to calculate temperature. RTD have accurate, stable, temperature readings, and platinium RTD can drift less than 0,05 celsius in 5 year time. IF using DIN standard platinium resistance change 0,00385 ohm for 1 celsius temperature change. If you use 100 PT sensor you need multiby it 100 so you get 0,00385*100=0,385. Because change is nearly linear 109,24 OHM would be 24 celsius. temperature. That is 9,24 OHM increase in from 0 celsius 100 OHM. When you divide 9,24 OHM By 0,385 9,24/0,385=24 celsius. So that way you get temperature.

There is 2, 3 and 4 wire configurations for RTD. 2 Wire configuration can cause serious lead wire resistance errors, if wire resistant is constant it can be offshet and not affect temperature measurement. Wire resistance vary with temperature, as conditions change, resistance change which introduce errors. If the wire is long errors might be sinificant. 3 Wire configurations we have 3 leads instead of 2. It reduce lead wire errors by using 3 lead wire, i have found worst case is resistance offset equal of 10% of single lead resistance. Optimum is use 4 wire configuration, it can be used if high accuracy is needed or measuring device is far from sensor. In 4 wire configurations inbalance is cancelled. I think that means self heating is removed so it don't affect temperature measurements. I got following message when i ask about this :

"[Engineering Support-Kevin] Errors caused by resistance imbalance between leads are cancelled out in a four-wire RTD circuit. Four-wire RTDs are used where superior accuracy is critical or if the sensor is installed far from the receiving device. In a four-wire RTD one pair of wires carries the current through the RTD the other pair senses the voltage across the RTD. 2- and three-wire RTDs require heavier lead wire because thicker wire, by creating less resistance to the measured signal, reduces measurement distortion. Therefore lighter gauge wire, less expensive, may be used in four-wire RTD applications."

Thermistor in other words thermally resistant. Thermistor measures temperature using resistance same way as RTD. Thermistor has narrowest temperature range about -100 celsius to 100 celsius. Usually thermistor resistance is specified 25 celsius. Decline in resistance might be many % per 1 celsius. So they might detect smaller change in temperature than RTD, but that might also make them less stable as smaller changes temperature are measured if compared to RTD. Those have higher resistance than RTD so they are more sensitive to temperature change than RTD. They are about as fast as thermocouple. In temperature measurement is NTC used which is abbreviation of negative temperature coefficient, which means resistand decline when temperature decline. Using common sense if you look at resistance at 25 celsius which might be 100 000 ohm and temperature range which it intended, calculating maximum temperature-minimum temperature and resistance divided by that number you might get some number which might show how sensitive it is temperature change. I think better would be to see if you have table showing resistance at some temperature, i have seen some of those. I have read resistance decline might be nonlinear but predictable. If i remember right those suffer if used outside of their intended temperature range.Those cost less than RTD.

So generally it looks like this way : RTD is most accurate, slower response time and little drift over time. Thermocouple are cheaper, has wide temperature range, are fast, but can drift when using them. Thermistor has narrow temperature range, are fast and has high senstivity.

Now back temperature data logger. Many temperature data logger have 0,1 celsius resolution. I have seen temperature data logger 0,001 celsius resolution. Some temperature data logger have display, so you can see measurements without computer. Some can be record temperature when set temperature is reached. You have software where you set how often you want temperature measurements. Some use just browser i think.

Now let's look at DAQ. DAQ is abbreviation and comes from data acquistion. There are multifunction DAQ and module DAQ. I don't know much much about those, but i have 2 words which can tell something : signal conditioning. DAQ is device which you can measure temperature and physical phenomenon. DAQ needs computer and can have much readings per seconds, say thousands. Idea is to get DAQ and then get sensor in it which then measure that sensor is intended to do. I have found one DAQ which also had data logger and uses USB memory and took power from electrical outlet. I don't know does those DAQ give measurements in techical numbers in temperature or celsius. I guess it could have celsius if someway choosen in software.

Now i have said something about general about temperature measurements. Those was general information, if you decide to get some device look those details before buying it so you know what you get. Those are not perhaps right in right all the way, but something information i have found and understand using common sense. I am not technical so ther might be mistakes. This is some idea about getting temperature data as someone has think it here before, hopefully it helps somebody in this thing. This got little bit long this writing, but i wrote this so some idea can be got from getting temperature data.

Comments (2)

  • ken_adrian Adrian MI cold Z5
    9 years ago
    last modified: 9 years ago

    boy i didnt read all that..

    but if every garden is its own micro climate.. then getting the info from ANYWHERE OTHER than your own garden..

    would seem.. just about useless ...

    eh??? [otherwise known as .. wouldnt you think???]

    ken

    ps: i am downhill from all of my surrounding neighbors.... i think the subdivision developer 40 years ago ..... pushed all the soil up to those houses to make walkout basements .. i usually have severe frost or freeze damage just 6 to 8 feet below .. those peeps;;... a mere 100 feet away dont.. because cold air move downhill and pools in my low garden ... and no gubment weather service will report said temp fluctuations .... so if i want pertinent info.. i have to generate it for myself ... just like.. frankly.. everyone else ... i wish you luck.. on relying on the gubment to provide for you .... so i think the OP is really onto something ....

  • dennis1983
    Original Author
    9 years ago
    last modified: 9 years ago

    Hi

    I forgot to say about RTD that another way to use longer lead wire is using PT-1000 so resistance is bigger, but i think better idea would be using 4 wire configuration. Temperature sensing part of sensor is the tip of sensor. Some sensors might be put around something for example teflon to protect it, that might not be as wide range tolerant of temperatures as sensor itself. In temperature data logger i forgot to say one thing to look is what temperature range it records? Some temperature data logger is wireless so temperature data can be downloded without getting it to computer, if you have device to it. I think it is not big deal to get temperature data logger inside to download the data. Sorry i made mistake above thermistor information, i need to correct it. Sorry for the inconvience, was not intend to give wrong information. i said resistance decline when temperature decline. i meant resistance decline when temperature rise.

    Ken_adrian hi

    It depends how useful temperature data from other place is for you. If you would know how much your temperature difference is from weather station in airport, you could then take it into calculations, but do you know it? It might be different in different times and might be affected by wind and other things if they are different than weather station in airport.

    Yes it is said cold air flows downwards. Temperature fluctuation, you mean how much temperature vary in daily? As i said there is hourly weather data, minimum and maximum temperature from weather station in airport in U.S.A. It is up to you what are you thinking is important temperature information for you.

    I think i write here little more about thermistor. Thermistor can reduce their resistance 3-6% for 1 celsius decline. Thermistor are quite sensitive to temperature change, i saw on internet they can detect temperature change of 0,02 celsius (If device allow to show that resolution i think). I think using common sense higher the chance of resistance is for let's say 1 celsius temperature change more easily it is detected, thermistor can respond temperature change quite fast. Because thermistor have high resistance, it allows to use longer lead wire without affecting much of temperature reading. Thermistors have tolerance 0,1 and 0,2 celsius in 0-70 celsius and 0-100 celsius. Thermistors drift when they are subjected high temperature above 100 celsius. If thermistors are used below their maximum specified temperature, drift is minimal. In thermistor you need to look specified minimum and maximum temperature, also look 25 celsius resistance. If temperature is between 25 celsius and 100 celsius drift can be 0,02-0,15 celsius after 12 month continous use between 25 celsius and 100 celsius. If thermistor are subjected closer to their maximum temperature they begin drift more. So they do drift more than RTD, because RTD don't have this temperature limit if you stay with in temperature measurement range in RTD. Thermistor might be good choice if your temperature range is not high and you stay in specified minimimum and maximum temperature. As thermistor resistance decline when temperature rise, think what temperature you use it for. In outside in garden in U.S.A about 40 celsius would be enough. Many thermistors have higher to about 100 celsius maximum rating. If we think 1 celsius increase in temperature reduce say 1% resistance, higher 25 celsius resistance is better because that 1 celsius change would be then bigger. Bigger change would mean in common sense more easily detected smaller change in temperature, but i guess it don't mean perhaps much in gardening if you get 0,01 celsius more of temperature. They are accurate, sensitive sensors and cost less than RTD.