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Our First Look at Thermistor details.

Thermistor Resistance vs. Deg F.

Today we are going to take our first look at a real thermistor and start looking at the problems we will have trying to work with it. The thermistor I have chosen to work with is a 10KΩ at 25 deg C (room temperature) thermistor.  It is a reasonable priced one, but as  you will see in the future there are always hidden costs with low cost.
You should download the data sheet for this one from the Jameco website. and have the data sheet available as I go through the specifications.

Thermistor Resistance entered into a spread sheet.

First, 10K was chosen because as we have found out working with op-amps that is the best range of resistance to work with op-amps.  Also 10K draws very little current and that means little heat has to be dissipated.   The very first thing I did once I looked at the data sheet was look at table at the bottom of it displaying the  resistance at the various temperatures.   I entered this data into a spread sheet and converted the temperature in Deg C into Deg F because as I described in the previous post, I can “feel” Deg F because it is used day-to-day here in USA.  I then graphed the resistance vs temperature and created the plot shown in the first picture.   In my normal vernacular:  “It is pretty obvious that Y=mX+b ain’t gonna get it.” .  We do not have a linear (line) relationship in this situation.

Resistance vs. Temperature with Resistance plotted on a log scale.

As I look at the data in the on the data sheet and in the spread sheet I created, I very quickly notice the resistance seems to be increasing by powers of 10 as the temperature gets colder.  This kind of increase is called an exponential increase and has been popularized and called “the hockey stick” in popular language lately.  To test this conclusion, I did a second graph of the same data, but I used a logarithmic scale to display the resistance.   This time the resistance almost looks like a line on the chart.  (“Almost” is a key word here… that means I still don’t have it completely understood, but that will not matter anyhow.)

This is the point where it is necessary to “zoom out” and think about what we are trying to do with this thermistor.  First, lets talk about what is going on inside the thermistor.  This type of thermistor is called a NTC thermistor and NTC stands for Negative Temperature Coefficient.  Most materials increase in resistance as the temperature increases, but NTC thermistors do just the opposite.  This happens because the increase in temperature excites the materials used and those materials then hold less tightly to the electrons.  The flow of free electrons within a material when a voltage is placed across the material is what we call electrical current the ratio of V/I = Resistance.   An approximation or  “rule of thumb” that applies to many chemical, biological, and electrical processes is that the process doubles for each 10 deg C increase in temperature.  One of the processes we are all familiar with is the amount of current our automobile battery is able to produce to start the car.  it always fails on the coldest, most miserable day of winter.  Any function that doubles for each increase to an input will be an exponential function.

Temperature Gauge on my old (and very dirty) Honda.

The first step we will have to do to try to force this very bent graph into a straight line is to ask ourselves how much of the range of temperatures do we really need. The second question to determine how close to absolute accuracy do we really need.  For example, on my old beater car I use to commute to work the gauge does not even display any numbers, but I know the place on the dial that is normal and if I see much variation I will become concerned.  I have no idea what that magic temperature number is.

The intention of my final use of the thermistors is take some measurements of some heat collecting solar energy devices.  The coldest outdoor temperature in the area where I live is right around 0 F and a few times I have seen negative numbers.  Initially, I set the “range of interest” down to -22 F (-30 C).  In the opposite direction I set the limit to just above the boiling point of water, because the boiling point will be a convenient calibration point.  This is a very wide range and will come back to visit me in a bad way very quickly.  The range is shown in yellow on the spread sheet.

I hope to use several of these devices to measure temperature of the solar panels (inlet and outlet) and the temperature of the storage unit and record these temperatures throughout the day using a computer.  Originally, I was going to use a low cost PLC to do this, but since that time an experimenters controller, the Arduino, has become popular.  This comes with 10 bit A/D convertors built in.  These will accept a 0 to 5 V input.  That will be where the output of this temperature instrument is sent and where scaling will need to be designed for.

So summary of our specifications for now.  We will measure between -30 deg C to 110 deg C .  We hope to have an accuracy of +/- 2 deg C after the A/D conversion.  We can accept a greater variation of accuracy of the analog output, but we must have a voltage resolution of at least 5/512 V (5 V divided by 9 bits resolution) for each 1/2 deg C.    These are goals and there is a lot of movement allowed in these specifications and further compromises may be necessary as we continue working with this.  In other words, we may find it is impossible to reach these goals cheaply and the expense is not worth the effort.

Now back to the data sheet.   We have more bad news.  The tolerance of the thermistor at 25 deg C is +/- 10% and the slope of the line drawn from 50 deg C to 25 deg C has a tolerance of +/- 2%.  What this means to us is we will have to calibrate each thermistor and the associated circuit individually.   We have our work cut out for us!

The thermal time constant (t.c.) specification of 20 seconds tells us how long it takes for the thermistor to reach a new temperature if it is taken from a place of one temperature and placed into a place of another temperature.   I will explain time constants at a later time but for now know that multiply the t.c. by 5 and the thermistor will accurately be at the new temperature.  20 X 5 = 100 seconds, so the thermistor will be at the new temperature in a little over 1 1/2 minutes and this will be fine for anything we will use.

The final number will will concern ourselves with is the thermal dissipation constant.  Because the thermistor is a resistance it will have a power loss across it.  This will cause the thermistor to produce heat.  That will do two things.  First it can cause us to have inaccurate readings because we are measuring the temperature due to self heating.  Second, if it is a very large amount it can burn out the thermistor.  We will use this and calculate it, but it will not be much of a problem for us because of the 10KΩ base value.

This post grew very long, and we have yet to start work at trying to use this device.  Remember, the devil is in the details.

Gary.

 

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4 comments to Our First Look at Thermistor details.

  • […] into something we can use. The plot show in the first picture is the one we created in “Our First Look at Thermistor details” and was created by simply plotting the resistance values in the data sheet after converting […]

  • Hey I was reading toguhrh your Arduino weather station project and I’m really interested. I have a TX21U-IT sensor and have bought several of the stations for friends/family as well. I don’t have an Arduino, but I do have a Xilinx CoolRunner II development board that I think could work just as well.I’m looking for the RF receiver. The sensor I have runs at 915 MHz instead of the 435 MHz yours runs on. It appears that the RF receiver you’re using is 435 MHz amplitude-shift keying (ASK). Do you know if all La Crosse weather station transmitters are ASK or could they be FSK?If I get the thing running (or not) I’ll let you know. Thanks, and congrats on a great blog with some awesome projects!Lance

    • Gary

      I think you have me mixed up with another blog. One of the reasons I am taking a week off was to think about where I am heading this blog. I doubt if I go very far into the Arduino area. There are lots of blogs and articles about that and most of those guys have a greater passion for it than I do. I guess my “turn-on” is more basic science, understanding things, and construction. I probably will talk some about general programming, but only where it relates to those subjects. Maybe a “rant” I need to have in the future about technology… do we serve it or does it serve us?
      If you find some good blogs please e-mail the links to me and I may put them in Resources or start a blog roll.

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