Help with selecting the proper thermistor for temperature gauge

[tkc100]

I have a simple digital temperature gauge with a single thermistor.
I would like to mount a selector switch and several additional thermistors to monitor the temperatures in different places.
Is there anyway to determine the type of thermistor I have so that I could match it?
If that is unlikely, difficult or impossible, does anyone know of a simple digital temperature gauge (assembled or kit) with a know value for the thermistor?
I am monitoring the internal temperature of a refrigerated compartment and it would also be beneficial to have an adjustable preset warning.
Any help or guidance you can give me in this regard will be greatly appreciated.
Thanks!

 

[MrDEB]

here is an artical where numerous temp sensors are connected on one wire (each sensor has a 64 bit code)
Swordfish Tutorial - DS18B20 Temperature Sensor
I myself am building a temp sensor for 4 separate re-fridge units.
the LCD display shows temp and plan on implementing LEDs (4 green, 4 red) that flash if tooo cold(green) or too warm (red)
looks pretty easy to build, very few parts and the sensors are accurate to 1/2C or better.

 

[tkc100]

The temperature sensor you posted a link to is way more than I need.
The one you are planning to build is more like it.
Thanks!

 

[MrDEB]

its the same one. just adding 3 additional sensors, the 8 leds and wala! hopefully it works as planned.

 

[Reloadron]

Thermistors come in a variety of flavors but a simple test would be to measure the resistance of what you have at a temperature of about 25 C (77 F) and note the resistance. That is the base point. You should see a resistance like 500Ω, 1K, 5K, 10K, or something. That would be what you want. That is a quick and simple way to see what you have and what you need.

Ron

 

[crutschow]

Are you sure it's a thermistor and not some other type of temperature sensor?

 

[Mike odom]

Thermistors come in a variety of flavors but a simple test would be to measure the resistance of what you have at a temperature of about 25 C (77 F) and note the resistance. That is the base point. You should see a resistance like 500Ω, 1K, 5K, 10K, or something. That would be what you want. That is a quick and simple way to see what you have and what you need.

Ron

that only tells half (or less) of the picture. It gives you the 25C reading. For instance, there are over 358 thermsisitors at Digikey with a base (25C)reading of 10K. Then you look at the 85C temp (or some other). This gives you the exact thermsistor you are using. This is how they are rated, 85C/25C. For instance, I used the PT103J2 from U.S. Sensor to build a temperature display for Budweiser. It has a 25C resistance of 10K and is made of "Curve J" material. (Beta 0-50 is 3890).

Since thermistors are very non-linear, I added a 47K resistor in parallel to somewhat linearize it. Then I did a spreadsheet to assign temperature values to ADC readings. I also used an AVR which gives me 10 bits. In the circuit, the top line goes to the Aref and one ADC input, the other two are two sensors connected to ADC inputs.

In the photo, I am using it to check ambient temp for testing another board.

 

[Reloadron]

Point well taken and thanks. My bad in not suggesting taking a few other readings including at 85C to better get the true curve. I never gave it enough thought.

Slick setup you have there too, I like that.

Budweiser? I liked that part but prefer Stella Artois brewed by the new owners of Anhauser Bush.

Ron

 

[MrDEB]

NEW OWNERS of Budweiser --bah humbug. The beer is not that great (Budweiser) but the advertising department made a BIG boo boo by droping the sponsorship of Kenny Bernestein

 

[Mike odom]

Point well taken and thanks. My bad in not suggesting taking a few other readings including at 85C to better get the true curve. I never gave it enough thought.

well, a few points won't get you the curve... it's not as simple as 85 ohm reading/25c ohm reading, there's a whole tangled formula the use to figure it out. Taking a few readings on the curve can help you pull out which one it is by comparing it on a chart of thermistor curves.

Slick setup you have there too, I like that.

Thanks, the sad part is, the board I'm testing in the photo was sent back off a railroad bridge in Detroit. They said "every time a train approaches the bridge it starts showing errors". I hooked it up and put it outside to test it as I wanted it in ambient environment in case temperature had something to do to cause the error. Besides the output transformer they broke sending it back, it worked 100%.... two days later they called and said nevermind, they found a wiring error. This was on a bridge that was in use for over 3 weeks!!! I'll never ride the Amtrak again!!! I put a lot of effort into self testing and detecting errors, but I didn't know I could detect system errors as well...

Budweiser? I liked that part but prefer Stella Artois brewed by the new owners of Anhauser Bush.

Ron

these were used in HOOTERS to display your frothy cold brew temperature (apparently, Budweiser tastes like piss when warm)...

 

[Reloadron]

This is funny:

these were used in HOOTERS to display your frothy cold brew temperature (apparently, Budweiser tastes like piss when warm)...

Now totally off topic but when cold Budweiser also taste like piss (not that I ever sampled piss). I spent considerable time in Europe during my younger travels and most Europeans who had tried Bud as well as other American brews told me they taste like warm puppy pee, so go figure.

That sucks about that nice little unit. Trains? Even going about troubleshooting something like that is a challenge.

I figure if the OP can get a few temps maybe with luck the thermistor can be matched up. Maybe?

Ron

 

[tkc100]

You guys have way more expertise and knowledge than I do.
Sounds like it would be difficult to determine the exact thermistor used in the temperature gauge I have.
Has anyone seen a simple circuit diagram for a digital temperature gauge.
As I said before it would really be nice if it had an adjustable warning (visual or audible) should the temperature exceeds a given point.
I'm pretty handy a building things and understand most of the basics but some of what you all take for granted I have to learn.
Thanks for all the input.

 

[Mike odom]

This is funny:



Now totally off topic but when cold Budweiser also taste like piss (not that I ever sampled piss). I spent considerable time in Europe during my younger travels and most Europeans who had tried Bud as well as other American brews told me they taste like warm puppy pee, so go figure.

That sucks about that nice little unit. Trains? Even going about troubleshooting something like that is a challenge.

I figure if the OP can get a few temps maybe with luck the thermistor can be matched up. Maybe?

Ron

certainly.. that's how I bought thermistors for it two years later... data sheet? data sheet? which one did I use???... so I looked up my ADC value chart to see what the 100F value was and the looked at a few curves until I found it...

or get enough points... the problem is, that's hard to do, w/o an environmental chamber... and even with... we built the sensors that go into the vertical launch missile tubes on submarines. To calibrate the temp sensor (it gives tube pressure, tube temperature, and presence or lack of seawater and when they fire the missile they throw it away and install a new one) we put our sensor in the center of a block of aluminum that had thermistors around it in a hex pattern. I think the block held 4 or 8 sensors in it at a time. Then we put it in the chamber and ran it down to -30C and looked for the 'hump' in the cycle (temp falling, holds steady, rises, holds steady, falls) as it maintained temp and input all thermistor values into a computer program and it gave us the average block temp.

As a hobbyist, you only have access to 2 really good thermal calibration points. Freezing and boiling points of water. Since I believe he said his intended use is for refridgerators, then the ice bath calibration is perfect as this is in the range he will be measuring. But he won't have two outside points for calibration, so as the temp veers from freezing, so will the accuracy. I suppose if you borrowed some liquid nitrogen from the physics lab?????? Anyway, I tested mine in an ice bath but boiling water was out of my intended range, but it tracks within two degrees of my metal oven thermometer I have it by. One time while testing I had it on the porch and the weather channel said Lenexa temperature was 7F and I looked at the display and it was at 07... right on right on!

 

[Mike odom]

You guys have way more expertise and knowledge than I do.
Sounds like it would be difficult to determine the exact thermistor used in the temperature gauge I have.
Has anyone seen a simple circuit diagram for a digital temperature gauge.
As I said before it would really be nice if it had an adjustable warning (visual or audible) should the temperature exceeds a given point.
I'm pretty handy a building things and understand most of the basics but some of what you all take for granted I have to learn.
Thanks for all the input.

well, let's see, if you want simple I have an opamp design that puts out a voltage proportional to temp using the PN junction of a transistor as the sensor. This is very linear as semiconductor devices leak a current that is PTAT (proportional to absolute temperature). I scaled it to give a voltage of 1.00V = 100F. That way, to display the temperature you just hook it to a cheap voltmeter.

The problem is, National Semi conductor has the same thing in a chip. See the LM34. I've also used these puppies on at least 4 different boards. Hook one of these into an AD port and your micro knows the ambient temperature. Or you could hook one into a voltage comparator and use this as your alarm output. P.S. my temp display has this alarm feature. In the picture you can see buttons on the left. Set, up, and down. When the temp gets above the setpoint, the display flashes and it closes a relay.

 

[Mike odom]

Sounds like it would be difficult to determine the exact thermistor used in the temperature gauge I have.

not at all. You'll just have to do some homework. Here's another cal point I forgot to mention... your tongue. You can measure the 98F resistance by holding it in your mouth, under your arm, etc. The mouth works the best. Make sure it's clean, though. That and the ice bath will give you two resistance values vs temperatures. You could then look thru a few charts and find it.

 

[Mike odom]

Sounds like it would be difficult to determine the exact thermistor used in the temperature gauge I have.

sometimes it helps to go back and reread your first post.

You have a temp gauge...
so you can unhook the thermistor and install a pot, one end and wiper to replace it. adjust the pot until the temp reads 100. do it again for several other temps. This is how you can characterize your thermistor. If you can't find it, post the values and I'll take a look.