What resistor do i use?

Hallo

I got a thermistor temperatur sensor, and I have measured resistance at different temperatures.

3c = 25000 ohm 25c = 10000 ohm 33c = 6100 ohm 70c = 2100 ohm 100c = 960 ohm

and then I used Thermistor Calculator to calculate

and used "The Elaborate Code (cleaned up a bit)" LINK

my question is, should I always use a 10k resistor, or how do I know what size I need?

/Bruno

my question is, should I always use a 10k resistor

Usually, yes. You are making a [u]Voltage Divider[/u]. At "nominal room temperature" (25 degrees C) two the resistors are equal so you get exactly half-voltage out of the voltage divider. That makes a very handy starting-point or calibration-point, etc.

A voltage divider is not linear so it's easiest to stick with a "standardized" value so you can use an existing algorithm (or chart) to find the temperature.

If you are making an oven or freezer or something where room temperature isn't "interesting" you might want to use a different resistor value, but you'd have to tweak the algorithm accordingly.

okay i need it to messuer at 50-100c.
what then?

A thermistor with pull up or pull down resistor has the most voltage variation on the tap if the resistor value is the same as the thermistor. The most A/D values per degree C. Outside that ideal thermistor/resistor "ballance", values per degree C are getting increasingly less. So it makes sense to use a resistor that is the same as the thermistor in the middle of the "temp of interrest". In your case about 2k. But.. A lower pull up will also increase self-heating. And you have to adjust the Steinhart-Hart maths line for the new resistor value. Maybe easier to use a DS18B20. Equal number of A/D values per degreeC over the whole temp range (-55 to +125). Leo..

Yes the DS18B20 is very nice, but i cant finde any with treads like This.

is there others like is, but with threads?

brunokc: Yes the DS18B20 is very nice, but i cant finde any with treads like This.

is there others like is, but with threads?

I don't know what you mean by "threads" - that word means either the fibers clothes are made from, or a series of posts in a forum.

My first intuition is to think you're talking about the wires or leads, but DS18B20 is easier to find with the wires on it than without, so that doesn't make sense (search ebay for DS18B20)

I think 8mm thread or 1/8 npt thread.

sorry, my English is not so good.

DrAzzy: I don't know what you mean by "threads" - that word means either the fibers clothes are made from, or a series of posts in a forum.

There's more meanings than that, I think the appropriate one brunokc is talking about is screw thread.

You can avoid self heating by only applying voltage to the thermistor when you take a reading (a few milliseconds).

A 10k pull up resistor should give you enough A/D values per degree C at ~85C (~3) to measure the cooling water temp of a car in < 0.5C steps.
Leo…