Making sense of temps - need a guide

Hi, I am building an oven using some reclaimed parts.
110vac, 400-Watt Electric heating element.
Glass encapsulated NTC thermistor

The inside aimed at temp was 400-F. Can someone better with numbers than me, that means pretty much everyone, tell me how to go about normalizing the NTC-glass output to get it to read 400 when the inside air is 400 etc.

I tried a simple delta-offset, but that did not work too well, so I am thinking there must be a percentage thing that will work, but well beyond my skills.

Using this code I found online for the NTC. It’s German, I think but works as is.

// Pin that the thermistor is connected to
#define PINOTERMISTOR A0
// Nominal temperature value for the thermistor
#define TERMISTORNOMINAL 100000
// Nominl temperature depicted on the datasheet
#define TEMPERATURENOMINAL 25
// Number of samples
#define NUMAMOSTRAS 10
// Beta value for our thermistor
//#define BCOEFFICIENT 4390
#define BCOEFFICIENT 3950
// Value of the series resistor
#define SERIESRESISTOR 100000

int amostra[NUMAMOSTRAS];
int i;
void setup(void) {
Serial.begin(9600);
analogReference(EXTERNAL);
}

void loop(void) {
float media;

for (i=0; i< NUMAMOSTRAS; i++) {
amostra[i] = analogRead(PINOTERMISTOR);
delay(10);
}

media = 0;
for (i=0; i< NUMAMOSTRAS; i++) {
media += amostra[i];
}
media /= NUMAMOSTRAS;
// Convert the thermal stress value to resistance
media = 1023 / media - 1;
media = SERIESRESISTOR / media;

//Calculate temperature using the Beta Factor equation
float temperatura;
temperatura = media / TERMISTORNOMINAL;     // (R/Ro)
temperatura = log(temperatura); // ln(R/Ro)
temperatura /= BCOEFFICIENT;                   // 1/B * ln(R/Ro)
temperatura += 1.0 / (TEMPERATURENOMINAL + 273.15); // + (1/To)
temperatura = 1.0 / temperatura;                 // Invert the value
temperatura -= 273.15;                         // Convert it to Celsius

Serial.print("The sensor temperature is: ");
Serial.print((temperatura*1.8)+32);
Serial.println(" F");

delay(1000);
}

I get these numbers.
NTC = NTC glass-encapsulated thermistor
Inside = air-temp inside using a K-type thermocouple
K-type = a second K-type thermocouple clamped at the same spot as the NTC-glass.
using a dual read-out meter for…

NTC      inside         K-type
67		67		68
72		67		77
113		72		128
123		92		141
161		140		178
170		188		188
189		246		211
200		275		226
213		300		237
230		343		253
244		370		265
250		379		272
262		400		283
268		410		288
275		420		295
286		426		299

Try a series resistor close to the thermistor resistance at 400F, measure the NTC resistance with a DMM when the oven is at 400F.
Where does the inside temp come from? Where is that sensor located in relation to the NTC?
Where is the NTC located? It seems to read pretty close to the K TC?

756E6C: Try a series resistor close to the thermistor resistance at 400F, measure the NTC resistance with a DMM when the oven is at 400F. Where does the inside temp come from? Where is that sensor located in relation to the NTC? Where is the NTC located? It seems to read pretty close to the K TC?

Thank for the reply. The NTC measures 112K at room temp, around 70-F. I'll check the resistance at 400-F and get back to you.

I have one of these. https://www.amazon.com/Thermocouple-Thermometer-Dual-channel-Backlight-Temperature/dp/B018QHQSB8/ref=sr_1_4?ie=UTF8&qid=1487888413&sr=8-4&keywords=k-type+thermocouple+meter

1 K-Type to the meter is clamped alongside the NTC so it is seeing the same heat as NTC. They are both mounted on the original reclaimed bracket which is a 16-G strip of steel attached to one wall of the oven. So both the NTC and #1 see the same heat. I was thinking of making the bracket out of aluminum, but close tracking of temp as it is rising, is not important as long as they can agree on the oven set-temp when reached.

2 K-Type to the meter, inside temp, is dangling inside the oven and the K-type junction is at about mid-point from all sides and reading the air temp. It seems quite consistent no matter where I have it as long as it is not too physically close to the actual element.

756E6C: Try a series resistor close to the thermistor resistance at 400F

Thanks, There is already a 100K (SERIESRESISTOR) series resistor the German software above, showed.

At room temp the NTC is 112K, (I changed TERMISTORNOMINAL to that)

At 400-F the NTC is 3.3K. So what needs changing or adding to?

I appreciate your help.

Well, usually you get the best accuracy when the series resistor is the same resistance as the NTC [u]at the temperature you are interested in[/u], you would have to adjust your math formula to suit. The 120 degree difference between the oven wall and interior don't seem right though, are you losing THAT much heat through the wall? Have you tried swapping the 2 K TCs? How about insulation? First though, you may be able to get the NTC to track closer to the K type by playing with the BCOEFFICIENT value, also, what do you get if you dangle the NTC close to the inside K type?

Thanks for hanging in here. I am going to take the brute-force approach and map the temp to the indicated. The temps are not that critical, but 286-F for 426-F actual is way off.

I had tried messing with the resistance figures and Beta in the Sketch, but it didn't make a great deal of difference in linearity.

The oven has good insulation so turning off the power to the element, the "inside" would only drop 5-F over ten minutes while the other two sensors caught up. If I let the temp "soak" for five minutes they closed the gap as the steel bracket evened out. I will make an aluminum bracket to replace the steel and also throw out the NTC and buy a K-type+MAX6675 amplifier to use as it has much faster response as well.

I am guessing the NTC was originally used as it is a lot cheaper to buy and interface in a commercial product.

Thanks for your help.

The sensor will work fine if you take a few minutes to calibrate it properly.

Here is a website that will help you do that: http://www.thinksrs.com/downloads/programs/Therm%20Calc/NTCCalibrator/NTCcalculator.htm

Instructions here