Type of resistor

I am reverse engineering an old board to make a new one using an Atmega chip, and I'm uncertain about this particular resistor. See photo.
I desoldered it from the board, and found a number 2211, which coincides with the ohm reading I get from it of 2.2K. But is it a special type of resistor that is important for me to use? The other dozen or so resistors on the board are your typical 4 band type. Also, I'm curious as to what the 0521J and 0524J mean. I thought at first that was the value of it, until I found the 2211 on the under side.

Without seeing the actual cct., guess these are precision resistors, maybe used in a linear amplifier application.

RN55 are 1% metal film resistors 1/8 Watt. See https://www.vishay.com/docs/31027/cmfmil.pdf

0521 and 0524 are probably week-of-manufacture. 2005 week 21 & 24 respectively.

The tolerance I'm not sure of from that datasheet - perhaps there's some more digits/letters
on the resistor, you say it says RN55D2211.....0521J - there should be a letter after the 2211
that's the tolerance code.

BTW 2211 means 2.21kohms.

These are likely better than 1% as most cheap metal film resistors are already 1% tolerance,
so I'd suspect they are more likely to be 0.1% or so.

RN55 are available in tolerances 0.1%, 0.25%, 0.5%, 1%, and tempco's 25, 50 and 100ppm,
these are 100ppm/C due to the "D" in the part number if you look up the table in that

Thanks! Precision resistors -- likely, so I'll just get good precision resistors to use for them. They are used in conjunction with a temp sensor. Which brings me to my next much worse problem: I'm not sure what this sensor is.

What I'm reverse engineering is heater controller for a military heater. The board is quite simple, nothing but LEDS, Resistors, Caps, a couple transistors, and an LM2901N IC. But there's this temp probe. (See photos) After some hours of googling, I'm thinking it's an NTC Thermistor. But I can't find anything that looks like it on the web. If anyone has any idea what it is, I'm kinda at a loss. I'm hoping I can perhaps replace this whole board with an Arduino chip when I get it all figured out.
Using my meter, somewhere around room temperature (70 degrees F), it it's resistance is about 3K. I put it in ice water, it was about 8.8K, and in boiling water from the microwave, about 200 ohms.

What I'm seeing as a standard NTC Thermistor is like 10K at room temp (25C)

So maybe it's not an NTC Thermistor?

Any help will be greatly appreciated. These controllers cost $900, and I'm needing several. There's about $3.00 worth of parts on the PCB, and then this temp sensor. So I'm hoping to be able to make them.
ps. there's a few numbers stamped on it 615 & 520 but I couldn't make heads or tails out of them on google.

Temp Sensor 1.png

Temp Sensor 1.png

My best guess the sensor is a Platinum RTD (Resistor, Temperature, Dependent).

Best test is measure with a good ohmmeter. At 0°C it should be either 100 ohms or 500 ohms.

Heating it will increase the resistance.

See this RTD Chart

If you have a 500 ohm sensor just multiply the chart by 5.

If the resistor is in the 5k to 10k area and the resistor decreases with increase in temperature then you have an NTC resistor (NTC = negative temper coefficient resistor).

If the resistor is in the 5k to 10k area and the resistor decreases with increase in temperature then you have an NTC resistor (NTC = negative temper coefficient resistor).

Im thinking it must be an NTC thermistor. Here are some test samples taken with my Fluke meter.
8.8K @ 0° C
2.5K @ 25° C
200 ohms @ 100° C

But I’m not finding much at DigiKey or other major suppliers that match this. Most anything that is a panel mount probe like this is 10K @ 25° C. I’m just about finished reverse engineering the circuit. If I can’t find an exact match to this sensor, I’m hoping I can possibly adjust the resistors in the circuit to allow the use of a different NTC THERMISTOR.