Types of temperature probes for thermostat and solar thermal equipment

PTC or NTC type temperature probes

They are temperature-sensitive resistors.
Their functionality is based on the change in internal resistance as the outside temperature rises or falls, as does the voltage internally.
Obviously this is completely analog technology. 

This is the type of sensor or thermal probe used in current consumer electronics.
It is easy to find in the electronic components market.

Advantages of analogue NTC or PTC probes

• They are easy to find in electronics stores.
• They do not require a special thermostat and can be tested with an electronic multimeter.

Disadvantages of analogue NTC or PTC probes

• The reading is not very precise.
• The quality and length of the cable can vary the sensor reading.
• The passage of natural time can vary the accuracy of the sensor.
• The thermostats that use these probes can be configured in order to fine-tune their accuracy, but it will not be easy if the probe is already installed in a difficult-to-access location.
• If several comparative probes are used, as is the case with a solar heater, the difference in precision can make our lives relatively complicated.

Digital temperature probes

They are not resistors like the previous ones.
It has a very similar appearance to a transistor.
They are pure digital devices as if they were micro computers.
The device that manages it literally asks what its temperature is, and this digital probe answers with the exact data.

They are widely used in recreation and learning about domestic electronic uses and home automation applications.
The most popular among digital probes is the DALLAS B18B20.

It is very popular in the world of Arduino.

The Dallas Ds18b20 digital probe looks like this:

These little devices are made up of 3 legs. Two of them are for supplying voltage (5v).
The 3rd leg is the one used for digital communication with your thermostat or any similar device that manages it.

Here is the identification of the legs of the ds18b20 sensor:

Advantages of Dallas 18B20 digital probes

• Nowadays they are easy to find in the digital market, usually in stores such as Aliexpress, Amazon, Ebay, etc.
• Poor quality, aging or excessive length of its wiring will not affect its reading accuracy at all.
• Thermosters do not need to calibrate or fine-tune the readings of these probes, because they are real and really accurate data.
• Generally, their minimum and maximum temperature range is wider than analog probes.
• Many probes can be placed in parallel on the same cable, since internally they function as a telecommunications network. (you could even put several probes in a hard-to-reach place, so you can alternately select another one in case of a fault)
• The device that manages the probes can configure their order without performing physical operations, since each probe has an internal digital address.

Disadvantages of Dallas 18B20 digital probes

• They need special thermostats for the use of these digital probes.
• They cannot be tested with an electronic multimeter, a compatible device is needed, or an arduino or raspberry with the code loaded for this use.
• The reading is much slower than an analog probe. If you use many probes and set them to high precision, the Latency time may be a bit high.