Controlling the temperature with high precision

Question

I need to control the temperature with high precision, and for this task would be good a temperature controller with pt-100 sensor, something like this one PID Temperature Controller ITC-100. As I see that, it can be very simple: there is an input (pt100 sensor), an output - to the heater element (my heating element needs 12V supply). However, I still have some things which are unclear for me:

1. Is it generally supposed that there is a power source for the heater element which I also put in the circuit or usually the temperature controller gives the voltage at the output ? Or is that voltages supposed for the external SSR, not my heater?
2. In many sources I have seen circuits with external SSR. However, such temperature controllers already have internal relay. Should it be possible without any problem just wire output to my heater (because in all schemes output goes to the external relays)?
3. Wires soldered to the pt sensor shouldn't influence much to the measurement, right?

Answer 1

3. Wires soldered to the PT sensor shouldn't influence much to the measurement, right?

These PT sensors are based on electrical resistance, so extending the leads will cause a slight change in the readings. You can minimise this with good connections and thick wire. (Thermocouples are different, you would need to use the right sort of wire for those.)

Answer 2

1.Is it generally supposed that there is a power source for the heater element which I also put in the circuit or usually the temperature controller gives the voltage at the output ? Or is that voltages supposed for the external SSR, not my heater?

2.In many sources I have seen circuits with external SSR. However, such temperature controllers already have internal relay. Should it be possible without any problem just wire output to my heater (because in all schemes output goes to the external relays)?

Typically, the decision to use the controller's internal power source, internal relay or an external relay would be based on the requirements for the heater. If switching the heater's load is within the capacity of the internal power or relay, it's simpler to just use that. (Remember, simpler is almost always better since there are fewer things to fail.) On the other hand, if you have a 480V heater, you're probably going to need an external relay, mechanical or SSR. In this case, the internal relay can handle 250V/3A or 120V/6A.

Wires soldered to the pt sensor shouldn't influence much to the measurement, right?

You can extend the leads on these sensors. There will be some effect on the measurement due to the added resistance of the extensions. How much this affects your measurement is going to depend on how much resistance you add with your extension wires. I would expect this to be minimal unless you're running a really long extension. It's likely that any such error can be compensated for when calibrating the system when it's setup in its final configuration.

There's a lot of information on these controllers here: http://ink-bird.com/asset/file/ITC-100_%20Manual_%20V1.0_6.30.pdf

If you look at Section 7 of the manual I linked above, it shows sample system diagrams of the various modes in which the controller can be set up. It also indicates which model (There are 3) of the controller they are using for that setup.

Answer 3

3. Wires soldered to the pt sensor shouldn't influence much to the measurement, right?

A "pt100" sensor has a resistance of about 100 ohms (hence its name!) and changes resistance by about 0.385 ohms per degree C. Your question doesn't say what you mean by "high precision" but those two numbers should give you an idea of what lead resistance is acceptable for your application.

If you want to use long leads and still get "high precision", it would be better to use a 3-wire RTD probe, as shown in the ITC-100 documentaton. That configuration uses a Wheatstone bridge type of circuit, and only the difference in resistance between the three leads contributes to the error. Three-wire probes can use leads up to about half a kilometer long.