PID controller

1. Why does the output LED of the PID controller blink?

The blink is normal. That is how the PID works. If the controller decide 10% is needed to maintain 216 °F, it will be on 0.2 sec and off 1.8 sec continuously (0.2/(0.2+1.8)=10%).

2. How long does it take for a controller to auto-tune?

The auto tune will heat the system to the SV, shut heater off to let it drop to below SV (normally not to that low), then heat again. It will do this for three times. If the system takes long time to cool, the auto-tune will take a long time.
For relay, cycle time would be at least 20 sec. It will take longer to auto-tune.

3. Why I can not set the cycle time to 1 second for SSR output?

The controller does not allow it to be set to 1s unless it is on/off mode. The reason is that for country with 50 Hz power, setting to 1 sec does not allow the SSR to be fired reliably. For 50 Hz, there are 100 half cycles. Each takes 10 ms. When cycle rate is set to 1s, 1% is a 10 ms pulse. Since most SSR has a zero crossing detection, the SSR might not get triggered for a 1% output when the trigger is issued at the zero crossing voltage. For the 60 Hz, the situation is a little better. Based on these considerations, 2 sec is the best option for precision control.

4. How to calibrate a controller?

A simple calibration can be done with ice water mixture. To calibrate in ice water, you need >70% ice and <30% water, equilibrate in a thermos (or stirred beaker) for 10 minutes before reading. If the reading is not 0 °C or 32 °F, you can offset the controller with the Pb (SYL-2342, SYL-2352, SYL-4342, SYL-4352), PSb (SYL-2362, SYL-1512) or SC (WS and WSD series). Both ice and water should be from distilled or purified water. This calibration method is best for applications that are close to the water freezing point. If the sensor is used at very high temperature, it can be calibrated near the application temperature with a NIST traceable sensor. You can also calibrate the sensor in the boiling water. However, that is a little tricky to do. You can easily get 5 °F off. You need to find the boiling water temperature at your location first. If you search for the “Boiling Point of Water Calculator” you will find many of website that shows how. After calculation, you can use the method discussed in this link.

It is not recommended to calibrate the sensor in the air. Because the mass of the sensor, it might take a long time for the sensor to equilibrate with the air, especially when the temperature difference between the sensor and air is very small.

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