Time Constants

When it comes to Energy Efficiency, temperature is an important property. A significant amount of energy is used to heat things up or cool things down. And on top of that, movement can be translated into heat through friction. Therefore being able to accurately determine the temperature is very important. Most people would know about accuracy, and it is a fairly easy concept to understand. But there is another important concept that can be just as important when it comes to measurement, and that is a Time Constant.

Imagine that you are driving in your car, driving at a constant speed. Ideally your speedo should be displaying a constant speed. Now, imagine that you were able to get into your car and drive, and the 0-100 time was zero. That is, you were suddenly going really fast. Then imagine that it took a few seconds for the speedo to catch up. This delay is caused by the Time Constant. In physical systems, there will be a delay if you put a thermometer into boiling water. And then there will be a delay in it cooling down when you remove it from the water. What you would find is that the thermometer gets to the final temperature much quicker in water than in air.

Whilst the thermometer is heating up or cooling down, it is actually not all that accurate. In order to get an accurate reading of a temperature, the temperature needs to have been stable for three to seven ‘Time Constants’. And the time constant depends on the thermal mass of the device, and the thermal conductivity of the ‘fluid’ surrounding the device. A small thermometer in water will have a faster time constant than a big thermometer in the air. According to one Blog, the time constant of the DS18B20 temperature sensor in Air is a bit over three minutes. This means that if the temperature changes, you need to wait about 10 minutes before you get a result within 1% of the difference between the old and new temperatures.