Temperature is one of the important parameters to all living things on earth, especially for those non-constant animals, like fish and reef. Most aquarium enthusiasts have both heater and cooler installed on their tank; however, are you sure they work together and can keep your fish tank within a range of temperature as a constant temperature system?
Believe it or not , it’s impossible to have the same readings from different temperature sensors, even from the same device, the same tank, the same location. Why? It is because thermal conductivity of sensors cannot be the same, just like identical twins who may look almost alike, but there is still miniscule difference in height, weight or personalities.
The point I am trying to make is that when you have temperature sensor of heater and cooler from different devices, different readings might be a big problem for your tank. For example, most of reef keepers who expect to maintain a constant temperature of 25 °C, will setup 24°C as the starting point to initiate heater and 26°C to start the cooler.
Normally, the tolerance of sensor probe is at least 0.5°C in comparison with real temperature and if you include the cable into consideration, the total tolerance of sensor might be more than 1°C. With two sensors in action would mean you will have the exponential tolerance risk of 2°C in your tank, and with this setup , how can you be confident that your tank temperature can be precisely kept between 24°C ~ 26°C?
3 ways to eliminate the reading issue
Of course, there are ways to eliminate the reading issue. One is to calibrate both sensors. Two is to have one sensor for both heater and cooler. The third and best way is to calibrate a sensor and use it for both heater and cooler controllers which can give the most reliable reading to have the heater or cooler to be activated correctly at all times and solve the uncertainty once and for all. Unfortunately an extremely accurate sensor will be pricey and hard to get right off the shelf.
Therefore the alternative and economic solution would be to get a 2 stage temperature controller so your tank always be in a constant temperature setting.