This page will contain links to frequently asked questions and specific topics of information.
The short answer is yes, you can.
The longer answer is yes, but make sure you know the pros and cons. You can leave any probe submerged in water, in fact, the industrial versions are designed for exactly that. The lab versions are primarily designed for use and then storage, but can also be submerged.
For EC probes, there really isn't anything bad that can happen to them from continuous submersion. There is nothing in their design that will degrade. If you frequently submerge and then remove them, you'll need to ensure they are cleaned and they don't develop calcium/hard water deposits as the water dries. These deposits may not dissolve very easily when the probe goes back in the water and will alter the readings.
For pH and ORP probes, it is best to think of them as disposable. They are inherently designed to slowly degrade. In these types of probes, there is a junction where a solution on the inside of the probe physically touches the water on the outside. Due to this water/water junction, the probes will slowly degrade over time.
The longer answer: You probably do. Cases where you definately will need isolation include using another EC, pH or ORP probe and when other electrical elements are in the tank (pumps, heaters, contact mixers). You may not need it in a small beaker with nothing else in it.
Isolation through μFire devices also provides a stable power supply which is vital for accurate measurements, so even if you feel like isolation isn't needed, it may be a better choice to use it for the stability, and you also gain isolation for some unplanned for use-case that would require isolation.
There are a lot of factors involved in this one. It depends on how roughly the probes are being used. Are they subject to large temperature swings, harsh chemicals, fast flowing water, or drying cycles? All of those things will contribute to a faster recalibration schedule.
How accurate do you need your measurements to be? With light use, you might not see a pH measurement move more than a few tenths in several months. Is that within your error budget? Only you can say, given your specific application requirements and specific set of environmental factors. We can give general guidelines, but there is no simple, accurate answer.
TL;DR For EC, it is required, for pH or ORP less so
For EC measurements, each C degree of temperature change will also change your EC reading by around 2%. Compensation formulas have been developed that are a good approxiation.
For pH and ORP, each particular solution will respond to temperature differently, so creating a temperature compensation formula is more difficult. You essentially need to know what you are meausuring and the temperature response curve. Sometimes this is possible and other times, it isn't. A sort of universal temperature compensation can be applied, but this only compensates for the resistance in the probe, which will also changing the readings. This resistance compensation is very minor and can be ignored unless you need accuracy to within a hundreth place.
Temperature compensation is most commonly used with EC measurements. It uses a formula that calculates what an EC reading taken at a certain temperature would be if it were taken at another temperature, usually 25 C.
As an example, if you took an EC measurement of a 1.413 mS/cm2 solution at 25 C, it should measure 1.413 mS/cm2. In that same solution at 20 C, it will measure 1.2904 mS/cm2.
Isolation in this context refers to galvanic isolation, the complete separation of one part of a circuit from another, while still allowing signals to pass through. This is accomplished in a number of ways, which for the purpose of this discussion, not really important.
What is important to know is that probes in contact with water will become part of a circuit since water is generally conductive. The circuit may contain some unknown or unwanted elements that alter readings by means of a flowing current. The current might be introduced by a pump in the water, or by a nearby probe. Galvanically isolating each probe will help to reduce or eliminate this effect.