Hydroponic dosers come in various forms from very simple basic dosers to really smart dosers that have other functionality.
All on-demand dosers have the job of adding fertilisers and/or pH correction fluid to the reservoir tank repeatedly until the set point(s) are reached. Most will have alarms so that a siren will sound if the EC or pH wander too far from the setpoint. They will also normally have a "shut-out" function so that if rediculous readings are obtained then dosing will be stopped and the alarm activated. Some of the more advanced dosers will also have a "maximum doses per hour" setting. If a pipe was to burst or a sensor stop working such that the controller was continually dosing it would soon exceed the "doses per hour" setting and the alarm would sound and dosing would be inhibited.
The other benefits of a smart doser is that they "understand" the interaction that takes place when fertiliser is added causing the pH to fall. This often leads to overdosing in simple controllers that do not take this into account. When starting with a fresh tank of water, a simple doser will repeatedly dose acid and fertiliser so that the readings head toward their respective setpoints. If the pH reaches its set point first, it will stop dosing but the further addition of fertiliser to correct the EC will have the side effect of lowering the pH below it's target value. Modern dosers will not do this and will hold off dosing excessive pH until the EC is close to target.
Other features that may be present in a doser are the ability to control the filling of a batch tank, the stirring and dosing of the tank and then the irrigation of multiple stations. This may be on a timed basis with different times for day and night or may use a solar integrator (radiation summer) for the day irrigations. The radiation summing method ensures that more water is applied on bright, hot days than in overcast weather. Batch dosers may also have a smart dose mode so that the dose applied is appropriately sized to suit the shortfall in nutrient concentration. For example, when starting with a fresh tank of water, the first dose will be very large so that it brings the tank up to, say, 80% of the final EC in one fell swoop. The next dose will be a lot smaller and may bring it to, say, 95% and so on. Thus, in just two or three doses a fresh tank of water can be fully dosed with fertilisers.
The more advanced dosers will also have some form of access from a PC. This may require proprietary software to be loaded onto the supervisory PC although the very latest systems will serve web pages from the controllers which can be accessed by any computer, tablet or smart phone providing it has a web browser. Using a web interface avoids problems with operating system incompatibilities and computer types and also allows easy remote access over the internet. Where this is done, strict password controls will be used to ensure unwelcome visitors can not gain access.
1) lack of maintenance - dirty old EC and pH probes - not calibrated
2) dosing tank without sufficient stirring - the heavy salts and acid fall to the bottom of the tank where they stay as a layer
3) lack of water through sample point - sample point too high for pump head.
4) dose size set either too small (wears out dose pump of solenoid through constant operation) or too big (tendency to overdose)
5) pH not dosing - smart doser inhibits dosing pH until EC is near its set point.