Should I add a backup high level alarm float?
That’s a good question and we say it’s definitely a good recommendation – even though the LIT & FOGRod have “internal backups”, where any problem with one (or even more) contacts will not stop the system working (read how we do it). So the high level float backup is only needed for complete failure of the LIT or of its power supply. The LIT is a very tough unit but everything electronic ultimately fails, even if it takes 30 years.
Here is our thinking on a redundant high level float..
Generally, adding redundancy isn’t trouble free, as many people who have spent time trying to design redundant systems will testify. It seems counter-intuitive, but often redundancy can add complexity and create more opportunities to fail.
However, the case of a high level float has two factors in its favor – first, it is low cost; and second, the “redundancy” is simple, because we have a very simple “switch” wired in parallel into an on/off circuit (e.g. red light on top of the panel).
Basically, floats are very reliable when they are not in wastewater, so as an occasional “once in a blue moon” alarm it can be expected to work. It is almost never in wastewater so you aren’t going to get a false “on” reading. It will just stay hanging there, in the “off” position.
Wiring it into the panel is also very simple. You don’t need a complex redundancy controller, or complex PLC software to determine which system has failed. You just wire the two cores from the one float into the red light circuit so when the float tips, the light comes on – easy to wire up, easy to understand and easy to test. Alternatively, you can wire it the same way into a telemetry system to call out an operator.
Using a high level float to start a pump (instead of generating an alarm) is a little more complex because now you want to turn the pump off at some point, otherwise you might run the pump dry and do some damage. Now you have two options, either run the pump for a fixed time or have an off float. In either case, you have a little more design work to do, but it’s still a pretty simple system. If you use two floats you definitely need to have the off float above the normal operation of the well, otherwise you will end up with the off float coated in grease and perhaps stuck in position when you needed it to take over.
In summary, if you can have a backup which is simple, easy to understand, easy to test periodically and won’t compromise normal operation then it’s a good choice. A high level alarm float definitely fits this category.