Your first question was: How deep does the water need to get in the sump basin before the pump should start?
That would depend on the switch design itself. Diaphragm switches are factory pre-set to a pressure that is created by certain volume of water in the basin. This might be 2”-3”, 6”-7” or some different value. You need to check the manual that came with the pump or write down the manufacturer / model number of your sump pump and try to find ON-OFF level range for the switch on the manufacturer’s website.
Sump pump basin turbulance
If you worry about turbulence you can prevent it by simply placing a small container (ie. a plastic jar, small bucket, etc.) inside the sump pump well and inserting the garden hose into it. Open the valve slowly to allow water fill-up the container and the well by flowing over the container’s rim.
You can also minimize water turbulence by only partially opening the garden hose valve and pointing the water stream onto the sump pump body so it loses its impact. You always have a full control over the amount of water that comes out of the hose; just turn the valve to a minimum if you want to create a “real life” trickle.
However, I don’t believe that diaphragm switches are susceptible to turbulence in the tank at all. The only way a stream of water could trigger the switch earlier is if you’d point it into the membrane of the diaphragm switch, this would act like a mechanical push.
Which switch is best? Sump pump diaphragm switch, Float switch, or electronic probe?
There are a couple of diaphragm switch components that might go wrong.
The membrane which usually gets harder over time and doesn’t respond to water pressure as it supposed to
The air tube that sometimes gets clogged or partially clogged which prevents proper response
How soon is that going to happen... unpredictable, the best bet is to stay within the warranty period and replace the switch.
Floating (tethered and vertical action) switches
work best if there’s sufficient room in the well and if there’s no debris floating around (turbulance might affect those switches). These too fail, simply because they are mechanical devices and have moving parts inside them. Sump pump
floating switches are fairly inexpensive so replacing one as soon as the warranty expires should keep the pump running properly.
Another group of switches called “flood free” has no movable parts and it’s activated by a probe that you can set at a desired level inside the sump pump well (you can simply attach it to the discharge pipe). As soon as the rising water touches the probe, it activates electrical outlet and activates the sump pump plugged into it.
No mechanical parts, just a probe and a circuit board, which can also fail at some point… unfortunately there’s no perfect solution for anything (this particular switch is claimed to be perfect
Things will eventually fail at some point of their life cycle, sometimes when they are needed most.