Are you talking about building a controller for $50? I'm not sure about other controllers but to upgrade a pre-existing profilux to measure salinity you're looking at close to $400 with the sensor, the sensor alone is around $250. A two pump dosing unit to add onto a profilux is around $250 so this would be a cheaper alternative and realistically do the job just as well. I'll admit the salinity measurement is really nice to have as the options for changing and maintaining salinity are endless but I've used these units before and they require a fair bit of attention. I had to adjust the gravity offset every few weeks and often change the set points for the salt control.
If I where to go with a saturated method I would choose the salinity controller but if I was going to go through the trouble of maintaining a large amount of saltwater pre-mixed to match the tank then I would opt for the simplicity of two dosing pumps. Unless the tank was fairly large as dosing pumps would take too long to change a decent amount of water.
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