Lets talk about F.O.B. S ! ( Foam on Beer Detectors )
There are many of these devices out there. They have been in use for many years in one form or another. The most basic of these resemble a small plastic cup with a ping pong ball inside.
The latest and most effective is a stainless steel glass cased version with a machined float device.
What they can do for your system is save you money, serious money . How serious the money saved is dependent on the distance or length of your beer line, jump lines ,trunk housing and the number of products you sell.
A good rule of thumb is One ounce of beer for every foot of 3/8 diam trunk line.
When a Keg becomes empty , actually before it becomes empty the supplied gas pressure begins to over carbonate and or push past the remaining beer and is drawn into the dip tube. ( the pipe that goes to the bottom of the keg ).
The beer begins to foam up and sales start to go down the drain.
The FOB will drop its float as soon as this is detected saving all of the product in the line from being emptied .
As an example : 70 feet of total product line = 70 ounces of beer . Standard systems will blow out all of the beer and replace it with froth and C02. Installation of the new Keg then meets up with the pressure on the line and the froth. This must be flushed out before a proper pour returns to the faucet on the beer tower or product shank & faucet device being used.
Almost doubling the amount of lost product each time the Keg is emptied.

By now you can see the real savings obtained in using these devices.
Trupor is in my view the best of these.
cost of installation is offset by the saved product of course . The length of time to recoup the cost is based on line length trunk housing bundle size and volume of sales.