Gauges and sending units can get very confusing.
In the photo above, this very nice Beneteau came with this engine panel, which has idiot lights for high water temperature and oil pressure, but no gauges. In an ideal world you want both, the gauges to show you normal temperatures/pressures and trends if anything changes, and the idiot lights/buzzer to go off if something catastrophic happens. This also builds in redundancy, because you have two pressure senders, two temperature senders, and the corresponding guages/lights at the helm.
To get a new panel from Yanmar with gauges cost $1700, and I was able to add the appropriate gauges and sending units for – ahem – considerably less:
With idiot lights, the task is pretty simple. The sending unit on the engine simply makes an electrical contact when the water temperature gets to a certain temperature or the oil pressure drops, and this connects (usally negative on a negative ground engine) juice to the idiot light/buzzer.
To get the right idiot light sender you just need:
a) the right size sender to fit the engine, usually measured in 1/8" to 1/2" pipe threads (NPT).
b) the sender to trigger at the right temperature or pressure. This can be fairly rough because we're looking for catastrophic drop in oil pressure or marked rise in cooling water temperature.
A typical water temperature sender:
For gauges it gets much more complicated. The sending units are variable resistors and they must:
a) be the right size to fit the appropriate orifice on the engine (hoping there is one, or one that can be teed into, without drilling/tapping some new hole in the engine).
b) have its resistance vary by the right amount for the gauge
c) have this resistance correspond to the right range of temperature/pressure for the gauge.
For example, you could have a sender that varies from 33 ohms to 240 ohms (the range for many Teleflex gauges) but this resistance is at temperatures ranging from 1000-2000 degrees, when your temp gauge only goes up to 240 degrees (an extreme example).
Likewise, you could have a sender for the 120-240 degree temperature range, which would be right for your engine, but does this at a resistance that won't even register on your temperature gauge.
I have been down these roads before.
Most chandleries carry lovely displays of all kinds of beautiful gauges, but carry nary a sending unit and they must be special ordered, because there is so much variation. If you buy your shiny new gauges, then go off to the auto parts store to buy the sending units, you, and the clerk, will be very confused very quickly. There are hundreds of automotive sending units, usually listed by make of car, and it's hard, though not impossible, to get the right sending unit.
In short, to keep things simple, when you buy a Teleflex/Faria/Moeller gauge, buy/order the corresponding Teleflex/Faria/Moeller sender, in the right size, at the same time. The company will list…somewhere…which sending units work with their gauges.
While my aftermarket gauges aren't on the panel, they're near it, and in easy view of the helm. They've got lights, and they've even got the right sending units: