There are 3 types of gauges and two ways to find out when it's time to refill
Gage type one: The best: is a float in the tank and a dial on the tank, The dial accurately displays the height of the float.. Ideally the float goes all the way to the bottom.. but not always.
Gage type two: A magnetic or adheasive LCD crystal strip that attaches to the side of the tank, to measure the level you first put a pan of water on the stove and using PROPANE heat the water to boiling (A tea kettle works very nice for this and is in fact what I'd recommend) Then pour the boiling or near boiling water over the tank, NOTE the spot where the color change is on the strip. ANd that is how full you are.
For portable tanks you can use a scale, not a gauge but a measurement of the weight of the tank, then there is a "Tare" weight stamped on the tank, Actual - Tare = Propane.
The last "Gauge" is a red/black thing which can be in-line or it can be mounted on teh changeover valve.. Green = "You have gas" Red means "Selected tank is empty" or "You are all out". Not really a gauge at all.
Do you need a gauge in both tanks?
With two tanks and proper valving no, you do not, you run one tank till it's dry, then haul it to the refill place while the other tank supplies gas.
