how does the gauge work; by sensing temperature (or) by sensing PSI???

The gauge on this is a mechanical float. This one will be accurate in telling you 3/4, 1/2, 1/4 full etc. I've tried the "pressure gauges" and found even with a tank at 20% level (checked by weight), the pressure gauge still shows in the green (over 1/2 full) where pressure is maintained by the gas. Those don't go into the red area until you're 5 minutes from empty and I think are just a confirmation that you've used it all. They can help with leak detection where a test you can do on any setup is this: Close all valves on stoves or heaters connected up, open the tank valve to pressurize the hoses (watching for the needle to go into the green), close the tank valve and wait a minute watching for the needle to drop back to the red. If you see it drop in 5-10 seconds, you have a BIG leak to find. If it holds for over a minute your setup is good. A drop of the needle 1/2 way over a minute shows a minor leak you may want to look for, but may not be a big deal (depending on long term setup or for a day while camping).

Any special precautions on first time filling?

There are no special precautions. Just close the valve after filling with propane so you can safely transport the tank home.

How many gallons of propane can it hold ?

If you look at the close up of the first picture, you can see "WC-47" stamped in the ring. Although that "WC" is "water capacity", after doing some math it turns out dividing that by 10 makes "4.7" gallons the max you should ever get in it. That same thing is true on other size tanks (like the 30 pound size say "WC-71" making 7.1 gallon max, etc). So that's an easy to remember thing that's always on it. Otherwise you can also tell how full it is by the weights. This one can handle "20 pounds" gas plus the tank also has stamped "TW-18.6" or such which is "Tare weight" or empty tank weight. So a full tank will weigh 20+18.6=38.6 pounds. Again simple to remember with the info on the tank. There seems to be conflicting info about tank proper fill specs. I've seen a chart from Flame King of someone saying "Tanks are only filled to 80%, therefore a 100 pound tank only holds 80 pounds gas, 20 pound tanks hold 16 pounds gas, etc". That is inconsistent with other brands of tanks including Worthington, Manchester, and more where the labeled "100 pound tank" holds 100 pounds of gas (and would be called a 125 pound tank if the 80% wasn't already taken into account). When I compare all of the common tanks from all of these OEMs, they are consistent in everything including dimensions, labels of "WC 47" stamped in the ring to show the size following DOT requirements, etc except for the final 80% fill level talk. To show my evidence of who's correct (besides 2:1 from those other OEMs showing only the Flame King says smaller numbers), there's others Q&A under the 100 pound tank asking "Is this a 30 gallon tank, a filling place sold me 27+ gallons in it?" If it was a 24 gallon tank at 100% (and intended for 20 gallons normally as the 80% level suggested by some), how could someone report 27 gallons in it? That's like saying "I got 6 gallons in a 5 gallon bucket" which is impossible. So I'm certain the tank size called out (like 100 pound) is the gas capacity with the tank being bigger than that to provide the 20% extra room. I'll even add a video of these smaller size tanks that I weigh in front of the camera showing the 30 pound tank with ~35 pounds gas (which again is impossible if it's 30 max for 24 normally). If you like getting the tank filled only ~65% level or such and needing to make more trips for refills, that's up to you, but seems crazy to me. I hope you don't get these overfilled (like I'm showing I have) as you can have problems from liquid propane going to your heaters or such, so feel free to watch for such. If a filling place says you owe for 5.5 gallons you can argue "This should only have 4.7 max in it, so either you're ripping me off or have overfilled it where I need to vent the extra, so why should I need to pay for what I can't use & didn't want?" The OEM has a sticker on the tank saying "If you fear an overfill, have the tank outside away from any ignition source and crack the bleed port until liquid spray stops" to bring it down to the designed safe 80% level. FYI the only reason I've accepted overfilling of my tanks is since I can refill some small 1 pound tanks at home where I want liquid propane for that purpose, so that's how I'm not "wasting" the gas but can drain it down from the overfilled and "unsafe" condition right after the refill. So showing I have seen overfilled tanks doesn't mean I'm invincible from possible problems, but rather I'm doing something about it to correct it (which needs knowing of it).

Material	Alloy Steel
Brand	Flame King
Item dimensions L x W x H	30.5 x 45.7 x 30.5 Centimeters
Exterior Finish	Powder Coated
Inlet Connection Type	Flange
Outlet Connection Type	Flange
Outlet Connection Size	0.75 Inches
Specification Met	DOT
Manufacturer	Flame King
UPC	850015730689
Item model number	YSN230b
Product Dimensions	30.48 x 45.72 x 30.48 cm; 7.73 kg
ASIN	B08XYH6TND

Flame King YSN230b 20 Pound Steel Propane Tank Cylinder with OPD Valve and Built-in Gauge, 20 lb Vertical

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