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Hey everybody: I just picked up the Viair 400P auto. Overall I’m very pleased with the unit – easy to use, decent price, light and compact, and fills up the tires very quickly. However the psi gauge is WAY OFF, like more than 5 psi. Which means I have to keep disconnecting the Viair and connecting an accurate gauge to see where I’m at. Not a huge problem I guess, but I’m paying for the gas-station-style gun with the built-in gauge, so if it actually worked it would be a lot better. Anyone else have this problem? Thanks!
 

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Pretty common. Just inflate to the overage on the VIAIR and check. I have 3-4 high priced gauges and there is a 4-5 pound difference between them. I've found Joe's to be the most accurate.
 

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You'd think for that price that it would be more accurate. I went the cheaper route and bought the ARB Tire Inflator with gauge for my Powertank and it didn't even work out of the box. Sent it back and bought the twice as expensive Powertank gauge and it works within +/- 1 psi.
 

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My Viair is off, although maybe not as much as @Sitka. Most industrial pressure gauges are only truly accurate mid scale. I've thought about changing the gauge out to one with the 30 PSI mid scale (60 PSI gauge) but never have because I have learned where 35 PSI really is on the gauge and fill the tire accordingly. If wishes were horses (or jeeps) then what I really would like is a longer feed hose between the tire and the gauge. Bending over to air up, my twice broken back starts really complaining around the third tire.
 

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Mine is a couple years old and its 2-4 PSI of consistantly
 

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Mine is more like 10 PSI off if reading the gauge while inflating. Good thing is is that it is very consistent and quite accurate when you figure in the calibration error. I fill mine to 40 PSI by the viair gauge and know for sure they are actually at thirty. I spot check them naturally but seldom need to adjust. Great compressor none the less. Used it regularly for four years now.
 

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You'd think for that price that it would be more accurate. I went the cheaper route and bought the ARB Tire Inflator with gauge for my Powertank and it didn't even work out of the box. Sent it back and bought the twice as expensive Powertank gauge and it works within +/- 1 psi.
I went the even cheaper route. Using the Harbor Freight 150psi compressors, which are supposed to be very similar to the Viair 400P at 1/3rd the price. The gauge on the HF is also about 10psi off while filling and I didn't think much about it. I just said to myself, it's a HF I'm not really expecting it to be perfect, I can make my own manual re-calibration if need be. I didn't realize it until the OP posted, how much more similar the HF is to the Viair 400P, right down to the inaccuracy of the gauges. Maybe the HF is just a rebranded Viair? LOL!!:rofl:
 

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I hope you understand that you have to hit the bleed valve on the side to get an accurate reading on the tire pressure (lower) and not see the fill pressure (much higher).
My gauge has worked great for a boron tube style gauge and seems pretty tough.

Remember too the air you pump in is hot and the tire needs to cool again for the onboard gauge to give a respectable reading.
 

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I hope you understand that you have to hit the bleed valve on the side to get an accurate reading on the tire pressure (lower) and not see the fill pressure (much higher).
My gauge has worked great for a boron tube style gauge and seems pretty tough.

Remember too the air you pump in is hot and the tire needs to cool again for the onboard gauge to give a respectable reading.
Exactly! A basic understanding of physics is all that is needed to understand a simple gauge will not read tire pressure while a higher pressure is being used to fill it. You either have to hit the bleed or turn off the compressor to get a reading. My Viair runs right in line with my high dollar gauges.
 

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I guess I didn't really understand that, but i do now. Thanks. Its a little confusing (for me) because when you pull the trigger on the gun the needle goes crazy high for a second and then settles back at about 40, which is where it stays while you are squeezing the trigger. Then only once you let go of the trigger does the needle settle further, to (roughly) the right number. Do I actually have to go and turn off the compressor as well? And what's the bleed?
 

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I guess I didn't really understand that, but i do now. Thanks. Its a little confusing (for me) because when you pull the trigger on the gun the needle goes crazy high for a second and then settles back at about 40, which is where it stays while you are squeezing the trigger. Then only once you let go of the trigger does the needle settle further, to (roughly) the right number. Do I actually have to go and turn off the compressor as well? And what's the bleed?
Some have a bleed valve on the chuck. When pushed with the compressor running it allows the air to equalize to the point of allowing a reasonably accurate reading. It's much simpler to carry the compressor with you to each tire if you're using a portable so you can turn it off to check pressure. If your compressor is mounted to the Jeep it's simpler to unhook the chuck and check pressure with a separate gauge. I prefer the portable.
 

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I guess I didn't really understand that, but i do now. Thanks. Its a little confusing (for me) because when you pull the trigger on the gun the needle goes crazy high for a second and then settles back at about 40, which is where it stays while you are squeezing the trigger. Then only once you let go of the trigger does the needle settle further, to (roughly) the right number. Do I actually have to go and turn off the compressor as well? And what's the bleed?
Sitka - The bleed valve is a button that you can push on the side of a Viair gauge handle. It is for letting out air for deflating after an overfill.

Everyone - Pressure gauges are always inaccurate during air flow due the venturi affect. (If you've never overhauled a carburetor you may want to google that.) This is true for any pressure gauge anywhere. With the Viair, it's true when filling with the trigger or deflating with the bleed valve. The only true pressure reading on a Viair is when the trigger is released without pushing the bleed valve.

As for the very high start pressure - with the trigger off the pump continues running until it hits its cut-off pressure. When you pull the trigger, the gauge ramps up toward that pressure. The volume of high pressure air is small because it is contained in just the pump chamber and the air hose. Hence, the pressure spike and the precipitous drop a second after the trigger is pulled as the pump pressure and tire pressure equalize.
 

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The trigger regulates the or, allows the air pressure from the pump to flow to the tire chuck (or tire if it is connected) and the trigger is pulled opening the hose past the trigger. With the chuck attached to the tire, the tire is like a big volume tank being filled by a low pressure pump. The pump only pumps at the present pressure of the tire (low pressure).

When you release the trigger with the hose and chuck attached to the tire the air flow is blocked from passing to the tire. Because the volume of the hose and handle of the pump is small (it now becomes the volume tank and fills quickly) the pressure increases rapidly until the cutoff pressure is reached and the pump stops.

The trigger holds that pressure back at that point from getting past the trigger valve. The small release valve on the side has to be temporarily pressed to allow the pressure to be equalized with the tire. Then then can you can see the true pressure of the air in the tire.

Once you see the actual tire pressure then you can make a decision to continue filling the tire or, disconnect the chuck. IF you decide to disconnect and leave the pump running you will see the pressure of the pump, filling the charge hose and handle again until the cutoff pressure is reached behind the trigger.

In essence, the trigger acts as an On/OFF valve and the small button on the side is a down stream/inline bleeder valve.
 

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Mine was off about 5 psi. I installed a different gauge and it fixed the problem. They definitely need to provide better gauges.
 

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I got the 450p. It's about 1 psi off on the high side. The gun looks to be a different one than the 400p. Maybe swap it out to the 450 style.
 
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