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Old 10-06-2003, 12:42 AM
Mr Goodfahrt's Avatar
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Join Date: Sep 2003
Location: Maryland
Posts: 136
Question Is Mityvac accurate as a Vacuum gauge?

I'm wondering if I can use my Mityvac/zitpopper to read vacuum or do I need a separate gauge for accuracy? Am on a budget and separate gauges seem to run around $30. The Mityvac seems to be reading low... around 12lbs when isolating the vacuum pump, but everything seems to be working ok.

Mr Goodfahrt (pronounced FAIRT... means "good trip" in German)

1982 300D Turbo Pastel Gray (off-white) 200k+(?)
newbie but willing
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Old 10-06-2003, 07:06 AM
Charlie Mitchel
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Migty Vac:

Yes sir you can use as vacume gauge.
I some time use both seperate gauge and Mighty
If you are chasing a vacume leak on 123 chassic.
Start with vacume from pump {21 lb.} hopefully.
Then check vacume to brakes booste,then to vacume shut off,then to tranny, then I would check to drives door. The drivers door is the master to all other doors and trunk and fuel doors.
Good luck.
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Old 10-06-2003, 07:44 AM
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Location: central Texas
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That might be a little optimistic.... did you mean 21 inches instead of pounds ? If my physics theory is correct one would have to be several thousand feet BELOW sea level to get 21 pounds... average total max where most of us live would be around 15 pounds depending on barometric pressure and perfect sealing.
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Old 10-06-2003, 10:18 AM
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Leather is correct. Since vacuum, like cold which is an absence of heat, is an absence of pressure it is impossible to have a vacuum reading higher than the surrounding atmospheric pressure. As you increase in altitude, vacuum readings will decrease and when you reach space, vacuum readings will be zero because there is no atmospheric pressure.

When you have a vacuum reading in inches of Mercury, it is how high the atmospheric pressure will support a column of Mercury when the air above the column is removed.

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Old 10-06-2003, 11:32 AM
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Join Date: Jan 2002
Posts: 18,350
JC Whitney sells a dash vacuum gauge for $9. I used to use them all the time on gassers for fuel economy regulation.
With all the vacuum systems on MB's, it wouldn't be a bad idea to install one permanently T'd into the different vacuum systems so you could do quick checks on the system. Shut off valves to each separate system with the gauge checking the main pump vacuum line all the time would be a nifty system.
1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
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Old 10-06-2003, 04:05 PM
Marshall Booth
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If the gauge is not reading correctly then when you pump with the line occluded you won't reach the 27+" Hg that is customary. The 12 psi (that I think you are reading) is probably about 12/14.5 or about 830 millibar or 0.83 bar! That's reasonable to high vacuum from the pump.

Precise vacuum reading are seldom required. A gauge that reads within 10-20% will usually be FINE.

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Old 10-07-2003, 11:29 PM
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Join Date: Mar 2002
Location: Rochester, NY
Posts: 333
No, the Mityvac gauge is not an "accurate" gauge, but it is great for "ballparking" your problem area. I truly can't think of a reason to have a precise gauge for troubleshooting your vacuum system. You are basically trying to insure your system is holding a vacuum when required. I manage a calibration lab here in Houston and half of our workload is pressure related test equipment. Periodically a customer will send in a Mityvac pump and gauge for calibration. We get a kick out of it and almost feel guilty about taking their money...we let 'em know they are throwing their money away. These types of gauges are typically 5% of reading...good ballpark gauge.

I appreciate your physics lesson. Never heard it explained quite like you did, but you sure did get your point across. Might try your approach with my new employee at work.



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