[DeTomaso] Rear Window

[Stephen]

Good evening Joe.  Good question.  In my initial e-mail on this science experiment I discussed the pressure gauge:  

 

When one talks about a vacuum, one is usually talking about a vacuum, relative to ambient atmospheric pressure.  So, if a vacuum gauge is hooked up to your intake manifold, it reads the difference in pressure between the atmosphere where the gauge is sitting, and the inside of your manifold, where the hose from the gauge is connected.

 

So, what does one compare to in a moving car?  The interior of the car?  That is what one would get if one puts a vacuum gauge in the cab and routes the hose to the area in question.  But, what does the pressure (or vacuum) inside the car do with the car moving down the road.  Open the windows, your ears pop, and the pressure changes.  Most of us don’t have an external air inlet (at least an intentional one) to the cab except for the windows.  

 

My solution is to use a fairly sensitive pressure gauge (a sphygmomanometer – gauge used to read blood pressure.  It reads in inches of mercury.  FYI, there are roughly 50 inches of mercury per psi).  I have connected the gauge with small-diameter plastic tubing to a 200 ml stainless sample bomb.  I wrapped the bomb in towels and placed in a cooler to minimize temperature changes.  See, the pressure in the bomb will change by roughly .03 psi (1.5 mm Hg) for each 1 degree F change in temperature.

 

I then pressured up the bomb and let it sit overnight to stabilize.  It ended up at 280 mm.  This is my “zero” at the then current barometric pressure.  

 

One has to view the pressure gauge as a delta-pressure device.  It shows the difference in pressure between the bomb (high pressure) and the atmosphere around the gauge.  So, if the reading on the gauge goes up, the differential is increasing, which means the atmospheric pressure is going down.  As in a wee bit of a vacuum around the gauge.  Flip side, if the reading on the gauge goes down, well, there is less differential between the 280 mm Hg in the bomb and the atmosphere around the gauge, so the pressure is going up.

 

OK – that is complex.  

 

While a gauge that can be read down to around 1 mm hg (0.02 psi) is pretty sensitive, I suspect we are talking pretty minimal vacuum levels.  Preliminary testing suggested a reading of 272 mm Hg at 80 mph.  The pressure around the gauge went up!

 

Subsequently several folks with more knowledge than I indicated that the engine compartment is pressurized at speed and that at least part of the area above the deck lid is under a vacuum when the car is moving at speed.  My test rig confirms this.  Ok, not a lot of pressure, but a little.  Now to review the videos and pictures from this evenings test runs.

 

Stephen Nelson

 

 

-----Original Message-----
From: Joseph F. Byrd, Jr. [mailto:byrdjf at embarqmail.com] 
Sent: Saturday, March 19, 2016 7:52 PM
To: 'Stephen' <steve at snclocks.com>
Cc: detomaso at poca.com
Subject: RE: [DeTomaso] Rear Window

 

Looking at the test equipment, what is the pressure gauge measuring?  Would that gauge indicate with respect to its ambient pressure so in effect it is measuring dP to where ever you have the other end of the hose?

Do you have some sort of "sensor" on the other end of the hose?

 

 

Joe

-------------- next part --------------
   Good evening Joe.  Good question.  In my initial e-mail on this science
   experiment I discussed the pressure gauge:


   When one talks about a vacuum, one is usually talking about a vacuum,
   relative to ambient atmospheric pressure.  So, if a vacuum gauge is
   hooked up to your intake manifold, it reads the difference in pressure
   between the atmosphere where the gauge is sitting, and the inside of
   your manifold, where the hose from the gauge is connected.


   So, what does one compare to in a moving car?  The interior of the
   car?  That is what one would get if one puts a vacuum gauge in the cab
   and routes the hose to the area in question.  But, what does the
   pressure (or vacuum) inside the car do with the car moving down the
   road.  Open the windows, your ears pop, and the pressure changes.  Most
   of us don't have an external air inlet (at least an intentional one) to
   the cab except for the windows.


   My solution is to use a fairly sensitive pressure gauge (a
   sphygmomanometer - gauge used to read blood pressure.  It reads in
   inches of mercury.  FYI, there are roughly 50 inches of mercury per
   psi).  I have connected the gauge with small-diameter plastic tubing to
   a 200 ml stainless sample bomb.  I wrapped the bomb in towels and
   placed in a cooler to minimize temperature changes.  See, the pressure
   in the bomb will change by roughly .03 psi (1.5 mm Hg) for each 1
   degree F change in temperature.


   I then pressured up the bomb and let it sit overnight to stabilize.  It
   ended up at 280 mm.  This is my "zero" at the then current barometric
   pressure.


   One has to view the pressure gauge as a delta-pressure device.  It
   shows the difference in pressure between the bomb (high pressure) and
   the atmosphere around the gauge.  So, if the reading on the gauge goes
   up, the differential is increasing, which means the atmospheric
   pressure is going down.  As in a wee bit of a vacuum around the gauge.
   Flip side, if the reading on the gauge goes down, well, there is less
   differential between the 280 mm Hg in the bomb and the atmosphere
   around the gauge, so the pressure is going up.


   OK - that is complex.


   While a gauge that can be read down to around 1 mm hg (0.02 psi) is
   pretty sensitive, I suspect we are talking pretty minimal vacuum
   levels.  Preliminary testing suggested a reading of 272 mm Hg at 80
   mph.  The pressure around the gauge went up!


   Subsequently several folks with more knowledge than I indicated that
   the engine compartment is pressurized at speed and that at least part
   of the area above the deck lid is under a vacuum when the car is moving
   at speed.  My test rig confirms this.  Ok, not a lot of pressure, but a
   little.  Now to review the videos and pictures from this evenings test
   runs.


   Stephen Nelson



   -----Original Message-----
   From: Joseph F. Byrd, Jr. [mailto:byrdjf at embarqmail.com]
   Sent: Saturday, March 19, 2016 7:52 PM
   To: 'Stephen' <steve at snclocks.com>
   Cc: detomaso at poca.com
   Subject: RE: [DeTomaso] Rear Window


   Looking at the test equipment, what is the pressure gauge measuring?
   Would that gauge indicate with respect to its ambient pressure so in
   effect it is measuring dP to where ever you have the other end of the
   hose?

   Do you have some sort of "sensor" on the other end of the hose?



   Joe