Figuring out pressure

[BenjaminT]

What is the best way to go about measuring absolute pressure at remote locations? I want to be able to test air pressure in and around my car to keep down-force close to neutral as I mod.

Before modding, the hood on my Mustang would dance around from too much air underneath it. I happened to remove the HVAC, leaving the hole in the firewall, and the hood now stays in place. I placed a sheet of paper over said hole and a steady stream of air would blow into the cabin.

First mod: ducting the radiator. The sheet would stay flat against the firewall and only blow off when wake from other vehicles washed under the car.

Second mod: custom diffuser that reaches back to the K-member. Sheet is planted.

While down-force is useful, too much hinders economy. Being able to remotely test the pressure will give me a real idea of what's going on and what needs to be done. A certain amount of sensitivity is needed in this gauge.

[rollercoaster]

Start here:

Pitot tube - Wikipedia, the free encyclopedia

[MGB=MPG]

MEASURING DEVICE=MANOMETER.. procure a sophisticated or fabricate a simple one [ u-tube]see below.. diy u-tube measuring in inches of water. put red dye in the water .
clear plastic tube of small dia. should suffice[ find in the aquarium supplies at the local pet store]

one end of the u tube is open to cockpit air .. the other end extends to the test site. end of tube *should not* be exposed to ram air effect like a pitot tube [that device measures air speed by ram ait effect] i presume you wish to measure over pressures [compression] or under-pressures [ lift?]
in other words the opening of the tube at test site should face away from the expected air flow or be perpendicular to it ..

at rest with equal pressures on both ends the level on both sides is the same .

at measurement speed .. assuming overpressure measured the upstream side is lower and the down stream side is higher .. the *inches of water * pressure measured is the difference in the height





Pressure measurement - Wikipedia, the free encyclopedia




U-Tube Manometer


and if that config is not sensitive enough use this


Inclining the tube manometer will increase the accuracy of the measurement.

[JRMichler]

Most hardware stores sell clear vinyl tubing by the foot.

At normal highway speeds, you should be seeing pressures on the order of 1 to 2 inches of water. And yes, inches of water is a standard unit of pressure measurement.

[MGB=MPG]

edit: if the area *to be Tested* is in view , like on a hood simple yarn strips there could give you an idea of whats happening with the air flow

on my *TO DO * list is the fabrication of a u tube
to remotely measure pressures around the auto. My induction is modified to supply air to engine from what i *presume* to be a high pressure area on the chin fin[forward fairing] Intuitively it is a high pressure area , but not measured .one wonders at a road speed of 60 MPH what the measured pressure would be.

i would like to measure the pressure at the convergent section of the fairing duct , in the pipe that goes to the air box , in the air box before the filter , and after the filter , finally at the convergent section of the carb throat before the throttle plate .
i measure manifold pressure[vacuum gauge ] in INHG [inches of mercury]

the theoretical value 60 MPH is .043 PSI or 1.19 INH2O or 30.2 MMH2O
using this tool for 88fps with an air density of 59oF - 0.0765 lbm/ft3

Dynamic Pressure

humm .043 PSI or 1.19 INH2O or 30.2 MMH2O

i wish i could just go to a tool room and check out a good digital manometer, the one i looked at cost @ $400

Inch of water - Wikipedia, the free encyclopedia

1 inAq = .036126 psi, or 27.68089 inAq = 1 psi.

1 inH2O = 248.84 pascals (60 °F)[2]
= 2.4884 mbar or hectopascals (60 °F)
≈ 2.54 cmH2O (4 °C)
≈ 0.0024558598569 atm
≈ 1.86645349124 torr or mmHg (0 °C)
≈ 0.0734824209149 inHg (0 °C)
≈ 0.0360911906567 PSI

[MGB=MPG]

please check my numbers and thinking
if i get a *GO* ill find some tube and implement
im in a sweat to know what the pressures are in my air box .. before and after the filter

reference:
1] U-Tube Manometer
2] Sine - Wikipedia, the free encyclopedia
3] Sine, Cosine, Tangent Chart & Calculator

with measurements expected on the order of 0ne inch of water a more sensitive device seems indicated than the vertical u-tube [see prior or ref 1]

for the vertical u-tube the formula pd = γ h = ρ g h applies using water as the working fluid and INAg as the measurement increment solving
pressure = Height
so one inch measure= one inch of water[ dynamic pressure measured]


for the inclined U Tube the formula pd = γ h sin(θ)
θ = angle of column relative the horizontal plane
with water working fluid
pressure = sine of the angle x height measured

examining ref 2 & ref 3 i find
angle 45 degrees = sine 0.70710678118654
angle 30 degrees = sine 0.5
angle 15 degrees = sine 0.2588

so does this mean that a U tube inclined at 30 degrees measuring 1 inch level difference is measuring 0ne half inch of water dynamic pressure at the test site?

and that a U tube inclined at 15 degrees measuring one inch level difference is showing 1/4 inch [+88 thousandths of an inch] inch water dynamic pressure at the test site ?


find a protractor . fabricate for a inclined u tube to measure 1.5 inches water
15 degree umm what about 6 inches

here is the one i really want .
Series 200 & 300 | Durablock® Solid Plastic Stationary Gage | Dwyer Instruments

would not that look spiffy on my panel
cost about $400 new, anyone happen to have one in a junk box they care to part with , i don't care if its in calibration or not

[JRMichler]

An inclined manometer is more accurate at low pressures, BUT it needs to be accurately leveled. That's not practical in a car where you are driving on a cambered road, turning, and accelerating. Try the simple U-tube first, make sure the two tubes are right next to each other, and see what happens. I suspect that it will be accurate enough for your needs.

If you want spiffy, get a Dwyer Magnehelic gauge.

[MGB=MPG]

i would like one of those dwyer instruments


Series 2-5000 | Minihelic® II Differential Pressure Gage | Dwyer Instruments
i see i can get a new 0-2 WC for 50$

When i was finished fooling with it in the Automobile it would be very nice on my air handler in the machine room.

this is where i am testing the DIY U Tube Manometer .

i tried inclined both 15 and 30 degrees, UNSAT there is some phenom does not allow the water in the tube to settle evenly ..i sould speculate about that but *why bother*
the vertical U-tube works like a charm measuring system pressure drop . it will be very handy there to assist in Filter Change decisions .
My system has been icing up lately and me and the HVAC tech been hassling about that .
Brand new trane 4ton with a 2 stage scroll compressor and a variable speed blower . when its in Eco-mode the scroll only runs at abt 66 % and the blower moves very little air. but when it ramps up i think it moves [or supposed to] 1800 CFM .HVAC Tech was blaming my coil ice up on my prefilters and filters. not the case i think and now i have a *proper * measuring device to substantiate my contention.
wit filter out and filter door open and the machine ramped up the pressure drop through the heat exchanger seemed excessive to me ,

i am told usual system design is 500 FPM on the filter Face and a pressure drop of .5 INH20 or WG.. im running twice that. even with the filter door standing open. ..

at any rate on to a portable one , i see theoretical dynamic pressure is 1 INH20 at 55 MPH , i sure can draw an inch on a card and hold the needle on *Double Nickle * I need a rider to read the card then ill run up to the next Even point ..78 MPH for 2 INH2o

If i get readings ill check the rest of the induction, especially pressure drop through my induction air filter ..
if i seem to get readings there ,
[not too awful accurate with a water U-tube. i cant draw lines on a card to .1 inch much less read them .
ill spring for the Dwyer
and record . if i can find a rider to read the gauge
ciao.

[MGB=MPG]

the fabricated U-tube

sensing tube / not shown is abt 1/8 dia ... i tested with a 20 foot run of sensing tube and it worked .
the sense tube fits inside a little larger tube , which fits in the biggest tubes , measurement tubes . the *U* at the bottom is the middle size to reduce radius.. filled with water and food color. i used some glue to seal where the water filled tubes joined. first effort leaked . maybe this will not

taped to a card , card with increments marked shoved in behind. Zero calibration is by sliding the increment card up or down

this should work pretty good for the OP ., will measure 10 inches of water over and under pressure. and cost very little

my intent with this device is to measure dynamic pressure at the engine air duct set into the Special Tuning chin fin on this machine.
i measured in the convergent sector of the duct , later intend to measure many other places

theoretical over pressures are :
45 mph = .67 inh20
50 mph=.83 inH20
55 mph =1.0 inH20
65 MPH = 1.4 InH20
78 MPH = 2.01 InH20

measured values in initial test

40 MPH 1.0 INH20 measured
50-55 =2.0 inH20 .. measured
i saw intermittent of 2.5 inh20 [ when i met traffic and hit the bow waves ,, 2 lane blacktop]

2" of water=0.0721 PSI




i will be testing many other places , but that will be another string
should this type of testing seem appropriate to this forum