MAP sensor calibration

[Donald Whisnant]

Instead of having to use charts to calibrate a MAP sensor -- charts that
have been derived from some "standard, ideal condition MAP sensor" -- why
not create your own chart for your own MAP sensor?  That way it will be
guaranteed to be accurate with your sensor ...  [I derive this from the
pressure transducers that we use at work to measure water level in a vat
of paper stock -- inches of water level is much much more sensitive than
inches of Hg!]  ...

Procedure:  Create a manometer from a couple of clear pieces of acrylic tubing.
And 2 clear acrylic 90's for the tubing...
For example, to measure to 30" of Hg, one side needs to be about 17" and
the other side needs to be about 34".  Diameter of the tube doesn't matter
except for cost (remember you must fill the tube with Hg!) ...


                Long Side- : 
                           :
                           :
                           :  : -Short side
                           :  :
                           :__:

Make you a ruler scale to go behind the long side (or just buy a yard stick)...
If you make your own scale, make the scale be 1/2"=1 measured inch, this will
simplify your measurements -- in other words, for each inch displaced downward,
you are displacing an inch on the other side upwards.  So, 1 inch movement
on one side is equivalent to 2 inches of pressure.  

Next, get a handheld vacuum pump (available at most hobby stores or mail
orderable from places like Edmund Scientific).  Get several feet of tygon
tubing -- with proper diameters and adaptors to fit over the end of the MAP
sensor, the end of the vacuum pump, and the end of the acrylic tubing.  This
tubing is available from most hardware stores.  Then get enough mercury from
your favorite chemistry/hobby supply store to fill the tube (the amount will
depend on the diameter you choose -- but a 3/4" tube usually requires almost
1 quart of fluid) ---  BE CAREFUL NOT TO SPILL THE MERCURY AS IT PRODUCES
VERY TOXIC FUMES!

Fill the manometer so that it is about an inch or so below the top of the
SHORT SIDE.  Set the inch scale behind the longer side so that the ZERO is
at the top of the fluid on that side (with the numbers increasing going
up -- indicating inches of vacuum)

On the LONG SIDE of the manometer, make a Tee fitting of the three units
(i.e. manometer, handheld vacuum pump, and MAP sensor)...  BUT, leave the
short side OPEN -- stoping up or blocking air flow into/out-of the short
side will make your reading inaccruate..

MAP SENSOR----T----:
              :    :
 Hand Vacuum__:    :   
    pump           :  :
                   :  :
                   :__:

                   ^___ scale is behind the long side

With a 5 volt power supply (could be just a simple 9V battery and a 7805
regulator), apply power to the MAP sensor and connect a volt meter to the
sensor's output.

Write down the voltage returned by the sensor for 0 inches of vacuum.  Then,
pump the hand unit until 1" is indicated (don't forget that 1" of movement
on 1 side really is 2 inches of vacuum).  Record the reading from the
sensor...  And repeat in desired increments until the desired amount of
vacuum is reached (or you run out of manometer)...

Note:  If the vacuum leaks down when you stop pumping, either your tygon
tubing has a leak, you have a bad stop valve on your pump (or it isn't shut
totally), or you have a bad MAP sensor.

Also Note: Make sure that the manometer is level...

When finished, pour your Hg back into its container (making sure not to spill
it) and save it for next time.

This technique works on almost any type of pressure transducer -- i.e. MAP
sensors, BARO sensors, altimeters, etc. etc...  To measure inches of water
instead of inches of Hg, just use distilled water instead of Hg....  In
fact, you can measure inches of Hg with water, but you will need a very
long manometer (or you will have to make a special inclined manometer) to
read it.  --  I don't remember the conversion factor right off, but it 
seems like there is about 27.+ inches of water per 1 inch of Hg!

You may also prefer to use glass tubing instead of acrylic tubing -- especially
if you are good at glass bending...  You can also buy ready made manometers
from Dwyer and other instrumentation/calibration suppliers -- in fact, they
may even sell Hg or an Hg substitute (i.e. some liquid with the same
density as Hg)...

To measure pressures instead of vacuums, just move everything from the long
side to the short side and replace the vacuum pump with a squeeze bulb (like
those used on blood pressure instruments -- available from most any medical
supply house)...

Donald Whisnant
dewhisna at ix.netcom.com