Using a differential manometer to measure pressure generated by small syringes.?

Question

I need some help for some medical research. i want to measure the "negative" pressure generated by small syrignes of about volume 5 -10 ml I have a digitron 2082 differential manometer for this. My problem is that there is some dead space between the syringe and the measuring membrane, and i feel that it may influence the results, becuase the air in the dead space may expand. how can i get rid of this dead space? can i fill it with water, or is there some other liquid that i can use with out damaging the manometer ?

mark mentions that the manual says about using a buffer solution. that is true ,but they don't emntion if th efliud can touch the sensor.

Answer 1

The dead space will not influence your reading. There must be some empty space, otherwise, your vacuum would be so tight that you would get zero pressure, or (almost ) perfect vacuum with the slightest displacemente of the piston..

When air is present, you will have to draw the syringe piston a little further to obtain the same vacuum. Yet, if you want to be sure, you CAN fill the whole system with water. Any instrument can deal with water.

With or without water, at some point the piston will start leaking air, and the negative pressure you created will stay constant from there on.

HOWEVER, the water in the siringe will add to the tightness of the piston seal and create a higher vacuum than if it would run dry.

As far as the risk of damaging the manometer, remember that you cannot create a negative pressure (vacuum) larger than one atmosphere, equivalent to 1kg/cm2, or 15 psia, or a 10 mt water column. Check the scale of the instrument (I am not familiar with that one in particular). You would be probably better off with an absolute pressure gauge, not a differential one.

In any case, you should have the instrument specs handy. Don´t use an instrument you do not understand.
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Answer 2

Pg2 of the instruction referr to using an isolation buffer with certain materials which can be water, mineral oil or air.
I would cantact the company and ask to be certain as the instruction above specifically mentions a 2000P. (Though this may referr to the series)