Pressure Xducer Help

Usually means there are two ports, one going to each side of the pressure sensing diaphraghm.

Sounds like you have the wrong type. You may need an "absolute" transducer rather than differential.

sensing diaphraghm.

That's the strange thing. The same pressure xducer comes in 2 formats: single pressure port and dual pressure port. So I guess the differential pressure parameter must apply also to the single pressure port format.

Sure. I suppose you could say that even in the single port part, there is still a "differential pressure" accross the diapraghm. It is just that one side is fixed.

I am basically looking to "map" a patient/athlete's respiratory pattern (breathing waveform). Articles on the web, when referring to respiratory applications of pressure xducers always seem to talk about DIFFERENTIAL pressure xducers. I have to figure this dilemma out.

However, for simple plotting of a pressure (regardless of application), "absolute-type" should do the trick. Or differential, with one side fixed as you mentioned.

There are three standard types of pressure transducer...

PSID DIFFERENTIAL pressure measures the pressure difference between the two ports.

PSIG GAUGE measures the pressure difference between one port and a second that is connected to ambient.

PSIA ABSOLUTE measures the pressure difference between one port and a vacuum reference.

More on my website.

--- There are three types of pressure transducers: absolute, gage, and differential.

They all have two ports, each port leading to one side of the pressure sensing mechanism, diaphragm, bourdon tube, whatever.

In an absolute pressure transducer the pressure in the reference port is pulled down to zero (well...) with a vacuum pump and the port is then sealed, leaving the working port always pushing against zero (well...) PSI. The output of the transducer is referred to as "PSIA", Pounds per Square Inch, Absolute.

In a gage transducer the reference port is brought to a pressure of one atmosphere and then sealed, leaving the working port pushing against 1 atmosphere. The output of the transducer is referred to as "PSIG", Pounds per Square Inch, Gage.

In a differential transducer neither port is sealed and the pressure in the working port will always be pushing against the pressure ambient on the reference port. The output of the transducer is referred to as "PSID", Pounds per Square Inch, Differential.

---

--- For respiratory applications, I suspect the reason the differential transducer is used is because the lungs will always be working against the ambient atmospheric pressure, and the transducer's output will reflect that.

-- John Fields Professional Circuit Designer

Key word: AMBIENT I get it now. Since ambient pressure does vary, a gauge pressure transducer could NOT have been used. The "reference" pressure needs a method to be altered. Hence, the differential pressure transducer is required.

P.S: This means that the branches of the wheatstone bridge are identical. Rigth? Either the top resistors resistance increase with a positive pressure change or the bottom resistors resistance increase with a positive pressure change. It is not the case where the diagonnally facing resistors resistance increase with a positive pressure change. Right?

-Roger

I had (erroneously) assumed

Your proposed configuration (see above) has more sensitivity.

-Roger

Pity that Google Groups doesn't use fixed pitch as is usual for Usenet.

--- OK.

One thing, though, since on a lot of pressure transducers one side of the diaphragm is subjected to the working fluid and strain gages can't be attached to that side, the array takes the form of:

. +V . | . +-----+-----+ . | | . [RV1] [RF1] . | | . +-E1 E2-+ . | | . [RF2] [RV2] . | | . +-----+-----+ . | . 0V

Where the RV's are the strain gage elements and the RF's are fixed resistors.

What will happen, then, when the diaphragm is bowed out from pressure on the working side is that RV1 and RV2 will be stretched, increasing their resistances, while the resistances of RF1 and RF2 will remain unchanged. That will cause the voltage at E1 to fall and the voltage at E2 to rise simultaneously, giving the best resolution possible for strain gages mounted on the same side of the diaphragm.

-- John Fields Professional Circuit Designer

I noticed that some pressure Xducers that have NO ports, are clearly marked DIFFERENTIAL. From their picture, it appears that the chip is resting on 1 side (looks sealed, it would have to be because it is resting against the PCB) and the other side has an aperture to the ambient pressure (that's side is OK. I mean I have no qualms with that side).

How can they label that as a differential pressure xducer? I would expect it to be a guage. I saw this phenomenon quite on several pressure Xducers. Can anyone explain.

I noticed that some pressure Xducers that have NO ports, are clearly marked DIFFERENTIAL. From their picture, it appears that the chip is resting on 1 side (looks sealed, it would have to be because it is resting against the PCB) and the other side has an aperture to the ambient pressure (that's side is OK. I mean I have no qualms with that side).

How can they label that as a differential pressure xducer? I would expect it to be a guage. I saw this phenomenon quite on several pressure Xducers. Can anyone explain.

Also a pity that no-one seems to tell the googlies how to properly include context - it's only a click away, after all.

First click "Show Options", then click _THAT_ "reply" link, then trim context sensibly.

And it is possible to set googlegroups for fixed width, but there's another case of lack of training. I don't remember what I had to do, it's been such a long time since I've used google.

Thanks, Rich

I've been looking at the thread, and I can't see what just a plain ordinary pressure reading will tell you about your customers' breathing rate, unless it's to be incorporated into some kind of pitot tube, to give actual flow.

Or are your flow rate sensors already taken care of? How about O2 and CO2 concentrations?

Thanks, Rich

When the reference port of a differential sensor is open to atmosphere, its output is PSIG.

Cheers! Rich

If the pressure sensor is to measure the sligth pressure variations at the input/output of nose cannula (...the hoses that are wired accross your jaw and feed O2 through your nose...), you should be able to tell when the patient is inhaling and when he is exhaling.

-Roger

On googles groups, when visualizing a thread, on the top rigth corner there are the following options: "Fixed font - Proportional font". It is self explanatory, I imagine.

-Roger