Calculating Pressure from transducer

Discussion in 'General Electronics Chat' started by drbenne, Mar 7, 2014.

Jul 30, 2013
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Hey guys,

I have a pressure transducer that I'm trying to obtain a pressure reading from and I'm having trouble in understanding the conversion process from the output of the transducer.

specs of the transducer:
Cole Parmer 07356-61 (I think that's the model, Half of it is scratched off and that's the closest model I could get from their website)
Output: 1-5 Vdc
Input: 10-36 Vdc
Range: 30psi (label says "30#&Vac")
I believe it's reading gauge pressure as there's a vent hose running with the electrical connections.

Right now I'm just trying to read ambient pressure which is reading ~2.0485 Vdc. (According to the NOAA website the pressure was 1007.5 mbar in my area at that time.)

I came across this formula on the National Instruments site but am stumped on one term:

Pressure = (Cfs/Vex)(Vmeas/CF), where
Cfs = Full scale capacity
Vex = Excitation voltage
Vmeas = measured voltage
CF = calibration factor

So far I have:
Cfs = 30 psi (range of transducer)
Vex = 24 Vdc (voltage of my supply)
Vmeas = 2.0485 Vdc (measured voltage on DMM)

Where i'm stumped:
CF = calibration factor

In the NI article I found, the theoretical example given stated a CF of 3mV/V. I couldn't find this spec in the datasheet obtained from Cole parmer for my transducer and was wondering if this could be obtained from the given data. I know I should know this but for the life of me I can't figure it out.

NI article: http://www.ni.com/white-paper/3639/en/

If more information is needed let me know and I'll try and provide

Thanks,

Dave

Last edited: Mar 7, 2014
2. GopherT AAC Fanatic!

Nov 23, 2012
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NI software allows you to add offsets and amplify signals so they are likely just describing what you can do in LabView software if you want to (need to) make a run-of-the-mill sensor more accurate. I don't recall a CF pin so I am pretty sure it is a software thing specific to their tools.

Looks like you found the right voltage and are moving down the right path.

drbenne likes this.

Jul 30, 2013
47
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ah okay. I see. The article does mention signal conditioning and needing hardware or software to convert the voltage. I guess this is a good time to mention that I don't have labview, at least my work doesn't, but I do have a Keithley 2001 DMM which I'm using to read the voltage and that does have a y=mx+b function that could possibly do the conversion.. that's where I think I need a push in getting a direct pressure reading

4. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
2,018
350
30#&Vac means PSI and Vacuum

Next problem: Reported barometric pressure is NORMALIZED to sea level. You have to apply altitude corrections.

Beware of absolute pressure and gauge pressure. Gauge has a port open to atmosphere.

They mixed the units: 30" Hg to 30 psi

They used ambiguous units of PSI and not PSIA or PSIG.

30" of Hg is atmospheric pressure.

I THINK the pressure formula you are using is for a bridge sensor.

So it should be a simple relationship: 0.491 154 152 23 PSI = 1.000 V and 30 PSI = 5V

Last edited: Mar 8, 2014
drbenne likes this.

Jul 30, 2013
47
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Ok, so I'm pretty certain it is a gauge pressure because of the clear tube running alongside the electrical connections and the datasheet saying specifically "Clear tubing = vent".

On the label it is also stated "Proof = 90psig" so terms may be in PSIG.

As for a bridge sensor, I'm not too familiar with these systems so I can't be sure on that either... again It was the closest thing I could find correlating measured readings to pressure.

And if I may ask, where did you get 0.491 154 152 23 PSI = 1.000V ?

Just to further clarify, this application is meant to measure vacuum pressure and not meant to go above atmosphere. I'm just trying to at least get an atmospheric reading to demonstrate that it is working properly.

6. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
2,018
350
Proof is fine - can withstand 90 psig

So, make sure you have absolute atmospheric pressure and not the relative reported variety or you include the sea level term. www.wunderground.com will list that.

Boo Boo

I used 1" Hg rather than 30 and Psi from: http://onlineconversion.com/pressure.htm and that may be wrong too.

Note that -30 in Hg is about a -1 atmosphere. so 30 in Hg is 1 atmosphere.

It does say compound sensor: http://www.coleparmer.com/Virtual-Catalog/US/1456

So, I'm wondering what is the transpositioning of units. Vacuum is sometimes measured in mm Hg. e.g. manifold engine vacuum in a car when you used to do it with a gauge.

Now they are MAP sensors or Manifold absolute pressure sensors.

I can definitely understand -14.7 to 30 PSI, but 30" Hg to 30 PSI baffles me UNLESS they mean 30" Vacuum.

so -30 in of Hg is -14.7 PSI.

So 1 V = -14.7 PSI and 5V is 30 PSI.

To linearize, use the two-point form of a line: http://en.wikipedia.org/wiki/Linear_equation

Again, look at the the cole parmer catalog page.

Also look here: http://www.wikihow.com/Calculate-Barometric-Pressure

where it does say that 30" of Hg is 14.7 PSI?

Still a bit confused.

Last edited: Mar 8, 2014
drbenne likes this.

Jul 30, 2013
47
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Thanks keepitsimple. I'm still having trouble understanding the altitude correction, so instead of waste anymore time in trying to figure that out I decided to hook up the sensor to an existing vacuum system with a digital readout. From that i'll just read two pressures and determine the slope and offset which should hopefully work.

8. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
2,018
350
drbenne:

This http://www.sensorsone.co.uk/altitude-pressure-units-conversion.html site should make more sense.

What altitude do you get from www.wunderground.com using your zip code if in the US?

Pressure is wierd concept and I hate Pascals. The number is too darn big, I had a boss that said numbers should be between 1 and 10. Like 10 kV and not 10,000 Volts.

I basically had to deal with the different types of gauges and pumping systems and pressures as low as 10E-9 Torr where Ion pumps are used.

Jul 30, 2013
47
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The elevation here is 5312 ft. (Albuquerque, NM, Sunport) Yea, the number of ways pressure is measured is beyond crazy. Ultimately I need the reading in millibars.

I did do the two point calculation for the -14.7 psi to 30 psi as you suggested and got the correct point-slope form but the conversion to millibars is where I started to get lost. The more I looked at it the more it confused me.

Jul 30, 2013
47
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Okay keepitsimple, So I think I was able to get a reasonable reading and see if it looks right to you.

I used the 30 "Hg as 100% vacuum and as my 0 point of 1 Volt.

I then used the ambient atmospheric reading as my 2nd point of 2.045 V.

I then pulled up the current (altitude compensated) pressure closest to my area, which was about 29.83 in. Hg. Then I found a table showing how many inches to subtract for my present pressure which gave me about 24.525 in. Hg. I then converted that to millibars which came to about 830 mb.

I then found an equation on engineering toolbox and did the necessary conversions and they came out almost pretty much the same (834 mb):

Air pressure above sea level can be calculated as

p = 101325 [(1 - (2.255770)(10^-5)(h)]^5.25588

where

p = air pressure (Pa)

h = altitude above sea level (m)

using the 2 point formula I then came up with the point slope of :

y = 794.258x - 794.258

it's close enough so I think I can live with it for now.

Thanks again

Last edited: Mar 12, 2014
11. KeepItSimpleStupid Well-Known Member

Mar 4, 2014
2,018
350
I think you have it.

Look at it another way as your compensated 30" of Hg is -24.525 in Hg for the min pressure and 30 PSI + 24.525" of Hg Note the mixed units - what got you into this mes to begin with.

Absolute pressure goes down as elevation goes up, so check.

(-24.525 in Hg) to (30 PSI + 24.525" Hg) corresponding to 1 and 5V. Now, lets get the units of interest right.

so, you can say that -24.525 is 0 absolute pressure unit of anything.

if you want millibar: 30 Psi = 2068.427184 millibar and
24.525 inch of mercury [0 °C] = 830.51182051 millibar

1V = 0 millibar and 5 V = 2068.427...plus 830.51...

And remember that you will probably have may be a 0.4 in of Hg variation (guess) so you have an ~13 millibar range which depending on how you did your conversion, you might be able to say +x or -y millibar.

Since your don;t have absolute pressure, rain and wind will also change the absolute value of pressure. Without another guage like Sunroad altimiter/pressure meter/compass etc you might be close enough for government work.

I dealt with torr and millitorr, PSI for gauge pressure and occasionally I'd like to look at things as atmospheres. Conceptually, I know what -15 to 90 PSI is.

And if you ever wondered why there are no many bolts on like a 10" diameter vacuum port, try figuring delta P and lbs/sq in * area to get lbs. 14.7 lbs/sqin * (PI*3.5*3.5) and you get a whopping force in pounds.

It's only when the electronic measuring stuff came about did it even matter,

So, at least two misunderstod concepts:
The implicit sign change for vacuum of -14.7 PSI
The barometric pressure is always normalized to sea level.
I think absolute (PSIA) and gauge (PSIG) was understood. Then there is PSID or differential pressure.
More different units than there are vegetables.

Last edited: Mar 12, 2014