It is currently Wed Feb 21, 2018 7:07 pm



Reply to topic  [ 34 posts ]  Go to page 1, 2, 3, 4  Next
 Formula help? 
Author Message

Joined: Sun Nov 04, 2012 7:13 pm
Posts: 16
Returning member with an update to my old bench.


Basically I got tired of reading water manometers and went electronic with an Arduino and an differential pressure sensor. Code is pretty much done and i get numbers close to what they should be however i need to fine tune.

The biggest thing i need to do is correct for depression, right now it's all over the place giving me some high readings. I usually manually bleed off the vacuum to try for 28" but if i could write that into the code life would be good. Is there a formula that works for this?


The next thing i need to do is correct for air temp, that's far down on the list since i'm testing in pretty much below freezing conditions right now. lol


Thanks guys, i appreciate it.


Dave


Fri Nov 23, 2012 11:48 pm
Profile

Joined: Mon Nov 05, 2012 12:50 pm
Posts: 12
you should put a pwm valve, there's a library on arduino to control pwm outputs, it's really simple to use


Sun Nov 25, 2012 6:19 am
Profile

Joined: Thu Mar 04, 2010 9:01 pm
Posts: 274
Most of the flow bench standardization formulae look very similar to this one for converting flow at a random depression to flow at a standard 28"
Attachment:
Flowbench fomula.JPG

There are several forum members that have considerable experience with Pitot tube style benches perhaps one of them will chime in with the specific formulae for temperature and barometric pressure corrections.

BTW, there are some real nice, relatively inexpensive weather sensors that would be simple to add on to your arduino such as the SHT11 temperature and humidity sensor and the BMP085 barometric pressure sensor. These could be useful in further automating your data collection.


You do not have the required permissions to view the files attached to this post.


Sun Nov 25, 2012 8:06 am
Profile
Site Admin

Joined: Fri Jan 08, 2010 3:35 pm
Posts: 1664
Location: Pennsylvania
86rocco wrote:
There are several forum members that have considerable experience with Pitot tube style benches perhaps one of them will chime in with the specific formulae for temperature and barometric pressure corrections.


I forgot all I knew about pitot style after I went orifice style :mrgreen:

This is ripped from the Old Tractorsport Flowbench Forum doing a search for "correction factor"

I'll try and show the formula I've used in the past:

Static pressure (ps) vertical manometer
Barometric pressure (pb) barometer gage
indicated temp (ti) thermometer

absolute temp (ta)=ti+460
Dry air density (d)=1.325*(pb/ta)
Correction factor (cf)=sqrt(.075/d)

Pressure velocity (vp) from your inclined meter
Velocity feet per minute (vc)=(4005*(sqrt[vp]))*cf
CFM's = (inside dia of your testing pipe in square feet)*vc

Hope this all makes sense, and u get a good idea of whatca need to do? Hopefully I copied this from my notes correctly also . . . Simply create a spreadsheet that does all the math for you and you're all set!

And actually, Rick and myself are exploring the idea of getting back into the flow element business and using the proven PTS DM design as low cost alternative to the higher priced Pitot style designs on the market. This would allow someone to start out with a simple shopvac style flowbench and upgrade if they would like along the line to a full-on PTS Orifice Flowbench and not lose their previous investment, other than a few pieces of PVC pipe for the flow element.

Right now it's in the discussion stages but would be some software coding and me getting setup to make flow elements of which I am able to do as I have built them in the past. Actually was instrumental in one of the designs I compete against on the market now. Glad I parted ways with them though . . . :D

_________________
Bruce

Who . . . me? I stayed at a Holiday in Express . . .


Sun Nov 25, 2012 11:07 am
Profile WWW

Joined: Thu Mar 04, 2010 9:01 pm
Posts: 274
Brucepts wrote:

Static pressure (ps) vertical manometer
Barometric pressure (pb) barometer gage
indicated temp (ti) thermometer

absolute temp (ta)=ti+460
Dry air density (d)=1.325*(pb/ta)
Correction factor (cf)=sqrt(.075/d)

Pressure velocity (vp) from your inclined meter
Velocity feet per minute (vc)=(4005*(sqrt[vp]))*cf
CFM's = (inside dia of your testing pipe in square feet)*vc



Just to make sure everyone's on the same page, I want to doubt check the units of measure: temperature in degrees Fahrenheit, barometric pressure in inches of mercury, air density in pounds per cubic foot, velocity pressure in inches of water.

Have I got that correct Bruce?


Sun Nov 25, 2012 11:51 am
Profile
Site Admin

Joined: Fri Jan 08, 2010 3:35 pm
Posts: 1664
Location: Pennsylvania
Yes, correct.

I forget about the metric system being out there.

And my math is for a pitot setup but the correction factor part would work for anything. The math actually comes from the net years ago.

_________________
Bruce

Who . . . me? I stayed at a Holiday in Express . . .


Sun Nov 25, 2012 11:59 am
Profile WWW

Joined: Sun Nov 04, 2012 7:13 pm
Posts: 16
Thanks, that formula looks like what i was thinking about using. I also found an old excel sheet online with a table of corrections in it.



PWM control of the motors would be great, in this case though i pulled my bench out of storage to test a few heads then it's likely going back for a while. Just doing a favor for a few friends and getting a few bucks on the side for porting.


This is my very first Arduino project, i've only had it for a few days.. lol

I'm using this sensor to monitor the Pitots: http://code.google.com/p/ardupilot-mega/wiki/Airspeed

and a GM 1 bar MAP sensor to monitor test pressure.

all the math is done internally so i get Depression and CFM spit out (CFM is averaged across 10 samples) to the serial monitor.


I want to make a couple small tweaks to the code but need to read up more first. For now it seems to be working well enough though.


Thanks,
Dave


Sun Nov 25, 2012 12:12 pm
Profile

Joined: Thu Mar 04, 2010 9:01 pm
Posts: 274
Brucepts wrote:
Yes, correct.

I forget about the metric system being out there.



Canadians, especially middle aged ones like me, are a little schizophrenic when comes to units of measure, we tend to use metric for some things and imperial for others, I'm okay with pounds, feet and inches but I tend to use liters, degrees Celsius and millibars or kilopascals for barometric pressure. Gallons are a nightmare for Canadians because there are U.S. gallons and imperial gallons, they're not even remotely the same and it's not always obvious from context which type of gallon is being referred to so I avoid both like the plague and use liters instead.


Mon Nov 26, 2012 6:26 pm
Profile

Joined: Sun Nov 04, 2012 7:13 pm
Posts: 16
Much better using corrections however I'm still not 100% confident in the numbers i'm getting out of it.


Bruce, (If you don't mind sharing the info).. your controller must use some averaging of samples. I've tried 10 and 25, 25 seems better but i'm curious how many you use?

Since i haven't gone fancy and used a screen for the arduino i'm using the serial monitor, this is a sample of what i get with averaging 25 samples. Don't mind the depression numbers, this was just with me blowing into the test pitot.

I test one lift segment at a time, basically watching the CFM values to even out (103.65 in this case) and write that down, and it's depression (which is raw data, not averaged) and correct on that.

Code:
CFM = 27.81
Depression WC = -12.18
CFM = 79.21
Depression WC = -11.62
CFM = 89.41
Depression WC = -12.18
CFM = 96.79
Depression WC = -11.62
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.18
CFM = 103.65
Depression WC = -12.75
CFM = 103.65
Depression WC = -12.18
CFM = 104.48
Depression WC = -12.18
CFM = 104.48
Depression WC = -12.18
CFM = 104.48
Depression WC = -12.18
CFM = 104.48
Depression WC = -12.18
CFM = 104.48
Depression WC = -11.62
CFM = 94.09
Depression WC = -11.62
CFM = 81.35
Depression WC = -11.62
CFM = 67.49
Depression WC = -11.62




Thanks,
Dave


Tue Nov 27, 2012 12:12 am
Profile

Joined: Sun Nov 04, 2012 7:13 pm
Posts: 16
Here is a quick shot of it actively correcting for depression:

Code:
CFM = 170.70
Depression WC = 28.43
Corrected CFM = 169.39


going to give this a try tomorrow after work on a head.


Tue Nov 27, 2012 12:45 am
Profile
Display posts from previous:  Sort by  
Reply to topic   [ 34 posts ]  Go to page 1, 2, 3, 4  Next

Who is online

Users browsing this forum: No registered users and 1 guest


You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot post attachments in this forum

Jump to:  
Powered by phpBB® Forum Software © phpBB Group
Designed by ST Software for PTF