Working with the Honeywell Bridge Pressure Sensor- 26PCBFA6D

pgbpsu · 2012-02-25 11:34

Does anyone have experience working with differential pressure sensors, specifically the Honeywell 26PCBFA6d?
http://sccatalog.honeywell.com/pdbdownload/images/26pc.series.chart.4.pdf

I'd like to use it to make water (maple sap actually) column measurements. I'm using this part to do something very similar to this:
http://www.practicalarduino.com/projects/water-tank-depth-sensor

I didn't so much as select this part as have it handed to me. A couple years ago digikey sent me these when I'd ordered what happened to be in the bin right next to them. When I brought it to their attention, they promptly sent me the correct parts and told returning the mistaken parts was a waste of their time. So I have 2 free samples.

My measurement is the height of maple sap collected and stored in an old milk tank. I need to know when to drive the 40 miles to the sugar bush and collect it before it starts running over. I currently have a system that measures the number of flips the vacuum system undergoes, but this is a bit finicky (hall effect sensor in just the right place) and it ignores the sap collected by gravity alone.

My question about this particular sensor is how to relate changes in voltage to inches of water- for all practical purposes, sap has the same density as water. I don't see anything on the above "datasheet" that specifies how much change in voltage I should see when the pressure difference between the two ports changes.

This unit will handle up to 5PSI which is over 130inches of water. My tank is ~72 inches. How best do I set this up to capture changes at or below 1" (=0.036psi) in water depth?

I was planning to feed this into an INA122 to do the gain and then into an MCP3202 12-bit ADC then into a prop which assembles the SMS text messages among other things.

Any input would be greatly appreciated.
Thanks,
Peter

dre · 2012-02-25 13:20

Peter,

I haven't worked much with pressure sensors but have worked with strain gages, and I believe this sensor is simply a wheatstone bridge configuration of strain gages. As such allow me to offer some comments:

The sensor will give an output of 50mvolts for a full scale pressure of 5 psi. [~ 130 inches of water]. Your application has a full scale of 72 inches or [72/130] x 50 mvolts. Now if the ADC is set up for a full scale of 5 volts you will want a gain of 5 /72/130] x [50 x 10^-3] or about 200 if my ascii math is correct. I use a INA125; the datasheet of which gives the gain set resistor vs desired gain. Using this gain as input to the ADC should result in a digital value of 2^12 counts [ 4048 if I remember] for a full scale input of 72 inches of sap.

Dr Tracy Allen [emesystems.com] has an excellent write-up on using a 12 bit ADC.

Hope this helps.
cheers,
David

pgbpsu · 2012-02-25 17:55

Hi David-

This is a great help. All I really needed to know, and in retrospect should have from the datasheet, is at 5psi the device puts out 50mV. From there I can get the rest. My math agrees with yours- I need gain of about 200. I haven't looked at the INA125 (but will) but it seems to be similar to the INA122. The datasheet specifies the resistor value for lots of different gains At this point I should be good to go.

I need to come up with 10V for this resistor bridge pressure sensor. But other than that I should be good. Thanks!
Peter

pgbpsu · 2012-02-25 18:06

Hi David-

Just looked at that INA125 chip. It's perfect. It will provide the 10V reference and do the gain. What a find. Thanks again.

p

dre · 2012-02-26 05:57

Hi Peter,

You're entirely welcome and thanks for the feedback.

I don't offhand know the amount of current the sensor takes but, to be safe, I would use the TIP29 to boost the bridge supply capability. This addition is shown in one of the figures in the INA125 datasheet.

Best of luck on your project. One of nature's greatest inventions is sugar maple sap.

cheers, David

pgbpsu · 2012-03-03 06:48

Hi David-

Are you still following this thread? I hope so because I've run into some trouble and I'm hoping you could give me a bit more advise. I've attached a hand-drawn sketch of what I'm trying to do. I went with the INA122 instead of the INA125 because the INA125 appeared to need greater than 10V which I didn't have.

I'm connecting all this to a Gadget Gangster board (http://gadgetgangster.com/find-a-project/56?projectnum=257) which will provide regulated 5V. The pressure sensor needs +10V so I'm using a MAX680 to turn the 5V into +/-10 which seems to work. That's included on the schematic. And when I measure the voltage between GND and output A and B of the pressure sensor I do get readings. In both cases they are near 5V. If I measure the voltage between the two outputs, I get something close to zero which goes UP if I blow on the hose connected to the pressure sensor and down if I suck on it. Does that sound right?

I wanted to feed OutA and OutB into the INA122 with a gain of about 200. But that doesn't seem to work. To test that I'm not lacking supply current to the pressure sensor, I removed the connection between the pressure sensor and the INA122. With the 5V supply and GND connected as well as the cap between Vcc and GND (C5), pin 5 to GND, pins 2 and 3 shorted and no gain resistor, I still get an output voltage between pin 6 and GND of 2.9V. That's not what I was expecting. I was expecting this to be zero (5*input differential=0).

Do I have the INA122 wired correctly? I followed the datasheet figure1 (which also shows up in another figure further along).

Any comments/suggestions would be greatly appreciated.

The parts I'm using:
MAX680CPA http://pdfserv.maxim-ic.com/en/ds/MAX680-MAX681.pdf
pressure sensor linked above
INA122 http://www.ti.com/lit/ds/symlink/ina122.pdf

Attachment not found.

dre · 2012-03-04 07:27

Hi Peter,

I'm replying without the benefit of datasheets or your sketch, which I could not open. Therefore I hope others will check my comments.

When you shorted the input pins were they connected to the sensor [as well as to the amp] so that you were forcing both inputs to be at exactly 5 volts? If so and the sensor ground and amplifier grounds were connected then the output of the amp should be zero. [This is one reason I like to amplify the analog signal; rather than use a high resolution ADC and eliminate the amp.]

Don't know anything about a Gadget Gangster board :-(.

With the sensor output not shorted, the sensor supply reading 10 volts, the grounds connected, and the gain resistor installed you should also see near zero output at zero differential pressure. The only thoughts I have for your seeing 2.9 volts is that maybe some grounds aren't grounded together. I am puzzled at your saying that the MAX680[?] outputs +/- volts. For both the sensor and the amp you need only the [+] supply and ground.

Also remember that the sensor only outputs some 30 milllivolts at your full scale output of pressure. You can see this on a digital voltmeter but not well.

What is the gain if you don't have a gain resistor installed? Maybe you need some resistor value there in order for the amp to work [will look at the datasheet].

If you want feel free to PM. But the forum has many who can offer much better help. Isn't there a symbol for unsolved threads?

Keep trying and asking,

cheers, David

dre · 2012-03-04 07:57

Hi again,

I got to thinking that maybe you were trying for a [+/-] output?? I was thinking that your application was for a + to 0 pressure range. If so then connect as per figure #5 of the INA122 datasheet, connecting the ground [pin 4] to the ref [pin 5].

How about trying the circuit without the voltage converter? For instance just use a fresh 9 volt battery to supply the sensor and the amp. That should be near enough to the 10 volt sensor supply requirement. And the amp specs note 'rail-to-rail' output

cheers, David

pgbpsu · 2012-03-04 08:38

Hi David-

Thanks for looking over this and responding. My last post wasn't very clear. I've done some more "snooping". Although I haven't solved the problem, I have simplified the question. Because I was having trouble with the INA122 I decided to pull everything else off the breadboard. So now I only have the 8-pin INA122. Here's how it's connected:

INA122 pin number
1 - No Connection
2 - connected to INA122 pin 3
3 - connected to INA122 pin 2
4 - GND
5 - GND
6 - output measured with voltmeter
7 - +5V and bypass cap to GND
8 - not connected

In this configuration I would expect measured output voltages between 0 and +5V. However, with no gain resistor the datasheet says the Gain should be 5. Since I've shorted pins 2 and 3 on the INA122, there is no voltage difference between these input pins and I'd expect an output voltage of zero. When I measure the output in this configuration, I get +2.85V. If I lower the supply voltage from +5V to +3V the measured output on pin6 is +1.12V. If I raise the supply voltage to 8V (9V battery) I measure +5.83V at the output. Removing the bypass cap does not change anything.

I don't see how this would be correct. Maybe I mis-understand the INA122 function/datasheet.

Are these output voltages, given my setup, what I should expect?

Thanks,
Peter

dre · 2012-03-04 09:40

Peter,

Sounds like you haven't sent a common voltage to the INA122 input pins. Do you have
any, say, 4 - 1 k resistors to make up an artificial bridge. Normally these resistors must be
matched to less than, say, 0.1% in order for the 'zero' output to be near zero. But if you
make up a bridge and short the outputs together then the input to the amp will be near 1/2
the bridge supply [say, 9to10 volts supply gives ~5 volts output from the bridge]. But, if
shorted together, the differential input will be exactly zero; and the amp output should be
close to zero. If this happens then try the next test.

Unshort the inputs and see if the amp puts out a large voltage[try with minimual gain - no
gain resistor?]. If you wired the amp in a single supply configuration the output may be
less than zero and therefore not shown. Try reversing the bridge outputs [reversing the
wires going into pins 3 & 2 of the amp]. One way or the other the amp should have a
large output - indicating that the bridge has an initial unbalance due to the tolerance of
the resistors making up the bridge.

Once you get an initial unbalance output you can, if you have a selection of resistors of
much greater value [or a large value pot*], try balancing the bridge by shunting one arm
of the bridge [the high value resistor in parallel with the bridge arm]. This should change
the amount of amp output.

* If you have a large value pot, especially one having 10 turns of adjustment, you can trim
the output to zero by connecting one of the pot limits to the bridge supply; the other limit
to ground and the slider to one of the bridge outputs. In this configuration you essentially
are shunting adjacent arms with adjustable resistance. Start with the pot slider at
midrange so that the shunting is about equal on both arms and then slowly trim the pot
and note the change in amp output.

It dawns on me that there are better explanations on the web. If wanted I can direct you to
one.

for now,
cheers, David

pgbpsu · 2012-03-04 10:31

Hi David-

Thanks again for being so patient and helpful.

You asked about sending a common voltage to the inputs of the INA122. I was trying to do that by shorting in input pins on the INA122. But I could be very wrong in thinking this looks like a balanced input to the INA122.

I don't have 4 1% resistors to build a bridge, but I do have my pressure sensor and it works. I know this because I can connect the Honeywell sensor to the 9V battery (Supply on pin1, GND on pin3, signal on pins 2 and 4). When I hook up my voltmeter across pins 2 and 4, I get 0.2mV. When I blow on one end (via a rubber tube), I can get the voltage measured between pins 2 and 4 to drop as low as -20mV. When I suck on the tube, it rises as far as 30mV.

I took these two wires (pin 2 and 4 of my pressure sensor) and attached them to pins 2 and 3 of the INA122. The setup on the INA122 is similar to my previous post- +5V supply, pins 4 and 5 grounded, no gain resistor. This time however the output of my bridge is connected to the input of the INA122 (pins 2 and 3). Moving the volt meter over to measure the voltage between pins 5 (connected to GND) and pin 6 of the INA122, I get 2.80V. If I blow or suck on the pressure sensor, there's no change in the output voltage from the INA122.

So it looks like I'm failing the first test you suggested. Regardless of whether I short the inputs to the INA122 or put the balanced bridge output into I always get 2.80V. Changing this input makes no difference. Without a gain resistor the output of 2.8V suggests an input of 0.56V. I found a 100K resistor and added that as the gain resistor which should result in a gain of ~7 the output then jumped to 3.11 which suggests the INA122 is seeing an input of 0.44V. When I remove the bridge from the input of the INA122, nothing changes.

I guess it's possible I've damaged the INA122. This doesn't seem right.

Peter

I'm going to hunt around and see if I have any normal op-amps that I can use to perform the function of the INA122.

dre · 2012-03-04 11:12

Me again,

Per paragraph 3 of your last entry: When you connect the sensor to the amp, do you have 1/2 the supply voltage [~ 2.5 volts] at each of the inputs to the amp [pins 2 & 3]? You should have. If ok and the amp output [between 5 & 6] is 0.56 or 560 millivolts then I suspect something about the sensor or amp chips. Leave everything the same and jumper [short] the input to the amp. Does the amp output go to 0.0 volts? If so then I suspect the sensor. These sensors should be balanced out [zero output for zero diff. pressure] very accurately. I'm purplexed ;-(

Your paragraph 4 testing does suggest that the amp is not working properly; except that the output changes when the gain is changed. My experience with the INA125 is that these instrument amplifiers are rugged. You can try a op amp with feedback to give a small gain or gain of one [voltage follower?]. However they are not too stable at high gains. You might order another INA122. [Hopefully this would become simply a spare.]

Let me leave for now with providing 2 links:

http://www.nakka-rocketry.net/strainlc.html#Circuit

This uses a INA122 in the +/- output configuration.

http://www.eidactics.com/Downloads/Refs-Methods/NI_Strain_Gauge_tutorial.pdf

This shows a wheatstone bridge balancing setup.

We'll pervail :-)
cheers, David

dre · 2012-03-04 11:43

Peter,
Read your test report again: Per paragraph 3, "I get 0.2mV". Can you measure 0.2 mVs?
If so then we can run some further tests.

But for now: per para. 4 if there is no change in amp output with pressure/vacuum; is
there still the -20mV to +30mV change in the input to the amp? That is; check to see that
the differential input to the amp remains the same after the sensor is connected and
pressure/vacuum is introduced. The sensor output better be the same irrespective of
whether it is connected. The sensor has an output which is not amplified so you are
measuring small voltages and connections could result in problems I suppose.

I sure hope someone more knowledgeable about low level circuitry comes to our aid.
cheers, David

pgbpsu · 2012-03-04 15:55

dre wrote: »

Peter,
That is; check to see that the differential input to the amp remains the same after the sensor is connected and
pressure/vacuum is introduced. The sensor output better be the same irrespective of
whether it is connected. The sensor has an output which is not amplified so you are
measuring small voltages and connections could result in problems I suppose.
cheers, David

Hi David-

This is an excellent question and one I should have thought of checking sooner. With the pressure sensor powered by 5V, I get +3.93V out of one half the pressure sensor (pin 4 and GND) and +3.93V out of the other half of the pressure sensor (pin 2 and GND). Measuring the voltage across the two output pins (pin 2 and 4) I get .2mV. When I blow or suck I can make this change. All this is just like above. When I connect these 2 output legs to the INA122 there is no change in behavior. If measured between the two sensor outputs (pins 2 and 4) I still measure small changes when I blow/suck. When I measure the output voltage of each leg with respect to GND I get ~3.93V. But the output of the INA remains 2.56V (no gain resistor so gain should be 5). The pressure sensor is putting out equal voltage on both halfs (confirmed with my voltmeter). When I blow or suck on it now, I now get a very small change in voltage on the output of the INA122. Min is 2.40V; max is 2.55. These are nearly 50mV swings on the sensor output and they only show up as .15V changes AFTER going through the INA122 with a gain of 5.

It's like there's a DC shift on the output of the INA122. I'm more confused than ever. I'm going to dig around for a simple op-amp. I don't need super precise measurements. I just want to know if the sap tank is overflowing. I plan to put this into a 12-bit differential amp. I think I'll probably just try to create the circuit internal to the INA122 (detailed in the datasheet) by using 2 op amps. It might be noisier but at least I'll have a better idea of what's going on.

David, thanks for all your help on this.
Sincerely,
Peter

dre · 2012-03-04 19:01

Hi Peter,
The only additional thing I can think of is that maybe the sensor should be powered by
more than 5 V. My concern results from your statement that the output resides at 3.93V. I
would think that it should be exactly 1/2 the 5V bridge supply. I can't read the specs from
the single page referenced and I would think that the 5V should be ok but will have a
lower output [Wheatstone bridge output is 'ratiometric'. That is, it's differential output is
proportional to the bridge supply.].

I remembered & found some pressure sensors. They are 136PC01G2 from Honeywell
and the suggested bridge supply is 10V. If desired I could hook one up to my INA125
amplifier and check out the output. I don't know if this will help, or that we can compare
experiments; or that I will be successful.

Let me know.
cheers, David

pgbpsu · 2012-03-05 09:24

Hi David-

I've found some 8-pin dip dual op amp chips so I'm going to try that route. It may not be as clean, but hopefully it will work. I believe I can still gain up my 50mV signal into 3.3v (which is what I'll run my ADC at). Next time I order parts, I might add another instrumental amp in the hope that I somehow damaged the one I have. I want to understand why/how this works, but at the same time, the sap is flowing and I need to know how much I've got. Solving this the "right" way may have to wait. At the moment I just need it to be solved.

Thanks again for all your effort on this.
Kindly,
Peter

dre · 2012-03-05 14:15

Hi Peter,

A quick reply:
I can not find a way to get a personal message to you so I will try to look into this thread periodically.

I also thought of 2 other ways of measuring sap levels: Parallax sells a 'height' measuring tape. It won't cover the entire 70 some inches but it would tell if the level is within some, I believe, 15 inches.

Secondly: There was a write-up by Parallax on using the 'Ping' sensor to detect water height. I started a project [never finished] to measure the water level in a small stream using this sensor. I can search out the write-up if you desire.

Again, much luck and keep in touch.

cheers, David

pgbpsu · 2012-03-06 07:22

Hi David-

I think you can reach me via private message. I know I've received a few in the past. However, I don't mind posting to this thread so others might learn from my mistakes.

Your 2 suggestions are valuable and ones I've considered. Believe it or not I have a ping sensor and wired it up to give measurements on the amount of oil left in my heating tank. I could search around and find a link if your interested. It work very well. In a pinch I could use this setup but it has a couple of drawbacks. First is exposure. The sugar bush is wet, cold, exposed, and wet. I'd be very concerned about the long-term stability of the system. It would have too many exposed parts. Not to mention it's be difficult to mount in the milk tank which we use as a collection/holding tank. But the real problem is the holding tank sits above the actual collection point. There's a small sump pump on a float that periodically pumps the sap from the collection tank in throughout he man-hole cover on the top of the milk tank. From there it falls into the milk tank. The surface of the sap in there is constantly bouncing and waving about. I think it could be difficult to get a decent reading. The walls are also stainless steel so I think the reflections would be crazy. I think these issues are all solvable. However, the measurement I really want is volume or at least, height of sap in tank which I can convert to volume easily. If I do these via the ping sensor, I've got to trust that my sensor is pointed straight down. With all the stuff that happens up in the woods this may not be true.

The second suggestion would give me a direct measurement and tell me exactly what I want to know. It's a bit short so I'd have to put several together to cover the whole range or go without data until the level rose to the point where my "tape" was positioned. This might be my next attempt if I can't get what I have working. I hope to have some free time to try a simple op amp setup with the pressure sensor. I don't see why that won't fit my needs, but then again, I thought the INA122 would be straight forward.

I'll keep this thread posted with my success and failures.
Peter

dre · 2012-03-06 10:39

Peter,

The secret of using the 'Ping' for liquid depth measurement, aside from weather proofing
it, is to have it measure the depth in a 'settling pond'. In my and Parallax's experiment the
sensor was placed atop a piece of 4" dia. plastic pipe, looking down into the pipe; and the
pipe inserted vertically in the stream or tank.

I have a copy of Parallax's write-up but I don't know how to add it to this thread. Here is
the title however:

[Water Level Measurement with the Ping))) Ultrasonic Distance Sensor (#28015) ]

Perhaps you will be successful at searching Parallax's files.

I don't have a photo of my setup but it was weatherproofed as it was to be mounted
outside off the side of a foot bridge. The pipe would have reached to the stream bottom.
The bottom of the pipe had small 'v' cuts to allow small amounts of water to enter and
exit thus seeing a depth in the pipe equal to the depth of the flowing water outside the
pipe, but without turbulence.

However I can relate more to strain gage based sensors, so I use them whenever possible.
[Before microprocessors I tried making a sailboat speedometer using gages mounted on a
small cantilever beam made from a piece of spring steel. But I didn't know how to get the
velocity from square root of the force. Only a few years later the US twelve meter boat
won; partly due to using a secret speedometer. It was, indeed, a cantilever beam, strain
gage based sensor which gave them the ability to sense a 1 knot full scale speed
variation; to more accurately adjust sail settings, etc. Oh well! ]

cheers, David

Working with the Honeywell Bridge Pressure Sensor- 26PCBFA6D

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