Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Pressure Sensor for water level measurement

Status
Not open for further replies.
I would say that is correct.
Post your records we may be able to help.
E
 
I would think so as Eric says. You have to understand that pressure can be measured as what would be called guage pressure and absolute pressure. Guage pressure is the difference between absolute and barometric pressure. Further, you have to realize that and barometric pressures reported by the weather station are normalized to sea level. So, even your city is at 5000 ft, the pressure reported by the weather surface assumes your at sea level.

Even so, with a know dept sensor, there will be a temperature gradient in the water. That MIGHT be harder to compensate for. You can see where a pressure sensor at a known depth and temperature gradient would increase the resolution. A known fixed depth at the average temperature might be the safest place for your reference.

In a simple correction, say the 20' sensor reads 19'6" and the bottom computes to say 40', normalize.

If temperature becomes an issue, then you might require a temperature profie or even measured temperatures at various depths and "piecewise" compute the dept as layers of water at some average temperature. It might be wise to measure the dept/temperature dependence and see what the gradient really is.

A sensor at a known depth should compensate for everything BUT a variation of temperature with depth.
 
I would say that is correct.
Post your records we may be able to help.
E
I purchased the sensor from an Indian Company called "Monad Electronics". I had some discussions with them before purchasing . They were confidently telling that I can get 5cm resolution for the 50 Mts sensor. They even sent a photograph of one installation they made. Now they say that they do the calibration combined with their digitizer. For eg if the depth is 10 meters, they will adjust the equipment to show 10 meters. Then I asked how they will handle atmospheric pressure variation. They say by the following methods.

1. Analog Signal conditioning
2. Digital filtering
3. Analog linearity correction
4. Digital/software linearity correction
5. Digital/software compensation
cleardot.gif
When the atmospheric pressure variation is unpredictable, how they can make such corrections with the above steps? When I asked the question to them , their answer is
"Yes sir, otherwise there will no instruments to measure water level using pressure sensor.

Instruments are available in market and they are measuring with required accuracy"

Is it true that such external variations which are unpredictable in nature can be compensated with the 5 steps mentioned above?
 
hi,
I would say without some direct method for correcting for atmospheric pressure, their 5 methods sound doubtful to me.

Look at this PDF, its the PTX type I have use, I have marked a clip showing the required air vent pipe that is within the connecting cable.
E
 

Attachments

  • Depth-and-level-brochure-BR-004E-8.pdf
    1.9 MB · Views: 186
  • AAesp01.gif
    AAesp01.gif
    121.1 KB · Views: 140
Is it true that such external variations which are unpredictable in nature can be compensated with the 5 steps mentioned above?
If you have an atmospheric pressure sensor as well as a non-vented water-depth pressure sensor that should be possible. Is your depth sensor the type that is vented to atmosphere, as Eric mentioned in post #11?
 
hi,
I would say without some direct method for correcting for atmospheric pressure, their 5 methods sound doubtful to me.

Look at this PDF, its the PTX type I have use, I have marked a clip showing the required air vent pipe that is within the connecting cable.
E
Thank you very much.:)
 
So, your first order problem is atmospheric pressure and you don't have a differential sensor, so you have the WRONG sensor. Atmospheric pressure is a huge correction, but it's inherently done using the right sensor.

The next error is g or acceleration due to gravity. It might have a shorter component of the tides and a longer component of the earths orbit, but you may be able to live with that one. The density of the fluid may change and it can give an error too.

Back to, a correct sensor at a known depth (hanging from a bouy) can correct for g, density and even barometric pressure if they are both read at the same time.

If both sensors were differential, the difference in normalized readings would be much smaller. e.g. sensor reads 40 feet, but it really 40'. You know the correction because you know the other sensor should read 10' and it probably reads 40/41*10. So, that reference compensates for everything. Well, almost. Temperature compensation was included in the sensor.

And hopefully density doesn't change with depth.
 
I wanted to send my readings. But when I paste it directly , all borders vanish. When I tried uploading as excel/ word files, I get the message that it is not an allowed format. Then how to send the chart? Pls help.
 
Use the Snipping tool in Windows Accessories. Save it as a .png
 
If you have an atmospheric pressure sensor as well as a non-vented water-depth pressure sensor that should be possible. Is your depth sensor the type that is vented to atmosphere, as Eric mentioned in post #11?
The readings I got are given below.
upload_2015-4-20_19-54-58.png


My inference from the above is as below. Please correct me if I am wrong
  1. The external pressure sensor and the other sensor do not follow a common pattern.
  2. It may not be possible to derive a correction factor because of reason 1
  3. The pressure sensor variation may be due to other reasons related to the sensor, not because of atmospheric pressure variation alone.
  4. We may have to go for a better pressure sensor.
 
Glad to help. You are welcome.
 
You can post actual tables, but that's harder.

Tell us a bit about the measuring system. Is this a voltmeter that reads to 2000 mV? Or is it a D/A converter? How many bits? two's complement? What are you using for a reference? V+/2 or a stable reference or something else? Have you looked at the AC noise present?

What I potentially see is a "voltmeter" on the 2V scale that reads to 2.000 +- 1 digit.

I could also be seeing the effects of lead lengths and high input Z. Voltages over long distances are best converted to a current. e.g. 4-20 mA.
 
A few random thoughts...

Is this system being powered by the PSU with the spikey output as discussed in another recent thread?
If it is, then some filter capacitors on the output of the power supply may help.

In your table of figures, the drift is worse at the start of the test.
Was the DVM switched on immediately before the first reading was taken?
Could the DVM suffer from warm up drift?

In your first post you state that you are looking for a resolution of 5cm in 50m, ie 0.1%
If you are using a 3 1/2 digit DVM, that has at best a resolution of 0.05%, when everything is taken into account in the DVM specification it could be far less accurate that you think. Even a Fluke.
Do you have a better DVM, maybe 4 1/2 digit, which you could try?

Noise on the signal could be a problem.
Try a simple RC low pass filter at the input of the DVM.
I would be inclined to try something with a 0.5s (or there abouts, not critical) time constant, and see if that makes a difference.

And finally, asking the blindingly obvious questions:
Was the pressure transmitter in the water?
Did the water level stay constant?
Were there any ripples (waves) on the water surface?

JimB

PS
Of course if everything else fails, you could always use the float valve recommended by Gary to keep the waterlevel in the dam constant.:D
 
The multimeter is likely +-1 digit e.g. 1 mV and your readings are very close to that AND there is a general increasing in the "depth" colum and general decreasing in the "atmospheric pressure" column, so in a sense they are tracking.

This NASA document pretty much spells it out, https://www.grc.nasa.gov/WWW/k-12/WindTunnel/Activities/fluid_pressure.html Depth is proportional to ABSOLUTE pressure not relative.

So, everyone understands each other. What is the pressure (mV and engineering units) read on both gauges when out of the water? Second question, what is your altitude above/below sea level?

Reality says, you should repeat the experiment with a column of water. I agree, it looks like something is seriously wrong from the density of the water changing over depth to the "force of water" over a dam? Two 4" pipes will give you 20' on my side of the pond.

Based on the NASA document, you should be able to compute the absolute pressure.
 
If it is, then some filter capacitors on the output of the power supply may help.
I will try to remove spikes.
Could the DVM suffer from warm up drift?
Don't think so, but still will check again.
Do you have a better DVM, maybe 4 1/2 digit, which you could try?
I found my ADC output also changing in the same way.
Was the pressure transmitter in the water?
Did the water level stay constant?
Were there any ripples (waves) on the water surface?
I used a 4" of pipe 3Ms height filled with water for testing purpose. The sensor is kept inside. Water level is kept constant
 
OK, so the sensor is sitting at the bottom or really close to the bottom of a 3 meter vertical column of water that's capped at the bottom, right?

If this was a test was it inside where line frequency noise would be present?
Did you use twisted pair shielded cable with the shield connected at one end only or something else?
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top