# How to measure pulsations in a tube?

## Main Question or Discussion Point

Hey guys/gals so here is my problem, I am trying to be able to passively measure the mechanical vertical deflection of fluid passing through an elastic tube approx 1cm in diameter with 0.5mm thick wall that is embedded in to the surface of a gelatin mold. Pressure of the flow through the tube varies from a high of 220mmHg down to no flow but can be anywhere between those two value. The flow through the elastic tube occurs in a pulsatile pattern.

I was thinking of just taping a flat strain gauge or thin filament type stretch sensor to the tube perpendicular to the direction of flow to measure the vertical deflection and then correlating the vertical deflection/pulse to the pressure of flow below.

The end readout I'm trying to get is just the pressure of the pulsatile flow in the elastic tube. The physical constraint is that it has to be something I can essentially tape to the surface of the tube pretty easily.

Any ideas? Geniuses out there. . . assemble!

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anorlunda
Mentor
the mechanical vertical deflection of fluid passing through an elastic tube
That's unclear. Is the tube horizontal or vertical? Are you talking about deflection in the direction of flow or the direction normal to flow?

Is the gelatin transparent so that you could view the tube with a video camera?

PhysicsBabexo7
That's unclear. Is the tube horizontal or vertical? Are you talking about deflection in the direction of flow or the direction normal to flow?

Is the gelatin transparent so that you could view the tube with a video camera?

Thanks for getting back to me.

The gelatin is not transparent. So you can't optically see the pulsation too easily. Also I think a camera would be too big anyway. Whatever gauge is used to measure the stretch would essentially have to be able to be embedded in a 1inch x 0.5inch piece of tape and then hook up an output wire to a monitor or computer.

The tube is horizontal running just beneath the surface of the gelatin mold.
Deflection in flow I mean with the pulsations there should be an expansion of the elastic tube perpendicular to the direction of flow as the pulse flows through the tube. Since the elastic tube is just below the surface of the gelatin mold it thought taping a thin gauge of some sort (don't know which "gauge" would really be using here or if a strain gauge is the best choice) across the elastic tube perpendicular to the direction of flow would let me measure that pulsation.

jrmichler
How elastic is your tube? At your pressures, a metal tube will not have enough strain to get good measurements. A rubber tube of the correct modulus will have enough strain, but a normal foil type strain gauge will stiffen it. Getting good measurements with a strain gauge may not be practical.

What is the maximum frequency component of your pulsations? That is also important.

What is the calculated / measured displacement of the tube at maximum pressure? If it's, for example, more than 0.1 mm, then a laser sensor might work. Use it to directly measure the diametral expansion of the tube. Micro-Epsilon (www.micro-epsilon.com) makes good laser sensors with resolution less than 1 micron.

PhysicsBabexo7
anorlunda
Mentor
If it is a one time measurement, then you could probably do it without the gelatin. I'm guessing the gelatin won't change the deflection very much. Without the gelatin present, you could use a video, even a cell phone video, to make a pretty good measurement.

If it is a continuous measurement, then you could put something analogous to a pin sticking up from the tube above the gelatin, then measure the wiggles in the pin.

There are also various types of proximity sensors, magnetic, capacitive, sonic, and so on. They would give continuous signals, but they would need to be calibrated.

In short, there seems to be many possible methods that sound more promising than a stain gauge.

PhysicsBabexo7
How elastic is your tube? At your pressures, a metal tube will not have enough strain to get good measurements. A rubber tube of the correct modulus will have enough strain, but a normal foil type strain gauge will stiffen it. Getting good measurements with a strain gauge may not be practical.

What is the maximum frequency component of your pulsations? That is also important.

What is the calculated / measured displacement of the tube at maximum pressure? If it's, for example, more than 0.1 mm, then a laser sensor might work. Use it to directly measure the diametral expansion of the tube. Micro-Epsilon (www.micro-epsilon.com) makes good laser sensors with resolution less than 1 micron.
Lets say the baseline pressure in the tube is 70mmHg and that has zero displacement of the near and far wall of the tube. By near I mean the wall of the tube closest to our gauge and the far wall is the side opposite that. When pressure in the tube increased to lets say 130mmHg with the pulsation, the displacements of the walls are as follows (this is the best numbers I can come up with):
Near wall moves up toward the gauge by 0.4mm. The far wall moves away by 0.35mm as well. I believe the difference in movements is due to the lower portion trying to move against the gelatin while the upper wall is pushing against maybe a 0.5mm thick layer of gelatin.

The frequency of the pulsations is up to 5Hz.

The laser is a neat idea but 1) I need something that is cheap to make for one time use and toss it in the garbage due to contamination 2) it needs to be something that can be taped across the tube in a rush so I wouldn't have the time to accurately place a laser. By thought with the gauge was that as long as some part of the sensor band was taped to the surface above the tube, that it would be able to detect and measure the pulsations with some degree of accuracy.

If it is a one time measurement, then you could probably do it without the gelatin. I'm guessing the gelatin won't change the deflection very much. Without the gelatin present, you could use a video, even a cell phone video, to make a pretty good measurement.

If it is a continuous measurement, then you could put something analogous to a pin sticking up from the tube above the gelatin, then measure the wiggles in the pin.

There are also various types of proximity sensors, magnetic, capacitive, sonic, and so on. They would give continuous signals, but they would need to be calibrated.

In short, there seems to be many possible methods that sound more promising than a stain gauge.

Thanks for the ideas,
Unfortunately we can't have anything penetrate the tube. So the reason why I was thinking a strain gauge was that 1) I need something that is cheap to make for one time use and toss it in the garbage due to contamination. I could use a Doppler device on a miniature scale but the cost was prohibitive. So I came back to a strain gauge or something dirt cheap. 2) it needs to be something that can be taped across the tube in a rush because in the use of it I wouldn't have the time to accurately place the more finely tuned items. In the project we only have about 3 seconds to place the device on the tube. My thought with the gauge was that as long as some part of the sensor band was taped to the surface above the tube, that it would be able to detect and measure the pulsations with some degree of accuracy.

anorlunda
Mentor
It would take calibrations or perhaps calculations to translate the strain gauge signal variations into vertical displacement values.

Here is an example of a sub $10 sensor kit for the Arduino computer. • Ultrasonic hc-sr04 distance measuring transducer sensor • HC-SR04 consists of ultrasonic transmitter, receiver, and control circuit. When trigged it sends out a series of 40KHz ultrasonic pulses and receives echo from an object. • ~Power supply: 5V DC; quiescent current: less than 2mA; effectual angle: less than 15°; distance: 2cm500cm; resolution: 0.3 cm~ • Package Content: 5pcs HC-SR04 This particular one with 3mm resolution is probably too coarse. But the point is that you should at least spend some time researching proximity sensors. They are many and varied, and many of them inexpensive. It would take calibrations or perhaps calculations to translate the strain gauge signal variations into vertical displacement values. Here is an example of a sub$10 sensor kit for the Arduino computer.
• Ultrasonic hc-sr04 distance measuring transducer sensor
• HC-SR04 consists of ultrasonic transmitter, receiver, and control circuit. When trigged it sends out a series of 40KHz ultrasonic pulses and receives echo from an object.
• ~Power supply: 5V DC; quiescent current: less than 2mA; effectual angle: less than 15°; distance: 2cm500cm; resolution: 0.3 cm~
• Package Content: 5pcs HC-SR04
This particular one with 3mm resolution is probably too coarse. But the point is that you should at least spend some time researching proximity sensors. They are many and varied, and many of them inexpensive.

I think the difficulty with this is that the measuring device would have to be taped down to the surface of the gelatin containing the tube. Sorry I should have said that above with the constraints of the project. It seems the proximity sensor would have to have some standoff distance to measure the wall movement.

Tom.G