# Measuring pressure diffferences

Discussion in 'OnBoard Electronics & Controls' started by rwatson, Jul 1, 2013.

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### rwatsonSenior Member

Say I have a 1:5 scale model boat, with an outboard motor.

Say I want to measure the thrust needed to achieve speeds between .5 and 5 knots.

Say I put some device between the foot of the motor and the transom, create pressure on some measuring device.

Knowing that pressure can be mathematically converted to force quite easily, if I know the diameter of the tube and density of the fluid, if I could record the height of the fluid in the tube at different speeds (on camera) I could calculate the force required to achieve the different speeds.

The big trick would be to figure out an easy to assemble mechanism to do this.

Any ideas or suggestions would be appreciated

has anyone seen any good methods ?

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### TANSLSenior Member

I do not know exactly where you're going but you should bear in mind that the pressure, CAN´T be converted mathematically into anything. The same pressure, depending on the surface to act, can lead to very different forces value. That is, in my opinion, the pressure, the same pressure, can be mechanically converted into forces of very different value.
Either I misunderstand you or you should care more concepts and how to express them in writing as, otherwise we will not be able to help.
Cheers

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### gonzoSenior Member

You can install a pressure cell between the motor and the transom. That will measure thrust directly.

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### rwatsonSenior Member

have a look at

http://www.cleavebooks.co.uk/scol/ccpress.htm

as an example.

As i read it, if you know the fluid density and diameter of a tube, say, then you can calculate the equivalent PSI, newtons, kilonewtons of force that created the height.

ie

"Pressure and force are related, and so you can calculate one if you know the other by using the physics equation, F = P/A. Because pressure is force divided by area, its meter-kilogram-second (MKS) units are newtons per square meter, or N/m2. In the foot-pound-second (FPS) system, the units are pounds per square inch, or psi."

http://www.dummies.com/how-to/content/how-to-calculate-force-based-on-pressure.html

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### rwatsonSenior Member

That would be my ideal solution, but the mechanics of setting it up, the type of cell, recording results etc is outside of my experience.

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### daiquiriEngineering and Design

What would be the type and diameter of the tube on the 1:5 scale model?
And what values of the pressure we're talking about?
Cheers

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### latestarterSenior Member

My visualisation of your proposal is to have something akin to the master cylinder on a car's braking system connected to a vertical pipe.

When balanced, the force from the outboard on the piston would equal the height of the liquid x its density x the area of the piston.

In practice it might be hard to set up.

In any case you will not get the answer directly, you will need to take moments about the outboard pivot i.e. force on measuring device x its distance from pivot = thrust x its distance from pivot.

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### rwatsonSenior Member

Indeed. Those two questions are very important.

Latestarters comments are equally important, and the 'master cylinder' concept is a good direction.

I was sitting. meditating a few minutes ago, in the 'rolls of paper' room. I thought that it might be as simple as putting some sort of rubber 'bulb', (maybe even some large diameter rubber tubing) filled with air or liquid, between the motor support leg ( see below), connected to a flexible hose to the interior of the boat.

As the pressure from the motor squashed the air filled rubber thingy, it would force air or liquid to operate some sort of gauge, or even a piston with coloured markings. I could then record the pressure with the GPS camera inside, that is recording the speed at the same time. Its the speed V pressure/force that is the info needed, after all.

All I would need to do then, would be to calibrate the force being generated with some static weights, to accurately identify the amount of force being applied.

How much pressure an electric outboard can create will be interesting to find out.

http://www.boatdesign.net/forums/hy...ulating-power-scale-model-outboard-46455.html

but it appears that the forces required to achieve the different speeds is the crucial info.

I suppose a car instrument would be way too large a capacity to use. I suspect I need a more sensitive measuring device.

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9. Joined: Nov 2007
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RW
You should be able to rent one somewhere

you could put a trim cylinder horizontal ( like a race outboard has) and just measure the pressure with cyl diameter and you have the force.
Then measure propshaft to cylinder to tilt tube for the leverage.

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### latestarterSenior Member

I like your rubber thingy and calibration concept.

Getting a frictionless seal on a piston system would be difficult and calibration gets rid of the intermediate measurements and calculations.

A liquid filled squashy bladder/pipe connected to a vertical measuring pipe, where the force is largely resisted by the stiffness of the bladder/pipe rather than the hydrostatic pressure due to the height of liquid, could be worth trying.

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### CDKretired engineer

There are a lot of semiconductor pressure sensors on the market. Look at this page for examples: http://www.alldatasheet.com/datasheet-pdf/pdf/90541/ETC/ADP1101.html

In principle any conductor (even a piece of wire) changes its resistance when stretched, but to cancel out thermal effects a bridge circuit is used. All 4 members are subjected to the same temperature changes, only one is mechanically stretched.
The simplest and cheapest ones are those used in kitchen scales; they look just like any ordinary transistor squeezed between two surfaces.

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### rwatsonSenior Member

Thats a great idea. I may be able to cannibilze a cheap set of scales to get the gear I need.

I was considering an even more low tech solution today, a bit of 'string' attached to a spring, with an arrow indicator.

I will visit some op shops tomorrow, and some discount stores, and see if I can come up with cheap digital scales.

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### daiquiriEngineering and Design

Actually no, the big trick would be:
1) to measure the pressure with an acceptable accuracy on a scale model and
2) to extrapolate the calculated data on the full-size boat.

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### rwatsonSenior Member

Link to a cheap pressure sensor I found

http://www.instructables.com/id/How-to-Make-a-Ridiculously-Cheap-Analog-Pressure-S/

Converting Pressure

http://www.convert-me.com/en/convert/pressure/millibar.html

Arduino Pressure

Pressure Sensor with bridge output

http://playground.arduino.cc/Main/SPX3058D

Arduino Forum
http://forum.arduino.cc/index.php/topic,148729.0.html

Round Force-Sensitive Resistor (FSR)

"These sensors are fairly low cost, and easy to use but they're rarely accurate. They also vary some from sensor to sensor perhaps 10%. So basically when you use FSR's you should only expect to get ranges of response. While FSRs can detect weight, they're a bad choice for detecting exactly how many pounds of weight are on them."

Accurate Arduino Pressure measurement

http://forum.arduino.cc/index.php?topic=53801.0

https://www.sparkfun.com/products/retired/8161

Tekscan Pressure Sensors

At the heart of every Tekscan pressure measurement system is an ultra-thin, tactile pressure sensor. Every Tekscan sensor is comprised of numerous individual sensing elements, or sensels. The sensel density represents the total number of sensels per unit of area. In order to use Tekscan's pressure sensors, you must have a Tekscan pressure measurement system as well as the corresponding sensor map (software driver) for the sensor.
http://www.tekscan.com/pressureSensors

http://www.tekscan.com/9801-pressure-sensor
PRESSURE RANGES
psi 5 35
kPa 34 241

Last edited: Sep 11, 2014

15. Joined: Aug 2007
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### rwatsonSenior Member

Note to self - this accurate electronic pressure sensitivity is all too hard.

Since i have to have a tilt control on the motor, it could have a spring loaded extension that I could calibrate.

It would be easy to control the scale of the measurement as well, by using longer springs.

I could even put simple detection contacts on the arm, to electronically log the pressure levels as well as record them on video.

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