So, I decided I was going to build a ramjet but before I started build it, I wanted a method to test the thrust of it. Having a rocketry background, I have a basic knowledge of load cells and their setup, but I wanted to do this on the cheap. I did a little search and came up with an analog thrust gauge that depending on what parts you get can measure thrust from a few pounds to dozens (if not hundreds) of pounds.
It is all centered around a water pressure gauge I picked up from Lowes (local home improvement store) that will measure water pressure from 0psi to 100psi(about US$6.00). Add a 1/4MIP to 3/8th barb connector (about US$1.00) and it can plug into a length of 7/16thOD 5/16thID vinyl tubing (US$0.18 per foot) I then took a trip to all the local drug stores to find a syringe but the largest I could find was 10ml (cc) (CVS gave me 2 of them for free...can't beat that) This would work, but not with the existing tube and connector I had, so I searched some more. At Tractor Supply Company, I found a 60ml (cc) syringe(about US$2.50 for a pair) that would fit the tubing and connectors perfectly. Filling the tube and syringe with water, I can now push on the plunger and watch the needle on the gauge move. The only problem was, since the syringe is so large, the movement wasn't very dramatic (up to 40psi by pushing all my weight on it) I have a thrust gauge, but it was too stiff for the low powered ramjet I was going to initially test. I then went back to Lowes to find some 1/4thOD 3/16thID tubing and 1/4MIB to 3/16th barb connector. Finding these, I made another gauge using the 10 ml (cc). This one I can press with my thumb and get it to go to 50psi. I'll need to calibrate them using different known weights, but I think that these will give me a cheap/easy thrust gauge for whatever I need.
I'll need to add these to a test stand, and thats my next project (maybe this weekend)
-Aaron
simple/cheap thrust gauge
Moderator: Mike Everman
simple/cheap thrust gauge
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larry cottrill
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Simple/Cheap Thrust Gauge
Aaron -
This kind of hydraulic gauge will work, but it has one serious downside: There is a minimum friction force at the syringe required to get the gauge to start moving. This would be what's known as a "systematic error in measurement". This is not a killer, but you should be aware of it, especially since you are trying to measure small thrust values. It will result in a measured value that is LESS THAN the actual thrust force. You might be able to get a reasonable figure for it by doing the following:
Put a little water or oil (whatever fluid you're going to use) in the syringe with a small length of clear tubing attached, and slowly get the air out into the tube so there's noting but liquid in the syringe. Using a small, accurate scale (like a postal scale), put the syringe, plunger side down, on the scale and 'tare' the scale (set it to zero), then gradually push down on the syringe body until the plunger "breaks loose" and starts to move liquid. This will happen somewhat suddenly, which is why you want to increase your pressure gradually until it happens. Note the scale force at that moment, when the syringe starts to slide down on the plunger - that's the 'static friction' of the device. This is the FORCE you would ADD to every measurement you make with that particular syringe.
As you've discovered, the smaller the syringe cross-section, the easier it is to get pressure! That's because pressure is force DIVIDED BY area. It explains why the hydraulic cylinders at the wheels in your car's braking system are fairly large, while the 'master cylinder' behind the pedal is fairly small. Each different syringe you use will have a different force-to-gauge pressure ratio, and you will need to determine that to use them.
L Cottrill
This kind of hydraulic gauge will work, but it has one serious downside: There is a minimum friction force at the syringe required to get the gauge to start moving. This would be what's known as a "systematic error in measurement". This is not a killer, but you should be aware of it, especially since you are trying to measure small thrust values. It will result in a measured value that is LESS THAN the actual thrust force. You might be able to get a reasonable figure for it by doing the following:
Put a little water or oil (whatever fluid you're going to use) in the syringe with a small length of clear tubing attached, and slowly get the air out into the tube so there's noting but liquid in the syringe. Using a small, accurate scale (like a postal scale), put the syringe, plunger side down, on the scale and 'tare' the scale (set it to zero), then gradually push down on the syringe body until the plunger "breaks loose" and starts to move liquid. This will happen somewhat suddenly, which is why you want to increase your pressure gradually until it happens. Note the scale force at that moment, when the syringe starts to slide down on the plunger - that's the 'static friction' of the device. This is the FORCE you would ADD to every measurement you make with that particular syringe.
As you've discovered, the smaller the syringe cross-section, the easier it is to get pressure! That's because pressure is force DIVIDED BY area. It explains why the hydraulic cylinders at the wheels in your car's braking system are fairly large, while the 'master cylinder' behind the pedal is fairly small. Each different syringe you use will have a different force-to-gauge pressure ratio, and you will need to determine that to use them.
L Cottrill
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El-Kablooey
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GRIM
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THRUST MEASUREMENT
Hello Everyone , first post for me
Thought I would share my “home made thrust measuring device” with you
This came about by not being able to locate a suitable spring balance at any of the local hardware stores (all way too big )
The photos should explain better than words can , this particular device is made from ½” and 3/8” copper plumbing pipe that I had laying around therefore zero cost,
larger devices could be easily made using suitable EMT , shower rail , whatever
It was calibrated using 1 kg bags of sugar from the supermarket , a mark with a felt pen , then a very light groove with the tubing cutter serves as the scale
Don’t get me wrong , load cells and analogue chart plotters are way better , but not always within the resources or budget of many people
Anyway that’s it for now
Thought I would share my “home made thrust measuring device” with you
This came about by not being able to locate a suitable spring balance at any of the local hardware stores (all way too big )
The photos should explain better than words can , this particular device is made from ½” and 3/8” copper plumbing pipe that I had laying around therefore zero cost,
larger devices could be easily made using suitable EMT , shower rail , whatever
It was calibrated using 1 kg bags of sugar from the supermarket , a mark with a felt pen , then a very light groove with the tubing cutter serves as the scale
Don’t get me wrong , load cells and analogue chart plotters are way better , but not always within the resources or budget of many people
Anyway that’s it for now
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