First, thank you for your genuine interest in this!
I'm going to answer the question as best I can, seeing I don't have all the info.
Studiot said:
This is more useful information.
Does this imply you can't insert anything into the flow?
Seeing as this is a preliminary study done by us (students) we are going to be using water as a liquid, but the long term goal of the project is using a liquid that has the same viscosity as blood. So in a way, it doesn't quite matter.
Studiot said:
What are the properties of the tube?
That is
Is it flexible enough to distort sufficiently to measure with a strain gauge?
It -should- be flexible enough to be measured with a strain gauge, but so far I haven't managed to find one that is thin yet flexible, at least not flexible enough for a small gauge to measure it. That being said, it is flexible enough to easily bend by hand, I don't know if that is enough for it to be detected by the gauge.
One idea I had was to put the gauge inside the tube, to be affected by the flow, but that would probably cause short circuit problems.
Studiot said:
Can strain gauges (you normally need more than one) be attached or is it some type of plastic that you can't stick to (strain gauges have to be mechanically bonded to the item being strained)?
If they can't be glued could a suitable collar be fashioned?
Hmm, the tube I have in mind is plastic of sorts. I have yet to try to glue a strain gauge to it.
Studiot said:
How fast will the pressure change?
Do you want absolute pressure or just variations?
What sort of speed and frequency of measurement is required?
As fast as a human's blood pressure could change, so anywhere between 60mmHG and 120mmHG.
As for the speed and frequency, it will depend on how fast the pressure could change. So every heart beat, so.. around 1-2Hz or so?
Studiot said:
For instance a resistance strain gauge will not be affected by the magnetic fields you mention. They can be manually balanced (slow + cheap) or electronically monitored by equipment ouside the mag field (fast, convenient but expensive).
I've been doing my research on the strain gauges and already found a strain gauge that is resistant to magnetic fields. I found that a constantan alloy, and consists of two identical grids, with one stacked directly above and insulated from, the other. That way they cancel out the noise.
Finding it has lead me to this question, as I don't know how to figure out it's measuring accuracy.
Studiot said:
What is the temperature range of the fluid?
Either 20-30 degrees or 30-40 degrees, depending if we go with room temperature or that of the human body.
Studiot said:
Anything else relevant you can think of?
Not really! Quite honestly you gave me a lot of questions to think about myself and which I'll be asking the prof I'm working with! So thank you for that!
Edit: Saw your reply after I posted Aleph!
AlephZero said:
You are talking about very small changes of pressure here. The order of magnitude is
1 atmosphere = 760mm Hg = about 100 kPa
So 0.1mm Hg is about 13 Pa.
That will produce a very small strain unless you have a flexible pipe, but a flexible pipe could give calibration problems by not staying circular (i.e. the strain gauge is stiff compared with the pipe).
You might be able to use a laser vibrometer (LDV) to measure the change in shape of the pipe directly (to an accuracy of nm, or even pm with special image processing software). They can measure steady-state as well as dynamic effects.
Yes unfortunately it has to be quite small.
I've never heard of LDV, so I will do some searching on that and get back to you!