jmw
Industrial
- Jun 27, 2001
- 7,435
In another forum, a question arose about caustic blending and mass flow meters.
It is recommended that an FAQ is posted based on some of the answers. I have decided to break this down a bit and test some of the material out here first, so would apreciate some feedback.
The first thing is to understand how a coriolis mass meter works. I was never convinced by those early diagrams of a man walking on a gramaphone record and leaning in toward the centre, and besides, as engineers I am sure most would like to feel things and take them apart before worrying about the physics.
Part fo the reason for this is that even in an ISA instrument handbook we see vibrating element density meters described as "coriolis density" meters. A more appropriate description is "digital density meters" following the lead from the API standards.
The fact is that coriolis effect and density measurement based on frequency are totally unrelated. In fact, it isn't even necessary to vibrate the tubes to obtain the coriolis effect. It happens to be a convenient way of constructing an industrial sensor. Density, of course, has everything to do with resonant frequency and is a useful benefit from designing mass flow meters to work the way they do.
If you want to understand why vibration is not essentail to coriolis effects but useful in a flow meter, try this experiment:
The conclusions will surprise, I think.
1. Go and find a garden hose and hold a generous loop of hose in one hand.
2. Then start to swing the loop gently backwards and forwards.
3. Now get someone to turn on the water and watch what happens.
Does the loop twist?
Before you let go of the hose, let us try one more experiment and see what we can deduce:
1. With flow through the loop, stop swinging it and allow the loop to come to rest.
2. Now get your assistant to grab the bottom of the loop and pivot it up so the loop is now parallel with the ground. OK, it will sag a bit in places so two hands are better than one.
3. Now ask your assistant to let go of the loop cleanly i.e. without imparting any twist, while you continue to hold the other side of the loop. The loop will now swing down to the vertical.
Does it twist as it goes?
Does the twist accentuate as the loop accelerates?
What do we conclude?
Please let me have some feedback.
JMW
Eng-Tips: Pro bono publico, by engineers, for engineers.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
It is recommended that an FAQ is posted based on some of the answers. I have decided to break this down a bit and test some of the material out here first, so would apreciate some feedback.
The first thing is to understand how a coriolis mass meter works. I was never convinced by those early diagrams of a man walking on a gramaphone record and leaning in toward the centre, and besides, as engineers I am sure most would like to feel things and take them apart before worrying about the physics.
Part fo the reason for this is that even in an ISA instrument handbook we see vibrating element density meters described as "coriolis density" meters. A more appropriate description is "digital density meters" following the lead from the API standards.
The fact is that coriolis effect and density measurement based on frequency are totally unrelated. In fact, it isn't even necessary to vibrate the tubes to obtain the coriolis effect. It happens to be a convenient way of constructing an industrial sensor. Density, of course, has everything to do with resonant frequency and is a useful benefit from designing mass flow meters to work the way they do.
If you want to understand why vibration is not essentail to coriolis effects but useful in a flow meter, try this experiment:
The conclusions will surprise, I think.
1. Go and find a garden hose and hold a generous loop of hose in one hand.
2. Then start to swing the loop gently backwards and forwards.
3. Now get someone to turn on the water and watch what happens.
Does the loop twist?
Before you let go of the hose, let us try one more experiment and see what we can deduce:
1. With flow through the loop, stop swinging it and allow the loop to come to rest.
2. Now get your assistant to grab the bottom of the loop and pivot it up so the loop is now parallel with the ground. OK, it will sag a bit in places so two hands are better than one.
3. Now ask your assistant to let go of the loop cleanly i.e. without imparting any twist, while you continue to hold the other side of the loop. The loop will now swing down to the vertical.
Does it twist as it goes?
Does the twist accentuate as the loop accelerates?
What do we conclude?
Please let me have some feedback.
JMW
Eng-Tips: Pro bono publico, by engineers, for engineers.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.