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I can't imagine the calculations that correct for the fact that many units of measure in this process is derived from IPK. Such as Amps, Volts, magnetic flux. It's a lot to get my head around. Lastly, if the watt balance, and atom counting approaches are able to redefine the kilogram, how will I know it happened? It would be an exciting day.
Do you also wiggle the test-mas when in weighing mode (with an amplitude similar to that of the velocity-mode) to compensate for inhomogeneities in the B-field from the magnet, or is this done some other way? Also, how do you determine g_n accurately? This is likely the biggest source of uncertainty, yes?
Mildly confused here. It seems to me that the downward force of the test mass is dependent both upon the mass and the local gravitational field. And the upward force produced by the coil and magnet are not affected by the local gravitational field. So how does this scale actually measure mass instead of the product of mass and local gravitational acceleration? In a conventional balance scale, the local gravitational field is canceled out because both the test mass and the reference masses are both affected by the same local gravitational field. That compensation doesn't seem to be present in the watt balance.
John Cochran You’re correct. The watt balance experiment requires knowledge of the local acceleration, g, at the location of the mass. In practice, this is done by measuring the absolute value of the local acceleration at a point near the watt balance. With a separate instrument, the tie, i.e., the difference in the local acceleration between this measurement site and the mass location is determined. By combining the two measurements the value of g at the mass is obtained. This procedure adds a small relative uncertainty to the watt balance experiment. With careful experimentation, the relative uncertainty of this component can be kept below 1 part in 100 million. The above procedure might seem complicated relative to a simple balance, where, as you rightly stated, the local acceleration drops out. However, the watt balance allows one to define the unit of mass without using an artifact. This will solve many problems that one has with an artifact-based system, e.g., access to the artifact, wear of the artifact, etc.
National Institute of Standards and Technology Okay. I understand that part...we can find out g. But what if that changes? Aren't we traveling through space picking up dust as we go and getting more massive. Thus, changing the constant g? Thanks!
kyle5555 Yes, the local acceleration, g, changes. Not so much because of the dust that Earth picks up, but because of the tides generated by the sun and moon, the air pressure, and the moisture in the ground. Ideally one would measure little g continuously to have an updated value all the time. It is also possible to use models to calculate these changes. Changes due to the tides and air pressure are well understood and can be corrected for.
Given that they need to measure the local g, it seems to me that a long tube pumped to vacuum that you precisely time the fall of an object over a measured distance ought to work. We can measure time to within about 2 parts in 10^14, so that part of the measurement in g is easy enough and as for distance, that would be a tad trickier since ideally, the distance measurement ought to be non-contact. Perhaps the breaking of a light beam going across the tube with the light being obstructed by a slit? In any case, it ought to be possible to measure the local acceleration quite accurately.
The magnetic flux density varies relatively by about 1 part in 10,000 for about 4 cm of the travel. The field profile is very uniform at the weighing position itself. There, it changes relatively by about 2 parts per 10,000,000 per mm .
Thanks for checking out our videos. Please add your comments and let us know what you think.We will be reviewing and then posting comments as long as they are on topic, respectful and do not promote specific products or services.
Thanks for this excellent video! What is the expanded uncertainty ( U ) in measurement of 1kg mass. standard?
I can't imagine the calculations that correct for the fact that many units of measure in this process is derived from IPK. Such as Amps, Volts, magnetic flux. It's a lot to get my head around. Lastly, if the watt balance, and atom counting approaches are able to redefine the kilogram, how will I know it happened? It would be an exciting day.
Do you also wiggle the test-mas when in weighing mode (with an amplitude similar to that of the velocity-mode) to compensate for inhomogeneities in the B-field from the magnet, or is this done some other way? Also, how do you determine g_n accurately? This is likely the biggest source of uncertainty, yes?
Hi nice 1..... so what are the sources of Uncertainties that you have identified and evaluated so far?
How to compensate the wheel friction?
Mildly confused here. It seems to me that the downward force of the test mass is dependent both upon the mass and the local gravitational field. And the upward force produced by the coil and magnet are not affected by the local gravitational field. So how does this scale actually measure mass instead of the product of mass and local gravitational acceleration?
In a conventional balance scale, the local gravitational field is canceled out because both the test mass and the reference masses are both affected by the same local gravitational field. That compensation doesn't seem to be present in the watt balance.
John Cochran You’re correct. The watt balance experiment requires knowledge of the local acceleration, g, at the location of the mass. In practice, this is done by measuring the absolute value of the local acceleration at a point near the watt balance. With a separate instrument, the tie, i.e., the difference in the local acceleration between this measurement site and the mass location is determined. By combining the two measurements the value of g at the mass is obtained. This procedure adds a small relative uncertainty to the watt balance experiment. With careful experimentation, the relative uncertainty of this component can be kept below 1 part in 100 million.
The above procedure might seem complicated relative to a simple balance, where, as you rightly stated, the local acceleration drops out. However, the watt balance allows one to define the unit of mass without using an artifact. This will solve many problems that one has with an artifact-based system, e.g., access to the artifact, wear of the artifact, etc.
National Institute of Standards and Technology Okay. I understand that part...we can find out g. But what if that changes? Aren't we traveling through space picking up dust as we go and getting more massive. Thus, changing the constant g? Thanks!
kyle5555 Yes, the local acceleration, g, changes. Not so much because of the dust that Earth picks up, but because of the tides generated by the sun and moon, the air pressure, and the moisture in the ground. Ideally one would measure little g continuously to have an updated value all the time. It is also possible to use models to calculate these changes. Changes due to the tides and air pressure are well understood and can be corrected for.
How it is possible to measure g with such precision?
Given that they need to measure the local g, it seems to me that a long tube pumped to vacuum that you precisely time the fall of an object over a measured distance ought to work. We can measure time to within about 2 parts in 10^14, so that part of the measurement in g is easy enough and as for distance, that would be a tad trickier since ideally, the distance measurement ought to be non-contact. Perhaps the breaking of a light beam going across the tube with the light being obstructed by a slit? In any case, it ought to be possible to measure the local acceleration quite accurately.
Neato! What is the variation in magnetic strength?
The magnetic flux density varies relatively by about 1 part in 10,000 for about 4 cm of the travel. The field profile is very uniform at the weighing position itself. There, it changes relatively by about 2 parts per 10,000,000 per mm .
Thanks