NIST-4 Kibble balance - photo

**Altair** · Mar 10, 2020, 04:42 PM

The NIST-4 Kibble balance was used to measure Planck's constant to within 13 parts per billion to help redefine the kilogram.

More:

[url]https://en.wikipedia.org/wiki/2019_redefinition_of_the_SI_base_units#Kilogram[url]

Previously:

Definition of kilogram, amp, and kelvin set to change forever

**NeiljohnUK** · Mar 11, 2020, 11:50 AM

For such a large chamber that's an awfully small turbo pump in the foreground.

**IntheGroove** · Mar 11, 2020, 04:43 PM

Once the dome is installed, that pump can probably get that to negative pressure rather efficiently...

**NeiljohnUK** · Mar 12, 2020, 05:49 AM

I work with high vacuum systems, for a chamber that size I'd expect a much bigger turbo and at least one double stage backing pump, though I suspect the intent isn't 10 to the -6 or harder, just absence of atmospheric interference.

**stephanb** · Dec 9, 2020, 09:36 PM

The pump size, or rather pumping speed is largely set by one of two things: Through-put, that is when the system is operating, how much gas is being added that needs to be pumped away in order to maintain the desired operating pressure. Many system like the Kibble balance have no gas being added. In that case, a modest size turbo pump can be used in the roughing stage to bring the pressure down to where getter pumping can be used. The smaller turbo pump is actually an advantage as it makes leak detection easier.