Capacitative Detection of Phase Separation in 3He-4He Mixtures near the Tricritical Point

We have used two different types of capacitative sensors for detecting the phase separation boundary in 3He-4He mixtures. We have used a parallel plate geometry to make bulk concentration measurements, and we have developed inter-digital capacitors (IDC's) to make local concentration measurements. The IDC's were developed as part of our on going effort to study the tricritical point in the absence of acceleration. The IDC's should be well suited for making measurements of the phase separation on the 3He rich side of the phase diagram where the minority 4He rich phase will grow out from the walls. The resolution and advantages of these two types of sensors are presented below along with preliminary ground based measurements of the phase separation boundary taken with the IDC's.