VT PBN sample holder issue

One of screws has been broken when it was unscrewed since the nut is stuck with the bolt thread. This screw should be replaced. Other parts look fine.

pBN VT heater test

Test of VT pBN heater plate

• Remove old sample, re-attach top plate, verify thickness of assembly and heater brushes
• 7.6 mm total, 2.9-3.0 from top of brush to bottom of top plate
• 17 ohm left brush (seen from handle) to ground. Right brush grounded.
• Sample Heating Selector to "Direct Current"
• Initial 1.6 V at 0.16 A (10 ohm), gentle pressure rise, manipulator TC increases linearly with time 22 to 28 C over 6 minutes, 0.25 W
• Next 3.24 V 0.40 A 1.3 W 8.1 Ohm
• 4.49 V 0.65 A 2.96 W 6.7 Ohm, temp rises slowly with time
• 5.73 V, 1.04 A, 5.96 W 5.7 Ohm, current rises to 1.18, pressure low 1e7 outgassing
• More tests
• Set 6.26 V 1.05 A Monday 9:23 am
• by 10:14 current limited at 1.49 A, 5.7s C 8.5 W, stage at 270 C
• pressure rose to 1e-7 then down to 2e-9
• increase current to 2.01 A, initial 7.43 V drops to 7.23 steady state, 14.5 W
• stage steady state 332 C. pBN heater uniform "cherry red" 
• contact bars also hot, darker red
• heat shield, top plate, sample straps do not glow
• pressure has dropped to 1e-9
• Set 1.49 A, 5.55 V increases to 5.70, initially 3.72 Ohm -> 3.83
• heater "incipient red" just at visible glow threshold
• steady state base 280 C
• 8.5 W

Photo of heater at 14.5 Watts, pBN "cherry red"

Approximate sample temp vs power for pBN VT holders