Pond Engineering's Primary Fixed Point Cells (Indium, Tin, Zinc, Aluminum and Silver on the ITS-90) combine outstanding precision, repeatability and accuracy with superb durability and ease of use to produce a uniquely convenient, reliable cell. Following the cell geometry described in NIST Technical Note 1265, these cells contain approximately 1.2kg of high purity metal (0.4kg for Aluminum and 1.6kg for Silver) in a machined graphite crucible with a graphite thermowell liner sealed within a quartz glass envelope.
Unique to Pond Engineering's cells is a set of graphite "springs" directly above the crucible. These springs allow the thermowell to float and the crucible to expand more than 1/8" without stressing the envelope. Two glass spacers above the crucible and thermowell liner maintain proper relative positioning of the thermowell liner. Without this precaution, mechanical forces or hydrostatic pressures developed during heating can easily rupture the crucible, thermowell liner, or glass envelope; destroying both cell and thermometer.