Method and device for distinguishing material by thermal conductivity

A thermal conductivity testing device comprising:

• a probe having a sample contacting head;
• means for applying thermal power to the sample contacting head;
• a temperature sensing electrical resistor for sensing the temperature of the sample contacting head;
• and an electrical circuit for determining the thermal conductivity of the sample as a function of the resistance change in the resistor with a change in thermal power to the sample contacting head between two levels, wherein the electrical circuit includes an amplifier which receives a signal indicative of the electrical resistance of the temperature sensing resistor, the gain of the amplifier is dynamically controlled to provide a predetermined output when the thermal power to the sample contacting head is at a first level, and the output from the circuit is derived from the output of the amplifier at that controlled gain when the thermal power is at a second level.

A method of determining the thermal conductivity of a test material comprising the steps of:

• holding a test probe in thermal contact with the test material until the test material approaches a first thermal equilibrium and sensing the resistance of a resistor in the test probe;
• subjecting the test material to a predetermined level of heat flow to or from the test probe until the test material approaches a second thermal equilibrium and sensing the resistance of the resistor in the test probe;
• and
• providing an indication of the thermal conductivity of the test material as a function of the difference in sensed resistances weighted by a sensed resistance by means of an amplifier which receives a signal indicative of the electrical resistance of the temperature sensing resistor, the gain of the amplifier being dynamically controlled to provide a predetermined output when the thermal power to the sample contacting head is at a first level, and the output from the circuit being derived from the output of the amplifier at that controlled gain when the thermal power is at a second level.

A method of distinguishing gems by determining the thermal conductivity of a gem comprising the steps of:

• holding the test probe in thermal contact with the gem;
• cyclically subjecting the gem to two levels of heat flow to or from the test probe;
• measuring the difference in probe temperatures at the two levels of heat flow;
• and
• providing a continuous indication of the difference in probe temperatures, that indication being updated with each cycle of heat flow to make the indication independent of changes in temperature of the gem which changes are slow relative to the cycle time.

A thermal conductivity testing device suitable for distinguishing a sample gem comprising:

• a probe having a high thermal conductivity sample contacting head of thermal conductivity of at least about two watts per centimeter per degree Kelvin for making contact with a sample gem, the thermal conductivity and thermal mass of the head being such that the head time constant for temperature change is in the order of one second;
• means for changing the heat flow to or from the sample contacting head cyclically between first and second predetermined levels, the cycle period being in the order of one second;
• a relatively high resistance thermal path from the sample contacting head to the main body of the probe, the thermal resistance of that path being less than the resistance of the surrounding environment for preventing excessive heating of the head when it is not contacting the sample;
• temperature sensing means for sensing the temperature of the sample contacting head;
• and
• an electrical circuit responsive to the temperature sensing means for providing an indication of change in temperature of the sample contacting head, the circuit including means for compensating for the temperature of the contacted gem and means for updating the indication with each cycle of heat flow.

A thermal conductivity testing probe having a small thermal mass tip with heating and temperature sensing means at the end of a larger main body of the probe, the probe characterized in that:

• the tip comprises a heat flow element, a separate temperature sensing element, and a high conductivity sample contacting head in close thermal contact with both the heat flow element and the temperature sensing element, the thermal conductivity of the sample contacting head being at least about two watts per centimeter per degree Kelvin;
• the thermal path from the tip to the main body of the probe is of relatively high thermal resistance but of less resistance than that of the surrounding environment for preventing excessive heating of the the head;
• and
• the thermal mass and thermal conductivity of the elements forming the tip are such that the head time constant of the tip for temperature changes is in the order of one second.

A thermal conductivity testing device suitable for distinguishing a sample gem comprising:

• a probe having a high thermal conductivity sample contacting head of thermal conductivity of at least about two watts per centimeter per degree Kelvin for making contact with a sample gem, the thermal conductivity and thermal mass of the head being such that the head time constant for temperature change is in the order of one second;
• means for changing the heat flow to or from the sample contacting head cyclically between first and second predetermined levels, the cycle period being in the order of one second;
• a relatively high resistance thermal path from the sample contacting head to the main body of the probe, the thermal resistance of that path being less than the resistance of the surrounding environment for preventing excessive heating of the head when it is not contacting the sample;
• and
• temperature sensing means for sensing the temperature of the sample contacting head.