Omega Speaker Systems CY670 Series manual Application Notes, Installation And Operation

CY7/670 Series Temperature Sensors

Application Notes

M-4447/0307

INSTALLATION AND OPERATION

Three aspects of using a temperature sensor are critical to its optimum performance:

•the proper electrical and thermal installation of the connecting leads that run to the sensor

•the actual mounting of the sensor to the sample assembly

•the measurement electronics used for reading and recording temperature data from the sensor

Connecting Leads

Although the majority of the CY7/CY670 series sensors are two-lead devices, measurements are preferably made using a four-wire configuration to avoid all uncertainties associated with lead resistance. This is done by using four connecting leads to the device and connecting the V+ and I+ leads to the anode and the V– and I– leads to the cathode as shown in Figure 1. The exact point at which the connecting leads are soldered to the device leads results in negligible temperature measurement uncertainties.

In a two-wire measurement configuration, the voltage connections (point A in Figure 1) are made near or at the current source, so only two leads are actually connected to the device. Some loss in accuracy can be expected since the voltage measured at the voltmeter is the sum of the diode voltage and the voltage drop across the connecting leads. The exact temperature uncertainty will depend on the temperature range and lead resistance. For a 10-ohm lead resistance, the diode voltage will be offset by 0.1 mV, which gives a negligible temperature error at liquid helium temperature but a 50 mK error near liquid nitrogen temperature. Note the PI and CY adapter can be used only in a two-wire configuration.

An excessive heat flow through the connecting leads to any temperature sensor can create a situation where the active sensing element (for the CY7/670 series this is the diode chip) is at a different temperature than the sample to which the sensor is mounted. This is then reflected as a real temperature offset between what is measured and the true sample temperature. Such temperature errors can be eliminated by proper selection and installation of the connecting leads.

In order to minimize any heat flow through the leads, the leads should be of small diameter and low thermal conductivity. Phosphor-bronze or manganin wire is commonly used in sizes 32 or 36 AWG. These wires have a fairly poor thermal conductivity yet the resistivities are not so large as to create any problems in four-wire measurements.

Lead wires should also be thermally anchored at several temperatures between room temperature and cryogenic temperatures to guarantee that heat is not being conducted through the leads to the sensor. A final thermal anchor at the sample itself is a good practice to assure thermal equilibrium between the sample and the temperature sensor. Note that the CU, CY, SO, and DI mounting adapters serve as their own sample thermal anchor.

I the connecting leads have only a thin insulation such as vinyl acetal or other varnish type coating, a simple thermal anchor can be made by winding the wires around a copper post or other thermal mass and bonding them in place with a thin layer of CYAV varnish. There are a variety of other ways in which thermal anchors can be fabricated; a number of guidelines can be found in detail in the following references.

Figure 1. Four-Wire Configuration for CY7/670 Series Sensor Installation