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Omega Engineering CY670 Series Installation Manual

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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