Omega Products CY7-670-CO Installation Manual

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

Sensor Mounting

General Comments

Before installing the CY7/670 series sensor,
identify which lead is the anode and which lead is
the cathode by referring to the accompanying
device drawings. Be sure that lead identification
remains clear even after installation of the sensor,
and record the serial number and location.

The procedure used to solder the connecting
leads is not very critical and there is very little
danger in overheating the sensor. If for some
reason the leads need to be cut short, they should
be heat sunk with a copper clip or needle-nose
pliers before soldering. Standard rosin-core
electronic solder (m.p. 180C) is suitable for most
applications. Applications involving the use of the
SD package up to 200 °C require a higher melting
point solder. A 90% Pb 10% Sn solder has been
used quite successfully with a rosin flux.

For all adapters except the CY, CU, and DI, the
leads are gold-plated Kovar. Prolonged soldering
times may cause the solder to creep up the gold­plated leads as the solder and the gold alloy. This
is not detrimental to the device performance.

When installing the sensor:

• Make sure there are no shorts or leakage

resistance between the leads or between the
leads and ground. CYAV varnish or epoxy may
soften varnish-type insulations so that high
resistance shunts appear between wires if
sufficient time for curing is not allowed. Teflon
spaghetti tubing is useful for sliding over bare
leads when the possibility of shorting exists.

• Avoid putting stress on the device leads and

allow for the contractions that occur during
cooling that could fracture a solder joint or lead if
installed under tension at room temperature.

The CY7/670 series sensor is designed for easy
removal for recalibration checks or replacement,
and the following discussions for each of the
adapters are geared in this direction. If semi­permanent mountings are desired, the use of OB­CY10 or OB-CY20 low temperature epoxy can
replace the use of CYAG grease. In all cases, the
mounting of the sensor should be periodically
inspected to verify that good thermal contact to the
mounting surface is maintained.

CY7/670-SD

The SD version is the basic package for the
CY7/670 series sensor line, from which all other
configurations are made using the appropriate
adapter. The base of the device has a gold
metallized surface and is the largest flat surface
on the sensor. The base is electrically isolated
from the sensing element and leads, and all
thermal contact to the sensor must be made
through the base.

A thin braze joint around the sides of the SD
package is electrically connected to the sensing
element. Contact to the sides with any electrically
conductive material must be avoided. When
viewed base down and with leads towards the
observer, the positive lead (anode) is on the right.

For a removable mount, the SD sensor can be
held against the mounting surface with the CO
adapter (see below) or similar clamping
mechanism. Any method of clamping the sensor
must avoid excessive pressure and should be
designed so that thermal contractions or
expansions do not loosen contact with the sensor.
For uses restricted to below 325 K, a thin layer of
CYAG grease should be used between the sensor
and sample to enhance thermal contact.

The SD package can also be bonded with a low
temperature epoxy. The sensor should be pressed
firmly against the surface during curing to assure a
thin epoxy layer and a good thermal contact. The
device may be removed in the future by using the
appropriate epoxy stripper.

The SD adapter can be soldered using a rosin flux
(non-corrosive) if extreme care is exercised.

1. Tin the base of the sensor using a low
wattage, temperature controlled soldering iron
that will not exceed 200 °C. Use only a
minimal amount of solder. Tin the surface to
which the sensor is to be bonded and again,
avoid an excessive thickness of solder. Clean
both the sensor and the mounting surface of
any residual flux.

2. Reheat the mounting surface to the melting
point of the solder, press the device into
position and allow the sensor to warm to the
melting point of the solder.

3. After both tinned surfaces have flowed
together, remove the heat source and let the
sample and sensor cool.

Under no circumstance should the sensor be
heated above 200 °C and the solder must be
limited to only the base of the sensor. Excess
solder running up the sides of the SD package can
create shorts. Repeated mounting and
demounting of a soldered sensor may eventually
cause wetting deterioration and ruin the thermal
contact to the sensing element, although the nickel
buffer layer should minimize these problems.

CAUTION

The preferred method for mounting the SD
sensor is either the CO adapter or bonding
with epoxy. Omega Engineering, Inc. will not
warranty replace any device damaged by a
user-designed clamp or damaged through
solder mounting.

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