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Temperature Sensors &
LST Matching Networks


Resistance thermometry is a widely employed method of measuring temperature, and is based on using a material whose resistivity changes as a function of temperature. Resistance Temperature Detectors (RTD's) have fast response time, provide absolute temperature measurement (since no reference junctions are involved), and are very accurate. Their measurement circuits are relatively simple, and the sensors, when properly installed, are very stable over years of use.

Micro-Measurements resistance temperature sensors are constructed much like wide-temperature-range strain gages. The standard sensors utilize nickel or nickel/manganin grids, although special-purpose gages are also available in Balco alloy or copper foil grids. These Micro-Measurements temperature sensors are bonded to structures using standard strain gage installation techniques, and can measure surface temperatures from -452 to approximately +500° F (-269 to +260° C). Because of their extremely low thermal mass and the large bonded area, the sensors follow temperature changes in the structural mounting surface with negligible time lag.

For a complete description of the operational characteristics Micro-Measurements temperature sensors and LST matching networks, see Tech Note TN-506, Bondable Temperature Sensors .

TG Temperature Sensors
TG Temperature Sensors are normally selected for measurements from -320 to +500° F (-195 to +260° C). The sensing grid utilizes a high purity nickel. Three basic constructions are offered:

  • ETG Sensors have a polyimide carrier for flexibility. It is available as an encapsulated gage with exposed solder tabs ( Option E ), or with integral printed-circuit terminals ( Option W ).
     
  • The WTG Sensor incorporates integral leadwires and a high-temperature epoxy-phenolic matrix (reinforced with glass fiber) which fully encapsulates the grid.
     
  • The WWT-TG Sensor is a slightly larger version of the WTG, but preattached to a 0.005-in (0. 13-mm) thick stainless steel shim. This gage can be welded or bonded to a structure.
     

The resistance at +75° F (+23.9° C) is 50 ohms + 0.3% for the ETG and WTG Sensors; and 50 ohms + 0.4% for the WWT-TG Sensors.

Maximum operating temperature for ETG Sensors with Option E is +450° F (+230° C), and +350° F (+175° C) for Option W. All other types are +500° F (+260° C).

Sensor Designations
ETG-50A 50 ohms; Special Purpose
ETG-50B 50 ohms; Special Purpose

WTG-50A 50 ohms; Special Purpose
WTG-50B 50 ohms; Special Purpose

WWT-TG-W200B-050 50 ohms; Special Purpose


LST Matching Networks
The temperature coefficient of resistance of nickel sensors is very high and nonlinear as indicated in the graph. The sensor resistance can be measured directly and converted to temperature with the charts supplied in Tech Note TN-506, but since TG Sensors are commonly used along with strain gages, special matching networks have been developed to use with strain gage instrumentation.


These LST Matching Networks are small passive devices encapsulated in a molded epoxy case. They are connected between TG Temperature Sensors and the strain gage readout instrumentation to perform the following three functions:


  1. Linearize the gage resistance versus temperature.
  2. Attenuate the resistance change slope to the equivalent of 10 or 100 microstrain per degree F or C for a gage factor setting of 2,000 on the strain indicator.
  3. Present a balanced 350-ohm half-bridge circuit to the strain indicator at the reference temperature of 0° F (Fahrenheit networks) or 0° C (Celsius networks).

In order to optimize performance, separate network designs are available for cryogenic and normal temperature ranges. Environmental temperature range of LST networks is -65 to +250° F (-55° to + 125° C). Standard strain gage instrumentation, such as the Vishay Measurements Group Model P-3500, is ideal for use with these sensors, eliminating the need to purchase separate readout devices.

Network Designations

Low Temperature Range
Network
Designation  		   Output
Slope  		   Sensor
Temperature
Range
LST-10F-350C     10 microstrain/° F      -320° to +100° F
LST-10C-350C     10 microstrain/° C      -200° to +25° C
LST-100F-350C     100 microstrain/° F      -320° to +100° F
LST-100C-350C     100 microstrain/° C      -200° to +25° C


Normal Temperature Range
Network
Designation  		   Output
Slope  		   Sensor
Temperature
Range
LST-10F-350D     10 microstrain/° F      -200° to +500° F
LST-10C-350D     10 microstrain/° C      -150° to +260° C
LST-100F-350D     100 microstrain/° F      -200° to +500° F
LST-100D-350C     100 microstrain/° C      -150° to +260° C



CLTS-2B Temperature Sensors
The Cryogenic Linear Temperature Sensor (CLTS) is recommended for best accuracy over the temperature range of -452 to +100° F (-269 to +40° C). The CLTS-2B is a small surface thermometer gage consisting of two thin foil sensing grids laminated into a glass-fiber-reinforced epoxy-phenolic matrix, and electrically wired in series. The two alloys are special grades of nickel and manganin that are processed for equal and opposite nonlinearities in resistance versus temperature characteristics. The CLTS-2B is fabricated with integral printed-circuit terminals to provide strong, convenient attachment points for the leadwires.

Because of its low thermal mass and thin construction, the CLTS-2B responds quickly and accurately to temperature changes in the surface to which it is bonded. Special design features protect the sensor from damage due to thermal shock, even during plunges from room temperature directly into liquefied gases, including LHe at -452° F (-269° C).

Sensor Designation

CLTS-2B Special Purpose


Note
Avoid prolonged exposure of the CLTS-2B to temperatures above +150° F (+65° C) as this may adversely affect characteristics of the manganin material. The maximum recommended curing temperature of the bonding adhesive is two hours at +200° F (+95° C).

CLTS-2B Sensitivity
The nominal resistance of the CLTS-2B is 290.0 ohms + 0.5% at +75° F (+23.9° C). The resistance decreases linearly with temperature, reaching a nominal value of 220.0 ohms at -452° F (-269° C). This represents a change of 70 ohms for 527 degrees F, or a slope of 0.1328 ohms per degree F; the corresponding slope on the Celsius scale is 0.2391 ohms per degree C. With proper instrumentation a resolution of 0.01° can be easily achieved. Data readout can be accomplished by directly monitoring resistance change with an appropriate resistance measuring instrument.

CLTS Networks
When used in conjunction with bonded strain gages, it is often most convenient to modify the CLTS output with a simple, passive resistance network that can be used with strain gage instrumentation as described with the TG Sensors. The sensitivity can be adjusted to 10 microstrain per degree F (CLTS-N-F) or C (CLTS-N-C); with a resolution of 0.1° when used with most strain indicators. This type of network also provides a high degree of leadwire compensation. Environmental temperature limits for CLTS Networks are -65 to +250° F (-55 to +125° C).

Network Designations
Cryogenic Temperature Range
Network
Designation  		   Output
Slope  		   Sensor
Temperature
Range
CLTS-N-F     10 microstrain/° F      -452° to +100° F
CLTS-N-C     10 microstrain/° C      -269° to +25° C