Lake Shore 370 User Manual
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Lake Shore Model 370 AC Resistance Bridge User’s Manual2.8.1 Scanner PerformanceUp to 16 resistors with up to 64 leads can be attached to the Model 3716/L Scanner. Up to 8 resistors with up to 32 leadscan be attached to the Model 3708 Scanner. The large number and potential length of lead wires provide an excellentpath for noise to couple into the measurement system. Proper installation combined with the noise reduction featuresbuilt into both the Model 370 and Model 3716/L and 3708 can minimize the effects but a full loaded scanner may neverbe as quiet as a single guarded and shielded resistance input. A measurable increase in noise is most likely whenmeasuring large resistance of more than 100 kΩ. Large resistances pose an additional problem because the guardsavailable for a single resistance input end at the scanner enclosure. Scanner and lead capacitance after that point canattenuate measurement signals for large resistors.On the other hand, the times 100 amplifier in the scanner can actually reduce measurement noise in installations thatrequire the Model 370 to be a significant distance from the test Dewar. The built-in amplifier in the scanner may improvemeasurements any time long input leads are required, even if only one channel is being used.Accuracy is not as big a concern as noise when using the Model 3716/L and 3708 Scanners. The Model 370 ACmeasurement eliminates any voltage offsets that may be present in the scanners relays or amplifier. The amplifier gain iscalibrated at the factory and can be recalibrated in the field to keep scanner gain error well below overall measurementuncertainty.Settling time is a concern any time multiple inputs are multiplexed and must be scanned. The channel change settlingtime of the Model 3716L and 3708 Scanners are virtually the same as the range change settling time of the Model 370.Additional time may be required for the instrument to autorange if that feature is enabled and the resistance on the newchannel changes significantly between scans.2.8.2 Controlling Temperature While ScanningThe close integration of the Model 370 and Model 3716 Scanner permits fast efficient scan channel changes making itpossible to both control temperature and take data with the same instrument. Firmware in the Model 370 managesautomatic switching between the control channel and scan channel and it will freeze the control heater output when thecontrol channel is not active. Even with these features, scanning during closed loop temperature control will never be asstable as a dedicated control instrument.The biggest causes of control instability are the subtle changes in cooling power that are present in low temperaturecooling systems. These changes are offset by changes in heater output during closed loop control to keep temperatureconstant. When the instrument is scanning other channels it cannot read the control feedback sensor for several secondsor tens of seconds at a time. When the control channel is inactive the control output freezes (creating open loop control)and changes in cooling power result in changing temperature. The amount of instability depends on speed and mass ofthe cooled load. Scanning while controlling should be reserved for the least critical temperature control applications. Allinstrument specifications for temperature control are for continuous closed loop control without scanning.2.9 TEMPERATURE MEASUREMENTTemperature measurement is a popular use of resistance bridges because most of the temperature sensors that canoperate at low temperatures are negative temperature coefficient resistive temperature detectors (NTC RTDs) and needthe flexibility of a bridge. Resistive temperature sensors must be treated with the same care as any other resistor beingmeasured at low temperatures. The Model 370 is well equipped to measure and control temperature using these devices.2.9.1 Sensor SelectionLake Shore offers a full line of cryogenic temperature sensors including several models that are well suited for lowtemperature work to 50 mK and below. Two of the most popular are Germanium and Ruthenium Oxide RTDs. The LakeShore Temperature Measurement and Control catalog gives detailed specifications of these sensors and should beconsulted as a reference when choosing temperature sensors. The Model 370 will work with resistive temperaturesensors from Lake Shore or any other manufacture. A few important selection criteria are discussed below.2.9.1.1 SensitivitySensitivity is often the most important criteria when selecting a temperature sensor because it dictates many of the otherspecifications including range, resolution and accuracy. Many NTC resistive materials like germanium have a beneficialcharacteristic of temperature sensitivity that makes them particularly useful at low temperatures. The resistance andsensitivity of these materials increase exponentially as temperature decreases. The sensitivity of the sensors is best at the2-14 Theory of Operation |
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