Omega Engineering CYD201 and CYD208 manual Measurement Errors Due To AC Noise

OMEGA Model CYD201/CYD208 User’s Manual

For temporary mounting in cold temperature applications, apply a thin layer of Apiezon® N Grease between the sensor and sample to enhance thermal contact under slight pressure. The preferred method for mounting the CY-7-SD sensor is the OMEGA CO Adapter.

CAUTION: OMEGA will not warranty replace any device damaged by user-designed clamps or solder mounting.

For semi-permanent mountings, use Stycast epoxy instead of Apiezon® N Grease. NOTE: Do not apply Stycast epoxy over the CY-7-SD package: sensor stress may shift the readings. In all cases, periodically inspect the sensor mounting to verify good thermal contact to the mounting surface is maintained.

2.4.4Measurement Errors Due To AC Noise

Poorly shielded leads or improperly grounded measurement systems can introduce AC noise into the sensor leads. In diode sensors, the AC noise shifts the DC voltage measurement due to the diode non-linear current/voltage characteristics. When this occurs, measured DC voltage is too low and the corresponding temperature reading is high. The measurement error can approach several tenths of a kelvin. To determine if this problem exists, perform either procedure below.

1.Place a capacitor across the diode to shunt induced AC currents. Capacitor size depends on the noise frequency. If noise is related to power line frequency, use a 10 µF capacitor. If AC-coupled digital noise is suspected (digital circuits or interfaces), use a 0.1 to 1 µF capacitor. In either case, if measured DC voltage increases, there is induced noise in the measurement system.

2.Measure AC voltage across the diode with an AC voltmeter or oscilloscope. Most voltmeters do not have the frequency response to measure noise associated with digital circuits or interfaces (which operate in the MHz range). For a thorough discussion of this potential problem, and the magnitude of error which may result, request the paper “Measurement System-Induced Errors In Diode Thermometry,” J.K. Krause and B.C. Dodrill, Rev. Sci. Instr. 57 (4), 661, April, 1986.

To greatly reduce potential AC noise, connect twisted leads (pairs) between the measurement instruments and the diode sensors. Use 32 or 36 AWG OMEGA Duo-Twist™ Cryogenic Wire, which features phosphor bronze wire twisted at 3.15 twists per centimeter (8 twists per inch). See the OMEGA Product Catalog or contact OMEGA for further information.

Installation 2-5