Omega Engineering CYTX231 Series, User Manual, Page: 17 / 38

Omega Model CYTX231 User’s Manual
Installation 2-5
2.3.4 Measurement 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 of the following procedures.
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 microfarad capacitor. If AC-coupled digital
noise is suspected (digital circuits or interfaces), use a 0.1 to 1
microfarad 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
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.