Measurement Errors Due To Ac Noise - Omega Engineering CYD201 Operator's Manual

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
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.
2.4.4

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 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
®
N
2-5