Page 196 - Mechanical Engineers' Handbook (Volume 2)

References 185

sure pulsations and velocity profile disturbances downstream of compound, out-of-plane el-
bows and bends. Carlander and Delsing proposed using the variance in a set of measurements
as a diagnostic clue to the existence of an error in the mean value, since an increased variance
in indicated flow accompanied the onset of errors due to pulsations.

Aliasing with Process Noise
Vermeulen et al. 83 examined the error introduced by aliasing of the noise from a throttling
control valve with the signal of an ultrasonic flowmeter. They report that, under certain
predictable conditions, the ultrasonic component of noise from the control valve can be larger
than the signal generated by a commercial ultrasonic flowmeter and, therefore, can swamp
out the meter.

Non-Newtonian Fluids
Non-Newtonian fluids display velocity distributions that are significantly different from New-
tonian fluids and, hence, would be expected to cause trouble with flowmeters that require
84
an assumption of the profile shape. Fyrippi, Owen, and Escudier tested a single-beam transit
time ultrasonic flowmeter in a non-Newtonian fluid and found significant errors.
Turbulence
A fundamental assertion used in developing the equations for transit time flowmeters is that
the flow field is the same during the up-wind and the down-wind pulse transits. This may
not be the case in a turbulent flow. This possibility was investigated by Weber et al. 85

Fouling
Lansing 86 Investigated the effect of fouling on the performance of a transit time ultrasonic
flowmeter.

REFERENCES
1. NIST ITS90 Thermocouple Data base; www.nist.gov.
2. R. J. Moffat, Experimental Methods in the Thermosciences, Department of Mechanical Engineering,
Stanford University, Stanford, CA, 1978, pp. 1–26.
3. R. J. Moffat, ‘‘The Gradient Approach to Thermocouple Circuitry,’’ in Temperature, Its Measurement
and Control in Science and Industry, Charles M. Herzfeld (ed.), Vol. 3, Part 2, Reinhold New York,
1962, pp. 33–38.
4. R. J. Moffat, Experimental Methods in the Thermosciences, Department of Mechanical Engineering,
Stanford University, Stanford, CA, 1980, pp. 2–3.
5. R. P. Benedict, Fundamentals of Temperature, Pressure, and Flow Measurement, 2nd ed., Wiley,
New York, 1977.
6. E. O. Doebelin, Measurement Systems: Application and Design, McGraw-Hill, New York, 1966.
7. M. Campari and S. Garribba, ‘‘The Behavior of Type K Thermocouples in Temperature Measure-
ment: The Chromel-P-Alumel Thermocouple,’’ Review of Scientific Instruments 42(5), 644–653 (May
1971).
8. R. E. Bentley, ‘‘Thermoelectric Hysteresis in Nickel-Based Thermocouple Alloys,’’ Journal of Phys-
ics D: Applied Physics 22, 1902–1907 (1989).
9. W. Schuh and N. Frost, ‘‘Improving Industrial Thermocouples Temperature Measurement,’’ NEWS
2002, www.watlow.com.
10. R. E. Bentley and T. L. Morgan, ‘‘Thermoelectric Effects of Cold Work in Pt/10%Rh and Pt/
13%Rh versus Pt Thermocouples,’’ Metrologia 20(2), 61–66 (June 1984).
11. O. A. Geraschenko and N. N. Ionova, ‘‘Thermal EMF of Plated Thermocouples’’ UDC 536,532,
Translated from Ismeritel’naya Tekhnika, No. 1, January 1966, pp. 65–66.