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Non Nulling Measurements of Flue Gas Flows in a Coal-Fired Power Plant Stack



Aaron N. Johnson, Iosif I. Shinder, Bernard J. Filla, Joey T. Boyd, Rodney A. Bryant, Michael R. Moldover


Exhaust flows from coal-fired stacks are determined by measuring the flue gas velocity at prescribed points in the stack cross section. During the last 30+ years these velocity measurements have been made predominantly using S-type pitot probes. These probes are robust and inexpensive; however, S-probes measure only two components of the velocity vector and can give biased results if the stack flow has significant yaw and pitch angles. Furthermore, S- probe measurements are time intensive, requiring probe rotation (or nulling) at each traverse point to find the yaw angle. The only EPA sanctioned alternatives to the S-probe are 5-hole probes (i.e., the prism probe and spherical probe) that also require yaw nulling. We developed a non nulling technique applicable to the spherical probe and two custom designed 5-hole probes that reduce testing time and may improve measurement accuracy. The non nulling technique measures all 3 components of velocity without rotating the probe. We assessed the performance of these 5-hole probes in a coal-fired stack at the high-load (16 m/s) and the low-load (7 m/s). For the spherical probes, the non nulling results and the nulling results were in excellent mutual agreement (< 0.1 %). For the custom probes, the non-nulling and nulling results were inconsistent: the differences were 5% at the high load and 10 % at the low load. We speculate that the nulling data for the custom probes were flawed because the non nulling data for all the probes accurately determined the yaw and pitch angles at high and low loads. Our results demonstrate that the non nulling technique can accurately measure flue gas flows in a coal-fired stack.
Proceedings Title
Flomeko 18th International Flow Measurement Conference
Conference Dates
June 26-28, 2019
Conference Location


Method 2F, non-nulling, spherical probes, stack flow measurements, nulling, Smokestack Simulator, coal-fired stack, non-nulling flow RATA


Johnson, A. , Shinder, I. , Filla, B. , Boyd, J. , Bryant, R. and Moldover, M. (2019), Non Nulling Measurements of Flue Gas Flows in a Coal-Fired Power Plant Stack, Flomeko 18th International Flow Measurement Conference, Lisbon, -1, [online], (Accessed June 23, 2024)


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Created June 28, 2019, Updated May 8, 2020