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A mixing system is provided comprising a first mixing tub, a first actuator to control flow of a carrier fluid, a sensor to determine a rate of flow of the fluid, and a second actuator to control flow of a dry material to form a mixture in the first mixing tub. The system further comprises a second

A mixing system is provided comprising a first mixing tub, a first actuator to control flow of a carrier fluid, a sensor to determine a rate of flow of the fluid, and a second actuator to control flow of a dry material to form a mixture in the first mixing tub. The system further comprises a second mixing tub, a controller, and a weight sensor to determine the weight of the first mixing tub. The controller modulates the second actuator based on the rate of flow of the carrier fluid, based on the weight of the first mixing tub, and based on one of a sensed height of the mixture in the second mixing tub and a weight of the second mixing tub to provide the mixture at least one of at a commanded density and at a commanded flow rate. The mixture may be provided to a well bore to perform a servicing operation.

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1. A mixing system, comprising: a first mixing tub;a first actuator to control flow of a carrier fluid into the first mixing tub;a sensor to determine a rate of flow of the carrier fluid;a second actuator to control flow of a dry material into the first mixing tub, wherein the carrier fluid and the

1. A mixing system, comprising: a first mixing tub;a first actuator to control flow of a carrier fluid into the first mixing tub;a sensor to determine a rate of flow of the carrier fluid;a second actuator to control flow of a dry material into the first mixing tub, wherein the carrier fluid and the dry material mix in the first mixing tub to form a mixture;an at least one weight sensor to determine a weight of the first mixing tub;a second mixing tub, wherein the first mixing tub is configured to flow the mixture into the second mixing tub; anda controller to automatically modulate the second actuator based on the rate of flow of the carrier fluid, based on the weight of the first mixing tub, and based on one of a sensed height of the mixture in the second mixing tub and a sensed weight of the second mixing tub to provide the mixture at substantially a commanded density and/or at substantially a commanded flow rate to a well bore, wherein the controller determines a weight of the mixture in the first tub based on using the weight of the first mixing tub to index into a look-up table and wherein the controller automatically modulates the second actuator based at least in part on the weight of the mixture in the first tub, wherein the look-up table takes account of mechanical coupling of the first mixing tub to other structures. 2. The mixing system of claim 1, wherein the controller further automatically modulates the first actuator based on the rate of flow of the carrier fluid, based on the weight of the first mixing tub, and based on one of the sensed height of the mixture in the second mixing tub and the sensed weight of the second mixing tub and wherein the controller automatically modulates the first actuator and the second actuator to provide the mixture at substantially the commanded density and substantially the commanded flow rate to the well bore. 3. The mixing system of claim 1, further comprising a weight observer to provide an estimate of the weight of the first mixing tub and a first estimate of the disturbance based on an output of the at least one weight sensor, wherein the estimate of the weight of the first mixing tub is a smoothed, unlagged value, and wherein the controller automatically modulates the second valve in part based on the estimate of the weight of the first mixing tub and in part based on the first estimate of the disturbance. 4. The mixing system of claim 1, wherein the first mixing tub is mechanically decoupled from the second mixing tub. 5. The mixing system of claim 1, wherein the controller automatically modulates the second valve in part based on an estimated density of the mixture, wherein the controller determines the estimated density by dividing an estimate of the weight of the mixture in the first mixing tub by a volume of the first mixing tub and wherein the estimate of the weight of the mixture in the first mixing tub is based on the weight of the first mixing tub. 6. The mixing system of claim 1, wherein the mixture is a cement slurry or a fracturing fluid. 7. The mixing system of claim 1, further comprising a height observer to provide an estimate of the height of the mixture in the second mixing tub and an estimate of the disturbance based on an output of a height sensor, wherein the estimate of the height of the mixture in the second mixing tub is a smoothed, unlagged value, and wherein the controller automatically modulates the second valve in part based on the estimate of the height of the mixture in the second mixing tub and in part based on the second estimate of the disturbance. 8. A mixing system, comprising: a first mixing tub;a first actuator to control flow of a carrier fluid into the first mixing tub;a sensor to determine a rate of flow of the carrier fluid;a second actuator to control flow of a dry material into the first mixing tub, wherein the carrier fluid and the dry material mix in the first mixing tub to form a mixture;a density sensor to determine a density of the mixture in the first mixing tub;a second mixing tub, wherein the first mixing tub is configured to flow the mixture into the second mixing tub;an at least one weight sensor to determine a weight of the second mixing tub;a controller to automatically modulate the second actuator based on the rate of flow of the carrier fluid, based on the density of the mixture in the first mixing tub, and based on the weight of the second mixing tub to provide the mixture at substantially a commanded density and/or at substantially a commanded flow rate to a well bore, andfurther comprising an at least one weight sensor to determine a sensed weight of the first mixing tub, wherein the controller automatically modulates the second actuator based in part on corroborating the density of the mixture in the first mixing tub determined by the density sensor using the sensed weight of the first mixing tub. 9. The mixing system of claim 8, further comprising a recirculation mechanism to recirculate the mixture in the first mixing tub and wherein the density sensor is collocated with the recirculation mechanism. 10. The mixing system of claim 8, further comprising a weight observer to provide an estimate of the weight of the second mixing tub and an estimate of the disturbance based on an output of the at least one weight sensor, wherein the estimate of the weight of the second mixing tub is a smoothed, unlagged value, and wherein the controller automatically modulates the second valve in part based on the estimate of the weight of the second mixing tub and in part based on the estimate of the disturbance. 11. The mixing system of claim 8, wherein the controller is one of an analog control system comprised of control blocks implemented by differential amplifiers, resistors, and capacitors and a digital control system comprised of control blocks implemented by computer instructions. 12. The mixing system of claim 8, wherein the controller further automatically modulates the first actuator based on the rate of flow of the carrier fluid, based on the density of the mixture in the first mixing tub, and based on the weight of the second mixing tub and wherein the controller automatically modulates the first actuator and the second actuator to provide the mixture at substantially the commanded density and at substantially the commanded flow rate to the well bore. 13. A mixing system, comprising: a divided mixing tub comprising a first tub and a second tub;a first actuator to control flow of a carrier fluid into the first tub;a sensor to determine a rate of flow of the carrier fluid;a second actuator to control flow of a dry material into the first tub, wherein the carrier fluid and the dry material mix in the first tub to form a mixture, the mixture overflows the first tub into the second tub, and the mixture is further mixed in the second tub;an at least one weight sensor to determine the weight of the divided mixing tub;a height sensor to determine a height of the mixture in the second tub; anda controller to automatically modulate the second actuator based on the rate of flow of the carrier fluid, based on the height of the mixture in the second tub, and based on the weight of the divided mixing tub to provide the mixture at substantially a commanded density and/or at substantially a commanded flow rate to a well bore, wherein the controller determines a weight of the mixture in the divided mixin tub based on usin the wei ht of the divided mixin tub to index into a look-up table and wherein the controller automatically modulates the second actuator based at least in part on the weight of the mixture in the divided mixing tub, wherein the look-up table takes account of mechanical coupling of the divided mixing tub to other structures. 14. The mixing system of claim 13, wherein the controller further automatically modulates the first actuator based on the rate of flow of the carrier fluid, based on the weight of the divided mixing tub, and based on the height of the mixture in the second tub and wherein the controller automatically modulates the first actuator and the second actuator to provide the mixture at substantially the commanded density and substantially the commanded flow rate to the well bore. 15. The mixing system of claim 13, further comprising a weight observer to provide an estimate of the weight of the divided mixing tub and a first estimate of the disturbance based on an output of the at least one weight sensor, wherein the estimate of the weight of the divided mixing tub is a smoothed, unlagged value, and wherein the controller automatically modulates the second valve in part based on the estimate of the weight of the divided mixing tub and in part based on the first estimate of the disturbance. 16. The mixing system of claim 13, wherein the controller automatically modulates the second actuator in part based on an estimated density of the mixture, wherein the controller determines the estimated density by dividing an estimate of the weight of the mixture in the dividing mixing tub by a volume of the mixture in the divided mixing tub and wherein the volume of the mixture in the divided mixing tub is based in part on the height of the mixture in the second tub. 17. The mixing system of claim 13, further comprising a height observer to provide an estimate of the height of the mixture in the second tub and a second estimate of the disturbance based on an output of a height sensor, wherein the estimate of the height of the second tub is a smoothed, unlagged value, and wherein the controller automatically modulates the second valve in part based on the estimate of the height of the mixture in the second tub and in part based on the second estimate of the disturbance. 18. The mixing system of claim 1, further comprising an at least one weight sensor to determine a sensed weight of the second mixing tub, wherein the controller automatically modulates the second actuator based on the sensed weight of the second mixing tub.