Flow rate accuracy

Belt-conveyor scales determine the amount of material being conveyed on a belt. A section of belt is weighed by placing the belt support rollers on a scale the belt speed is also measured. Weight and speed data are suppHed to a controller which integrates them to arrive at a material flow rate, often stated in tons per hour. The controller may display a flow rate, shut the conveyor down when a predeterrnined amount of material has passed, or it may be used to maintain a specified flow rate. Accuracy is limited because of the number of detrimental influences involved, eg, variable belt tension. 

The wedge design maintains a square root relationship between flow rate and differential pressure for pipe Reynolds numbers as low as approximately 500. The meter can be flow caUbrated to accuracies of approximately 1% of actual flow rate. Accuracy without flow caUbration is about 5%. 

Vortex-shedding flow meters typically provide 1% of flow rate accuracy over wide ranges on Hquid, gas, and steam service. Sizes are available from 25 to 200 mm. The advantages of no moving parts and linear digital output have resulted in wide usage in the measurement of steam, water, and other low viscosity Hquids. 

Mobile phases employed for the separations are housed in a cartridge and delivered to the LC columns through a set of binary HPLC pumps (Shimadzu Corporation), as shown in Figure 6.2. The pumps provide a flow rate accuracy of 2% or 2 fiL (whichever is greater) in constant flow pumping mode, with a flow rate precision of 0.3%. A degasser (two channels internal volume of 195 /.d. /channel) is also housed in the pump module employed to minimize the occurrence of air bubbles. 

Pump flow rate accuracy and gradient accuracy Detector linearity of response, noise, drift, and wavelength accuracy Injector precision, linearity, and carryover Column heater temperature accuracy... 

Pump flow rate accuracy and gradient accuracy... 

Pump Flow rate accuracy 2% of the set flow rate 6 months... 

Flow Rate Accuracy. One of the key performance requirements for the pump module is the ability to maintain accurate and consistent flow of the mobile phase. This is necessary to provide stable and repeatable interactions between the analytes and the stationary phase [8,9]. Poor flow rate accuracy will affect the retention time and resolution of the separation. The flow-rate accuracy of the pump can be evaluated simply by calculating the time required to collect a predetermined volume of mobile phase at different flow rate settings. For example, the flow-rate accuracy at 2 mL/min can be verified by using a calibrated stopwatch to measure the time it takes to collect 25 mL of effluent from the pump into a 25-mL volumetric flask. A calibrated flow meter can be used to determine the flow rate as well. The typical acceptance of the flow rate accuracy is listed in Table 11.1. A steady backpressure may be required, depending on the requirement of the system. 

Perform the pressure test before the flow rate accuracy test to make sure that the pump seal and check valves are functioning properly and there are no leaks in the system. 

Flow rate accuracy max. 0.1 mlmin (set value minus flowmeter display). 0 Flow precision max. 0.02 ml min (highest minus lowest measured value). 

Case study 4 shows several examples of problems caused by equipment malfunctions and their subsequent diagnosis and solution. The first one involved a situation of poor retention time reproducibility of a gradient assay. It involved the analysis of a complex natural product, using a narrowbore column (2-mm i.d.) at 0.5 mL/min. System suitability test showed retention times to be erratic and could vary by 1-2 minutes without any obvious trends. Flow rate accuracy was found to be acceptable, however, the compositional accuracy test failed (see Chapter 9 on HPLC calibration). The tentative diagnosis was that of a malfunctioning of the proportioning valve. After its replacement, the retention time precision performance was re-established. 

Flow rate accuracy The ability of the pump to deliver exactly the flow rate indicated by a particular setting. 

As discussed, the performance requirements for drug delivery vary with multiple factors drug, fluid restriction, and patient risk. Thus the delivery of potent agents to fluid-restricted patients at risk requires the highest performance standards defined by flow rate accuracy, flow rate uniformity, and ability to minimize risk of IV-site complications. These performance requirements need to be appropriately balanced with the device cost and the impact on clinician productivity. 

These devices automate the process of adjusting the mechanical flow regulator. The most common controllers utilize sensors to count the number of drops passing through the drip chamber to provide flow feedback for automatic rate adjustment. Flow rate accuracy remains hmited by the rate and viscosity dependence of drop size. Delivery set motion associated with ambulation and improper angulation of the drip chamber can also hinder accurate rate detection. 

Beyond improvements in flow rate accuracy, controllers should provide an added level of patient safety by quickly detecting IV-site complications. The IVAC Corporation has developed a series of controllers employing pulsed modulated flow providing for monitoring of flow resistance as well as improved accuracy. 

FIGURE 25.6 Impact of five variables on flow rate accuracy in four different infusion pumps. Variables tested included solution Distilled water and 25% dextrose in water, back pressure -100 and 300 mm Hg, pumping segment filling pressure -30 inches of water and -1-30 inches of water, temperature 10°C and 40°C, and infusion rate 5 mL/h and 500 mL/h. Note First and second peristaltic mechanism qualified for low-risk patients, while the third peristaltic device qualified for high-risk patients. 

Flow rate. The flow rate accuracy will be determined by the instrument used and will be documented in the supplier s operational qualification documentation. The impact of analyst error made during system setup should be validated and it would be appropriate to investigate the effect of small changes in the region of 10% of the target flow rate. The flow rate may be adjusted by as much as 50%, provided that there are no adverse effects on the chromatography (i.e., resolution, peak shape and retention time). Common causes of flow rate error will be discussed further in Chapter 10. 

Today s stationary infusion pumps are highly sophisticated electromechanical devices that allow the user to program the desired flow rate and infusion volume for one or more i.v. lines. They are equipped with several alarm features to indicate potentially hazardous conditions such as changes in the flow rate, air in the hne, or occlusion of the catheter. The pump mechanism is either a peristaltic compression and release of the administration line or motor-controlled piston movement. The price of these devices ranges from 2,000 to 8,000, which excludes the disposable i.v. sets. The majority of these pumps are used in a hospital setting, where they are operated and supervised during operation by the medical staff. Health-care providers also lease stationary infosion pumps to chronically ill patients for home use. A flow-rate accuracy of 3 10% makes these pumps highly desirable for administration of proteins with a narrow therapeutic-index. 

The sample flow rate should be maintained within S % of the manufacturer s specifled flow rate. Accuracy losses are apt to occur in such special applications, and such a system is recommended only for screening purposes. Consult manufacturers regarding limitations.)... 

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