In the measurement of fluid parameters, the measurement of flow is the most complex one. All types of flowmeters have their own characteristics and scope of application. In the flow measurement work, first of all, different types of flowmeters should be selected according to various factors such as the measurement purpose and the properties of the measured medium, the flow range of the fluid, the fluid parameters under the process conditions, and the installation environmental conditions.
Selecting a good instrument is the basis of using the instrument well. The task of instrument selection is to determine the type according to the specific situation and specific conditions, on the premise of meeting the technical requirements of use, starting from safety, economy and practicality, and according to the selection principles and methods. The type, model and specification of the instrument. This chapter will focus on the selection of Coriolis mass flowmeters, and will start with some basic technical terms and technical indicators of some typical products, and explain some basic principles and specific methods in the selection of flowmeters.
technical terms
Some special technical terms are often used in the selection work, such as: flow range, range, pressure loss, zero instability, accuracy, repeatability, sensitivity and flow factor, etc. These technical data are used in flowmeter selection. important technical parameters.
1. Flow range
Within the allowable error limit of the measuring instrument, the range of the measured value is called the measuring range. The highest value and lowest value of the measurement range are called the upper limit value and lower limit value of the measurement range, respectively. Within the allowable error limit, the flow measurement range of the flowmeter is called the flow range. The upper limit of the flow range is called the maximum flow, and the lower limit of the flow range is called the minimum flow.
2. range degree
The range is also called the turndown ratio. is the ratio of the upper limit value to the lower limit value of the measurement range. In flow measurement, the ratio of the maximum flow rate measured by the flow meter to the minimum flow rate is called the flow range or flow turndown ratio.
3. pressure loss
The irreversible pressure value caused by the fluid overcoming resistance (for example, flowing through the flow meter and resistance member arranged in the pipeline, etc.). The pressure loss of the flowmeter, that is, the pressure difference between the two ends of the flow sensor, is related to the properties of the fluid, the flow state of the fluid, and the structural parameters of the flow sensor. Different flowmeters have different pressure loss curves.
4. Zero instability
The zero-point instability is commonly known as zero-point drift, and in many product manuals, it is also called “zero-point stability”. Zero instability expresses the ability of the meter to measure true zero flow. That is, when the flow rate is zero, the flowmeter randomly outputs the amplitude of the signal. The zero-point instability is expressed in the engineering unit of mass flow kg/min or kg/h, which is one of the important technical indicators of the Coriolis mass flowmeter.
5. Accuracy
Accuracy is an index to measure the measurement error of the instrument, which indicates the degree of agreement with the true value of the measurement result. It is generally expressed by the combination of systematic errors and random errors in the measurement results.
When expressing the accuracy of the Coriolis mass flowmeter, the following three methods are commonly used:
(1) Relative error: The measurement error of each flow measurement point is expressed by the ratio of the absolute error of each measurement point to the true value of the measured flow.
That is: relative error = absolute error / measured true value * 100%
The accuracy of the flowmeter in terms of relative error, also known as “percent flow accuracy”. (2) Reference error: The measurement error of each flow measurement point is expressed by the ratio of the absolute error of each measurement point to the full-scale flow rate of the flow juice. That is: reference error = absolute error / flowmeter full scale flow * 100% The accuracy of the flowmeter expressed in the form of reference error, also known as “full scale percentage accuracy”.
(3) Flow percentage accuracy with zero instability: The measurement error at each flow point is expressed by the flow percentage plus the zero instability.
6. Repeatability
Repeatability refers to the degree of consistency between the measurement results when the same measurand is measured several times in succession under the same measurement conditions (such as the same method, the same observer, the same measuring instrument, and the same laboratory). Flow meters with good stability generally have better repeatability of measurement results. The deviation of good repeatability and good linearity can generally be corrected by linear correction method. For the deviation with good repeatability and poor linearity, point-by-point correction method or piecewise correction method can be used to correct it.
7. Sensitivity
The sensitivity of the instrument refers to the ability of the instrument to respond to changes in the measured value. The sensitivity of the flowmeter is expressed by the ratio of the output change of the flowmeter to the corresponding measured change.
8. Flow factor
The actual flow area of the Coriolis mass flow sensor is generally smaller than the flow area of the process pipeline with the same nominal diameter, and this actual flow area is not given in the product technical indicators. In order to calculate the actual flow rate of the medium passing through the sensor measuring tube, the flow rate factor is usually used for conversion. The physical meaning of the flow rate factor is that when water at room temperature is used as the medium, the flow sensor corresponding to the unit mass flow measures the flow rate of the medium in the pipe.
