GB2482699A

GB2482699A - Calibrating gas specific gravity meter

Info

Publication number: GB2482699A
Application number: GB1013453.4A
Authority: GB
Inventors: Simon Patrick Hunter Wheeler
Original assignee: Mobrey Ltd
Current assignee: Rosemount Measurement Ltd
Priority date: 2010-08-11
Filing date: 2010-08-11
Publication date: 2012-02-15
Also published as: GB201013453D0

Abstract

The invention describes a method of establishing a temperature correction for a Ni-Span C cylinder incorporated in a gas specific gravity sensor, the cylinder is characterised in that during manufacture validation points of time period versus temperature have been established to determine if the cylinder is fit for use. The method of the present invention is characterised in that these validation points are used to derive a temperature correction curve where the curve may be a polynomial curve that is at least cubic in order. During operation of the device a measurement of temperature may be taken and applied to establish a correction factor in real time. The correction of temperature effects is particular relevant at gas pressures below around 8 bar.

Description

IMPROVEMENTS IN OR REM TING TO GAS SPECIFIC GRA VITY SENSING

Field of the Invention

This invention relates to gas specific gravity sensing and, in particular, to a temperature correction method for use in instruments used for measuring gas specific gravity.

Background to the Invention

One known form of gas specific gravity sensor incorporates a Ni-Span C vibrating cylinder which is known for its minimal changes in resonant frequency due to temperature effects. An example of such a sensor is the model 3098 gas specific gravity sensor manufactured by Micro Motion Inc of Boulder, Colorado, USA (the 3098 instrument').

In the 3098 instrument the vibrating cylinder is incorporated in a gas densitometer.

This densitometer is surrounded by a constant volume reference chamber filled with a fixed quantity of gas. A diaphragm ensures that the pressure of the sample gas in the densitometer is equal to that of the reference gas, and the whole system is temperature stabilized.

Currently, the individual vibrating cylinders are characterized during manufacture, by recording the time period over a range of temperatures (-25°C, 0°C, 10°C, 20°C, 30°C, & 50°C) and producing a characteristic response curve (see Figure 1). If the response of an individual cylinder falls within set limits then that cylinder is deemed fit for purpose and can be used in the production of 3098 instruments. Other than this characterization process, it is assumed that there will be minimal changes in resonant frequency of the cylinder due to temperature effects and no subsequent use is made of the temperature data collected.

The accuracy of the 3098 instrument is known to be dependant on a number of factors including: i) The repeatability, accuracy and stability of the densitometer; ii) The compressibility of the gas in the reference chamber; iii) Operating temperature; iv) Calibration process; and v) Gas molecular weight.

Of these factors, the most significant are reference chamber pressure, and temperature. If the reference chamber pressure is held at a low value within the sensor (up to about 8bar A) then the effects of temperature on the vibrating cylinder become a significant error source as is evident from Figure 2.

It is an object of the invention to provide a calibration method which will go at least some way in addressing the aforementioned problem; or which will at least provide a novel and useful choice.

Summary of the Invention

In one aspect the invention provides a method of establishing a temperature correction for an instrument having a vibrating Ni-Span C cylinder which has been characterised by establishing validation points of time period versus temperature and determining if said validation points lie within an acceptable envelope, said method being characterized in that said validation points are used to derive a correction curve.

Preferably said validation points are used to establish a polynomial correction curve.

Preferably said correction curve is at least cubic in order.

Preferably the method further includes measuring temperature to establish a correction factor in real time.

Many variations in the way the invention may be performed will present themselves to those skilled in the art, upon reading the following description. The description should not be regarded as limiting but rather as an illustration, only, of one manner of performing the invention. Where appropriate any element or component should be taken as including any or all equivalents thereof whether or not specifically mentioned.

Brief Description of the Drawings

An embodiment of the invention will now be described with reference to the accompanying drawings in which: Figure 1: shows a typical prior art temperature calibration specification for a 3098 instrument; Figure 2: shows a typical graphical summary of total error as a function of temperature at various reference chamber pressures as currently experienced in a 3098 instrument; arid Figure 3: shows a typical time period compensation curve derived by application of the invention.

Detailed Description of Working Embodiment

This invention describes a method of calibrating a gas specific gravity meter which incorporates a vibrating Ni-Span C cylinder. Such an instrument is sold by Micro-Motion Inc of Boulder, Colorado USA as a 3098 Gas Specific Gravity meter and further details can be found at www2.emersonprocess.com.

Referring to Figure 1, each Ni-Span C cylinder is characterised during manufacture, under vacuum conditions, at a number of different temperatures. In the example shown values of the time period of the resonant frequency are recorded at temperatures of -25°C, 0°C, 10°C, 20°C, 30°C & 50°C. Providing the resulting validation points lie within an envelope defined by the positive and negative limit curves shown in Figure 1, the cylinder is considered fit for purpose. Once the characterisation process is complete no further use is made of the validation data.

Historically we have found that 3098 instruments incorporating a cylinder so validated, display satisfactory performances when the reference chamber pressure is at pressures above about 8 Bar A. However, at lower reference chamber pressures, the effects of temperature on the vibrating cylinder become a significant measurement error source. An example of the increasing total error/°C, with deceasing pressure, is illustrated in Figure 2. This is for natural gas applications only.

According to the invention, the six measurement points used to characterise a particular cylinder are used, in combination with a real-time temperature measurement device, to establish a correction curve.

In its most elementary form, the correction factor can be calculated from the linear interpolations shown in Figure 1. However it is preferred that a polynomial curve be derived to fit the measurement points. It is particularly preferred that the polynomial be at least a cubic curve.

The polynomial can be established using a curve fitting technique such as those embodied in an application such as Excel.

Example

The invention is applied to a 3098 instrument as follows: During manufacture, the 3098 is tested under vacuum conditions at different temperatures. The time periods are recorded as well as the exact temperatures at which the individual time periods are measured. Typical results are shown in columns A & B of Table 1.

Table 1

A B C D E

Time Period Temp Time Period Change from 20°C (as) Caic Time Period Difference 507.0085 -23.47 -80.3 -80.25277647 0.0472 507.0725 0.02 -16.3 -16.26890128 0.0311 507.0863 11.66 -2.5 -2.71012969 -0.2101 507.0888 19.75 0.0 0.319723438 0.3197 507.0861 30.06 -2.7 -2.848095098 -0.1481 507.0594 50.24 -29.4 -29.47303094 -0.0730 Having established columns A and B, the differences in time period between the 20°C point, and each other point, are calculated. The results are listed in column C. Using a curve fitting technique a curve is fitted to all the points of temperature vs time period change (from the 20°C point). As can be seen in Figure 3, a cubic curve has proven to be a good fit but other options are possible. Preferably the curve is higher in order than a quadratic.

Listed in column D are the calculated time period changes from the 20°C point which are established using the cubic equation and the measured temperature. The resulting figures can be compared with measured figures listed in column C, the differences being shown in column E. As can be seen the differences are extremely small.

The equation thus derived can be incorporated in the software of any electronics associated with the operation of a 3098 instrument and, by measuring temperature alone in real time, the effect of temperature on the time period of the 3098 can be predicted. This, in turn, means that a correction may be applied to the sensor to improve its overall performance.

Claims

Claims 1. A method of establishing a temperature correction for an instrument having a vibrating Ni-Span C cylinder which has been characterised by establishing validation points of time period versus temperature and determining if said validation points lie within an acceptable envelope, said method being characterized in that said validation points are used to derive a correction curve.
2. A method as claimed in claim 1 wherein said validation points are used to establish a polynomial correction curve.
3. A method as claimed in claim 2 wherein said correction curve is at least cubic in order.
4. A method as claimed in any one of claims I to 3 further including measuring temperature to establish a correction factor in real time.