ISO 9464:2008 pdf download – Guidelines for the use of Iso 5167:2003. Condensation chambers
For specific fluids and conditions, sudi as steam, special connection arrangements, condensation chambers.
etc. may be required. See ISO 2186 for details
6.2.3 Pressure measurement devices General
The accurate measurement 01 the differential pressure generated across a primary element is fundamental to the calculation of ulowrate in a circular cross-section conduit employing orifice plates, nozzles or Venturi tubes.
In the case of onfice plates, for the measurement of gas or where higher accuracy is required for liquids, it is necessary to determine The absolute static pressure of the fluid at the upstream pressure tappings. In addition to The calculation of the expansibility factor, the static pressure is required to determine, as appropriate, the downstream to upstream corrections for process parameters such as temperature and measured density.
When density is calculated using an equation of state, the sensitivity of the static pressure measurement is greater and the need to measure this parameter accurately becomes more acute. In many instances, gauge pressure transmitters are employed to measure the pressure of The fluid at the upstream pressure tappings. The absolute static pressure of The fluid is required for the flowrate and referral calculations and can be calculated from gauge and ambient pressure measurements. Instead of measuring the ambient barometnc pressure it is common to add the conventional reference pressure of 101.325 kPa (1,01325 bar) to the measured gauge pressure. However, when variations in atmospheric pressure result in a 0.1 % change In mass flow, it is recommended That gauge pressure instruments are replaced with absolute pressure instruments. Pressure transducers
The differential pressure across the primary device is most commonly measured using an electronic (or. more rarely, a mechanical) transducer connected via the impulse lines to the upstream arid downstream pressure tappings. The connection to the upstream lappmg may be routed to the differential pressure and the static pressure transducers when both units are installed as part of a metering device as illustrated in Figure 10.
The choice of pressure transducer depends upon a number of factors which include the following:
a) the required accuracy of the measurement system:
b) whether the measurement is to be made continuously or intermittently;
C) the characteristics of the flowing fluid:
d) the date acquisition system including the computation device;
a) the required mounting arid location for The transducer.
Mechanical pressure transducers, whilst less common with the advent of flow compuLeri. ‘e (ili ueu ill many process applications. These units consist of an elastic element which converts energy from the pressure system to a displacement in the mechanical measuring system.
The more commonly used electronic pressure transducers incorporate an electric element which converts the pressure to an electrical signal which can be easily amplified, corrected, transmitted and measured. Calibration of pressure transducers
To reduce the effects of amb4ent temperature changes to a minimum, it is recommended that the differential and static pressure transmitters be Installed In temperature-controlled enclosures.
Static pressure transducers are usually calibrated in situ agewist an appropriate pressure calibrator selected for the specific function
Differential pressure transmitters are often calibrated at atmospheric pressure, again using a calibrator which is deemed suitable for the purpose. For optimum accuracy, a transmitter should ideally be calibrated at operating pressure. It Is common practice to use a high-static deadweight tester for this application.
As previously stated, a high-static calibration may not be possible due to less than ideal environmental conditions or background vibration at the worksate. If this is the case, a correction for static pressure shift effect should be Øied either mathematically or via an interim calibration option such as footpnnting”.
The footpflnting method referred to above involves the off-line calibration of the transducer ii a controlled environment and the subsequent production of an atmospheric foolprint which is used as a datum at the wortcsite for tie periodic checkmg of the transducer against test equipment which is less environmentally sensitive than a high-static deadweight tester. Damping of pressure signals
See Annex B of ISOITR 3313:1998.
6.3 Measurement of temperature
8.3.1 General
The temperature at the upstream pressure tapping is needed m order to determine the density and viscosity of the fluid and to apply correction for thermal expansion of the device and The pipe.
The temperature of the fluid should preferably be measured downstream of the primary device
8.3.2 Fundamentals of measuring the temperature of a moving fluid
Since any immersion temperature probe only measures Its own temperature, the problem is to ensure that the representative temperature in the fluid Is the same as the temperature at the measunng probe. Heat can be transferred by conduction, convection and radiation.
Except for great temperature differences, most of the heal is transferred from the fluid to the temperature probe by conduction and convection.