Verification regulation of the liquid level measuring devices
1 Scope
This regulation is applicable to the pattern evaluation, prototype test, first verification, subsequent verification and in-service inspection of liquid level measuring devices.
The liquid level measuring devices include float type, ball float type, displacement type, pressure type, capacitive type, conductivity type and reflex type. Liquid level measuring devices and level meters manufactured in other principles may also be verified with reference to this regulation.
2 Normative references
The following documents are referenced in this regulation:
JJF 1001-1998 General terms in metrology and their definitions
JJF 1015-2002 General norm for pattern evaluation and pattern approval of measuring instruments
JJF 1016-2002 The rules for drafting program of pattern evaluation of measuring instruments
JJG 617-1996 Verification regulation of digital temperature indicators and controllers
JJG 882-1994 Verification regulation of the pressure transmitter
GB/T 17626.2-1998 Electrostatic discharge immunity test
GB/T 17626.3-1998 Radiated radio-frequency electromagnetic field immunity test
GB/T 17626.4-1998 Electrical fast transient/burst immunity test
GB/T 17626.8-1998 Power frequency magnetic field immunity test
GB/T 17626.11-1999 Immunity test of voltage dip, short interruption and voltage change
JB/T 9329-1999 Basic environmental conditions and testing methods for instruments transportation and storage in the transportation
For the purpose of this regulation, the currently effective editions of the above normative references shall be used.
3 Terms and definitions
3.1 set point error
difference between the measured liquid level and the set value of the liquid level, when the output variable is output as specified
3.2 switching value
input (liquid level) value measured when the output changes from one state to the other, in the up stroke (or down stroke) of the stepping control instrument
3.3 differential gap
difference of switching values between the up and down strokes at the same set point
4 General
Liquid level measuring devices are used to indicate and control liquid level in industrial process measurement and control system.
Liquid level measuring devices may be divided by functions into local-mounted type (on-site indication) and remote type (remote display and control). Remote liquid level measuring device usually converts the on-site liquid level into electrical signal and transmits it the monitoring place, or uses liquid level transmitter with display to achieve remote transmission and display; the liquid level is usually controlled by stepping control mode.
The liquid level measuring device usually consists of sensor, converter and indicator. The liquid level measuring device with control function is also equipped with a setting mechanism.
The working principles of liquid level measuring devices may be classified by the detection mode into the following categories:
a) buoyancy level measuring principle. Measure the liquid level by detecting the buoyancy applied to the vertical displacement element with constant cross section within the liquid level measurement range (e.g. displacement type and ball float type);
b) float level measuring principle. Measure the liquid level by detecting the position of the float (e.g. magnetic type). The position of the float may be detected by mechanical, magnetic, optical, ultrasonic, radiation methods, etc.;
c) buoy and cable level measuring principle. Directly measure the liquid level according to the position of the buoy (e.g. float type). The position of the buoy is mechanically transmitted to the indicator and/or transmitter by cables and pulleys or gear cam groups;
d) pressure level measuring principle. Measure the liquid level by detecting the differential pressure between the upper and lower points of the liquid level (e.g. pressure type);
e) ultrasonic and microwave level measuring principle. The liquid level is determined by detecting the time required for a beam of ultrasonic sound energy and microwave energy to be emitted to the liquid level and reflected back (e.g. reflex type);
f) gamma ray level measuring principle. Measure the liquid level in the principle of the liquid absorbing gamma rays when it is between the radiation source and the detector (e.g. radiation type);
g) capacitance level measuring principle. Measure the liquid level by detecting the capacitance between the two electrodes on both sides of the liquid (e.g. capacitive type);
h) electrical conductance level measuring principle. Measure the level of conducting liquid by detecting the resistance between two electrodes separated by the liquid (e.g. conductivity type).
5 Metrological performance requirements
5.1 Indication error
There are two ways to express the maximum permissible error of the indication value of the liquid level measuring devices:
a) The maximum permissible error of the indication value: ± (a % FS + b)
Where,
a——0.02, (0.03), 0.05, 0.1, 0, 2, 0.5, 1.0, 1.5, 2.0 or 2.5;
FS——the measuring range of the liquid level measuring device, cm or mm;
b——the resolution of the digital liquid level measuring device, cm or mm. For the analog liquid level measuring device, b=0;
b) The maximum permissible error of the indication value: ±N
Where, N —— the maximum permissible error, expressed in the length unit, cm or mm.
5.2 Return difference
The return difference of liquid level measuring device shall not exceed the absolute value of the maximum permissible error of indication value. The return difference of reflex and pressure type liquid level measuring devices shall not exceed half of the absolute value of the maximum permissible error of indication value.
5.3 Stability
If a liquid level measuring device with power supply works continuously for 24 h, its indication error shall still meet the requirements of 5.1.
5.4 Output error of liquid level signal
The output error of liquid level measuring device with transmitter function shall not exceed ±c% of the output range.
Where, c——0.2, 0.5, 1.0, 1.5, 2.0 or 2.5.
5.5 Set point error
For the liquid level measuring device with stepping control, its set point error limit is ±a′% FS (or ±N′).
Where,
a′——0.1, 0.2, 0.5, 1.0, 1.5, 2.0 or 2.5;
N′——the set point error limit, expressed in the length unit, cm or mm.
5.6 Differential gap
For the liquid level measuring device with stepping control, its differential gap shall not exceed twice the absolute value of set point error limit.
1 Scope
2 Normative references
3 Terms and definitions
3.1 set point error
3.2 switching value
3.3 differential gap
4 General
5 Metrological performance requirements
5.1 Indication error
5.2 Return difference
5.3 Stability
5.4 Output error of liquid level signal
5.5 Set point error
5.6 Differential gap
6 General technical requirements
6.1 Appearance
6.2 Influence of main power supply changes
6.3 Environmental temperature influence
6.4 Common mode interference influence
6.5 Power frequency magnetic field immunity performance *
6.6 Electrostatic discharge immunity performance *
6.7 Radio-frequency electromagnetic field radiation immunity performance*
6.8 Electrical fast transient/burst immunity performance*
6.9 Voltage drop and short interruption immunity performance *
6.10 Pressure resistance and airtightness
6.11 Insulation resistance
6.12 Insulation strength
6.13 Transport and storage adaptability
6.14 Explosion-proof performance
7 Control of metrological instruments
7.1 Pattern evaluation or prototype test
7.2 Initial verification, subsequent verification and in-service inspection
7.3 Treatment of verification results
7.4 Verification period
Annex A Pattern evaluation or prototype test items and methods
Annex B Verification record format of liquid level measuring devices
Annex C Format for inside pages of verification certificates
