GESTRA LRG 16-60 Installation instructions

Original Installation & Operating
Manual
Conductivity Electrodes
LRG 16-60
LRG 16-61
LRG 17-60
819878-00
EN
English

2LRG 1x-6x - Installation & Operating Manual - 819878-00
Contents
Content of this Manual................................................................................................................................... 4
Scope of delivery/Product package............................................................................................................... 4
How to use this Manual.................................................................................................................................. 5
Illustrations and symbols used...................................................................................................................... 5
Hazard symbols in this Manual...................................................................................................................... 5
Types of warning ............................................................................................................................................ 6
Specialist terms/Abbreviations...................................................................................................................... 7
Usage for the intended purpose..................................................................................................................... 8
Applicable directives and standards...........................................................................................................8
Admissible system components, dependent on the required safety level....................................................9
Improper use ................................................................................................................................................ 10
Basic safety notes ........................................................................................................................................ 11
Required personnel qualifications............................................................................................................... 12
Notes on product liability............................................................................................................................. 12
Functional safety, safety integrity level (SIL) .............................................................................................. 13
Check the safety function regularly..........................................................................................................13
Reliability data as per EN 61508 .................................................................................................................. 14
Function ........................................................................................................................................................ 15
Technical data .............................................................................................................................................. 18
Name plate/Identification ............................................................................................................................ 21
Factory settings............................................................................................................................................ 23
Overall view .................................................................................................................................................. 24
LRG 16-60............................................................................................................................................... 24
LRG 16 -61.............................................................................................................................................. 24
LRG 17-60............................................................................................................................................... 24
Dimensions of LRG 16-60............................................................................................................................. 26
Dimensions of LRG 17-60............................................................................................................................. 27
Dimensions of LRG 16-61............................................................................................................................. 28
Installation.................................................................................................................................................... 29
Additional installation notes ..................................................................................................................... 30
Example LRG 1x-60.................................................................................................................................31
Installation examples with dimensions....................................................................................................... 33
Conductivity measurement ......................................................................................................................33
Conductivity measurement and continuous blowdown control.................................................................. 34
Conductivity measurement and continuous blowdown control via a separate measuring vessel............... 35
Key Fig. 14 to Fig. 16............................................................................................................................... 36
Aligning the terminal box............................................................................................................................. 36

3LRG 1x-6x - Installation & Operating Manual - 819878-00
Contents
Functional elements............................................................................................................................. 37
Connecting the CAN bus system.................................................................................................................. 38
Bus line, cable length and cross-section .................................................................................................. 38
Example ..................................................................................................................................................38
Important notes on connecting the CAN bus system................................................................................. 39
Bringing into service .................................................................................................................................... 40
Changing the factory settings if necessary............................................................................................... 40
Notes on changing communication parameters “bd.rt, Id.Hi or GrP”......................................................... 42
Changing the baud rate ...........................................................................................................................42
Changing the limiter ID ............................................................................................................................ 42
Changing the controller group .................................................................................................................43
Changing the alarm limit value of the limiter function............................................................................... 43
Changing the cell constant....................................................................................................................... 44
Changing the temperature coefficient ...................................................................................................... 44
Using the “CAL” function..........................................................................................................................45
Using the “FiLt” function.......................................................................................................................... 46
Changing the scaling of 4 - 20 mA actual output at LRR 1-60.................................................................. 46
Changing the unit of the display value (µS/cm or ppm).............................................................................47
Triggering a display test manually............................................................................................................ 47
Comparing the measured value with the reference measurement of a reliable sample............................. 48
Checking the alarm limit value Al.Hi by triggering a test function..............................................................48
Lockout function...................................................................................................................................... 48
Start, operation, alarm and test ................................................................................................................... 49
System errors ............................................................................................................................................... 53
Causes ....................................................................................................................................................53
Indication of system malfunctions using fault codes................................................................................. 54
Errors without shutoff .............................................................................................................................. 56
Checking installation and function ...........................................................................................................57
Taking out of service/Disassembly.............................................................................................................. 58
Cleaning the conductivity electrode ............................................................................................................ 59
Monthly comparison of the measured values ..........................................................................................59
Cleaning interval...................................................................................................................................... 59
Disposal ........................................................................................................................................................ 60
Returning decontaminated equipment........................................................................................................ 60
EU Declaration of Conformity....................................................................................................................... 61

4LRG 1x-6x - Installation & Operating Manual - 819878-00
Content of this Manual
Product:
 Conductivity electrode LRG 16-60
 Conductivity electrode LRG 16-61
 Conductivity electrode LRG 17-60
First edition:
BAN 819878-00/08-2019cm
© Copyright
All rights reserved. Any misuse, particularly reproduction and dissemination to third parties, is not permitted.
The General Terms & Conditions of GESTRA AG apply.
Scope of delivery/Product package
 1 x Conductivity electrode LRG 1x-6x
 1 x Sealing ring, D 33 x 39, form D, DIN 7603-2.4068, bright annealed
 1 x Installation & Operating Manual
Accessories
 1 x M12 CAN bus connector, 5-pole, A-coded, with 120 Ωterminating resistor

5LRG 1x-6x - Installation & Operating Manual - 819878-00
How to use this Manual
This Installation & Operating Manual describes how to correctly use LRG 16-60/LRG 16-61/LRG 17-60
conductivity electrodes. It applies to persons who integrate this equipment in control systems, install,
bring into service, operate, maintain and dispose of this equipment. Anyone carrying out the above-men-
tioned activities must have read this Installation & Operating Manual and understood its contents.
 Read this Manual in full and follow all instructions.
 Please also read the instructions for use of any accessories.
 The Installation & Operating Manual is part of the equipment package. Keep it in an easily accessible
location.
Availability of this Installation & Operating Manual
 Make sure this Installation & Operating Manual is always available to the operator.
 If you pass on or sell the equipment to a third party, please also hand over the Installation & Operating
Manual.
Hazard symbols in this Manual
Danger zone/Dangerous situation
Illustrations and symbols used
1. Action to be taken
2.
 Lists
 Bullet points in lists
A Keys to illustrations
Additional
information
Read the relevant
Installation & Operating Manual

6LRG 1x-6x - Installation & Operating Manual - 819878-00
Types of warning
DANGER
Warning of a dangerous situation that results in death or serious injury.
WARNING
Warning of a dangerous situation that may possibly result in death or serious injury.
CAUTION
Warning of a situation that may result in minor or moderate injury.
ATTENTION
Warning of a situation that results in damage to property or the environment.

7LRG 1x-6x - Installation & Operating Manual - 819878-00
Specialist terms/Abbreviations
Here, we explain some abbreviations, specialist terms, etc., which are used in this Manual.
IEC 61508
International standard IEC 61508 describes both the type of risk assessment and actions taken to provide
appropriate safety functions.
SIL (safety integrity level)
Safety integrity levels SIL 1 to 4 are used to quantify risk reduction. SIL 4 is the highest level of risk reduction.
International standard IEC 61508 forms the basis for establishing, testing and operating technical safety
systems.
CAN (Controller Area Network) bus
Data transmission standard and interface for connecting electronic equipment,sensors and control systems.
Data can be sent and received.
LRG .. / URS .. / URB .. / SRL .. / NRG .. / etc.
Equipment and type designations of GESTRA AG, see page 9.
SELV
Safety Extra Low Voltage

8LRG 1x-6x - Installation & Operating Manual - 819878-00
Usage for the intended purpose
LRG 16-60/LRG 16-61/LRG 17-60 conductivity electrodes may only be used for measuring electrical
conductivity in liquid media.
LRG 1x-6x conductivity electrodes are used in combination with the URS 60/URS 61 safety control unit
as a conductivity limiter and in combination with an LRR 1-60 conductivity controller as a continuous
blowdown regulator and limit indicator in steam boiler systems and hot-water plants.
 To guarantee trouble-free operation, observe the water quality requirements as specified
in the Technical Rules (TRD) and EN standards for steam boiler systems.
 The equipment may only be used within the admissible pressure and temperature limits; see
“Technical data” on page 18 and “Rating plate/Identification” on page 21.
 Visualisation and operation take place via an operating unit of type URB 60 or SPECTOR
control .
Applicable directives and standards
LRG 16-60/LRG 16-61/LRG 17-60 conductivity electrodes have been tested and approved for use in the
area governed by the following directives and standards:
Directives:
 Directive 2014/68/EU EU Pressure Equipment Directive
 Directive 2014/35/EU Low Voltage Directive
 Directive 2014/30/EU EMC Directive
 Directive 2011/65/EU RoHS II Directive
Standards:
 EN 12953-09 Shell boilers, requirements for limiting devices
 EN 12952-11 Water-tube boilers, requirements for limiting devices
 EN 60730-1 Automatic electrical controls – Part 1:
General requirements
 EN 61508 Functional safety of electronic systems
Standards documents:
 VdTÜV Bulletin BP WAUE 0100-RL
Requirements for the testing of water monitoring equipment
Technical rules for steam boilers - as a knowledge base:
This Manual occasionally refers to the TRD regulations as a knowledge base.
These regulations have not been in force since 1 March 2019 and are no longer updated.
They are replaced by the TRBS (German Technical Rules for Industrial Health & Safety).
Always observe the current regulations (EU directives, EN standards, information from emplo-
yers’ liability insurance associations, etc.) in order to comply with the current state of the art.

9LRG 1x-6x - Installation & Operating Manual - 819878-00
Usage for the intended purpose
Admissible system components, dependent on the required safety level
In accordance with EU Pressure Equipment Directive 2014/68/EU and standards EN12952, EN12953,
EN 61508, and the technical rules of VdTÜV Bulletin BP WAUE 0100-RL, the conductivity electrodes can be
operated with the following system components, depending on the required safety level.
Conductivity
electrodes
Safety
control unit for
limiter
Control unit as
continuous blowdown controller,
limit indicator
or automatic intermittent
blowdown
Operating unit
SIL 2
as per
EN 61508
LRG 16-60
LRG 17-60
LRG 16-61
URS 60
URS 61 −
URB 60,
SPECTOR
control
without
safety level in line
with
EN 61508
LRG 16-60
LRG 17-60
LRG 16-61
− LRR 1-60
URB 60,
SPECTOR
control
Fig. 1
Key to Fig. 1:
LRG = Conductivity electrode
URS = Safety control unit
URB = Visual display and operating unit
LRR = Conductivity controller
To ensure the proper use of equipment during all types of use, you must also read the Installa-
tion & Operating Manuals for the system components used.
 You will find the latest Installation & Operating Manuals for the system components named
in Fig. 1 on our website:
http://www.gestra.com/documents/brochures.html

10 LRG 1x-6x - Installation & Operating Manual - 819878-00
Improper use
There is a danger of death due to explosion if the equipment is used in potentially explo-
sive atmospheres.
Do not use the equipment in potentially explosive atmospheres.
Do not bring any equipment into service that does not have its own specific name plate.
The name plate indicates the technical features of the equipment.

11LRG 1x-6x - Installation & Operating Manual - 819878-00
Basic safety notes
There is a danger to life from scalding if the conductivity electrode is removed under
pressure. Steam or hot water can spurt forcefully out of the equipment.
 Only remove the conductivity electrode at 0 bar boiler pressure.
There is a risk of severe burns when working on conductivity electrodes that have not
been allowed to cool. The conductivity electrode becomes very hot during operation.
 Always allow the conductivity electrode to cool.
 Perform all installation and maintenance work only on conductivity electrodes that have
been allowed to cool.
There is a danger to life from electric shock when working on electrical systems.
 Always switch off the voltage to the system before performing connection work.
 Check that the system is not carrying live voltage before commencing work.
Danger to life from faulty LRG 1x-6x conductivity electrodes due to the sudden escape of
hot steam or hot water.
Impacts or knocks during transportation or installation can cause the 1x-6x conductivity elec-
trodes to become damaged, whereby hot steam or hot water can escape through the pressure
relief hole.
 Avoid damage, for example from strong knocks to the electrode rods,
during transportation and installation.
 Before and after installation, check that the conductivity electrode is completely undam-
aged. When bringing it into service, check that it is leak-tight.
Attempts to repair the equipment will cause the system to become unsafe.
 LRG 1x-6x conductivity electrodes may only be repaired by the manufacturer, GESTRA AG.
 Only replace faulty equipment with identical equipment from GESTRA AG.

12 LRG 1x-6x - Installation & Operating Manual - 819878-00
Required personnel qualifications
Activity Personnel
Integration in control system Specialist staff Plant designer
Installation/electrical
connection/bringing into service Specialist staff
The device is an item of equipment with a
safety function (EU Pressure Equipment
Directive) and may only be installed, electri-
cally connected and brought into service by
suitable, trained staff.
Operation Boiler service technician Staff trained by the plant operator.
Maintenance work Specialist staff
Maintenance and modifications may only
be performed by authorised staff who have
undergone specific training.
Modifications Specialist staff Persons trained by the plant operator to work
with pressure and temperature.
Fig. 2
Notes on product liability
We the manufacturer cannot accept any liability for damages resulting from improper use of the equip-
ment.
Basic safety notes
Deficient maintenance and cleaning can lead to the conductivity electrodes becoming
damaged and/or to incorrect measurement results and false alarms.
 Once a year, perform a check on the conductivity electrodes using comparative
measurements.
 Adhere to the maintenance and cleaning intervals; see page 59.

13LRG 1x-6x - Installation & Operating Manual - 819878-00
Functional safety, safety integrity level (SIL)
In combination with the URS 60/URS 61 safety control unit, LRG 16-60/LRG 16-61/LRG 17-60
conductivity electrodes are suitable for use with safety functions up to SIL 2.
Theyare eachelementsofasafety circuitup toa ratingof SIL2 asper EN61508 intheSPECTORc
onnect system,and
can transmit alarm notifications.
Combining them with accessories produces a type B subsystem. The following information on the techni-
cal and safety characteristics in Fig. 4 are based solely on LRG 1x-6x conductivity electrodes.
Breakdown of safety function failure rates (safety shutoff)
LRG 1x-6x
LRG 1x-6x
Mechanical
LRG 1x-6x
Electronic
CANopen
URS 60,
URS 61 Safety circuit
Fig. 3
The CANopen interface
is a “Black Channel”
be ignored during calculation due to
the low failure rate of < 1FIT.
type, and can
Check the safety function regularly
The functioning of the conductivity electrode must be monitored once a year by triggering the test func-
tion (T1 = 1 year). The test function can be triggered on site via the integrated rotary knob of the terminal
box; see page 52.
Furthermore, it is possible to trigger the test function remotely from URS 60/URS 61 or the external oper-
ating units URB 60 or SPECTOR
control system.

14 LRG 1x-6x - Installation & Operating Manual - 819878-00
Reliability data as per EN 61508
Description Characteristic values
Electrode type LRG 1x-60 LRG 16-61
Safety integrity level SIL 2 SIL 2
Architecture 1oo1 1oo1
Equipment type Type B Type B
Hardware error tolerance HFT = 0 HFT = 0
Overall failure rate for dangerous undetected
failures λDU = < 20 * 10-8 1/h λDU = < 35 * 10-8 1/h
Overall failure rate for dangerous detected
failures λDD = < 5000 * 10-9 1/h λDD = < 5000 * 10-9 1/h
Safe failure fraction SFF > 96.0 % SFF > 95.0 %
Test interval T1 = 1 year T1 = 1 year
Probability of dangerous failure on demand PFD < 100 * 10-5 PFD < 160 * 10-5
Diagnostic coverage. Percentage of dangerous
failures detected by a test. DC > 95.0 % DC > 91.0 %
Mean time to dangerous failure MTTFd> 30 a MTTFd> 30 a
Diagnostic interval T2 = 1 hour T2 = 1 hour
Performance level (as per ISO 13849) PL = d PL = d
Probability of dangerous failure per hour PFH < 20 * 10-8 1/h PFH < 35 * 10-8 1/h
Ambient temperature as a basis for calculation Tu = 60 °C Tu = 60 °C
Mean time to repair MTTR = 0
(no repair)
MTTR = 0
(no repair)
Fraction of undetected dangerous failures that
have a common cause beta = 2 % beta = 2 %
Fraction of detected dangerous failures that
have a common cause beta d = 1 % beta d = 1 %
Fig. 4

15LRG 1x-6x - Installation & Operating Manual - 819878-00
Function
The equipment measures the electrical conductivity in electroconductive liquid media.
Measuring process - LRG 16-60/LRG 17-60
The LRG 16-60/LRG 17-60 conductivity electrodes work as per the conductometric two-electrode
measuring process. A measuring current is passed through the medium with a frequency adapted to the
measuring range. This causes a potential gradient between the electrode and measuring tube, which is
evaluated as measuring voltage.
Measuring process - LRG 16-61
The LRG 16-61 conductivity electrode works as per the conductometric four-electrode measuring pro-
cess. It consists of two current and two voltage electrodes. From the current electrodes, a measuring
current with a set frequency is introduced into the medium. This causes a potential gradient between
these electrodes. This potential gradient is picked up in the media by the voltage electrodes and is evalu-
ated as measuring voltage.
Temperature compensation of the measured values based on a reference temperature (25 °C)
The electrical conductivity is a function of the temperature. In order to base the measured values on a
reference temperature, an integrated resistance thermometer therefore measures the medium’s
temperature. The electrical conductivity is calculated from the measuring current and the measuring
voltage and is based on the reference temperature of 25°C through temperature compensation.
Compensation process
The measured value of the conductivity is corrected linearly depending on a set temperature
coefficient. The coefficient (standard is 2.1%/°C) is usually used for steam generating units with
constant pressure. The conductivity is determined for an ambient temperature (25 °C).
The gradients are then verified at operating pressure with a calibrated conductivity measuring device.
Application
LRG 1x-6x conductivity electrodes are used in combination with the URS 60/URS 61 safety control unit
as a conductivity limiter and in combination with an LRR 1-60 control unit as a continuous blowdown
regulator in steam boiler systems and hot-water plants.
Automatic self-test
An automatic self-test cyclically monitors the safety and function of the conductivity electrode and
measured value recorder.
The data are transferred to the URS 60/URS 61 safety control unit in the form of a Black Channel data
telegram using the CANopen protocol on the basis of a CAN bus as per ISO 11898.

16 LRG 1x-6x - Installation & Operating Manual - 819878-00
Function
The following information is transferred as data telegrams:
 Measured value of the conductivity, temperature-compensated
 The temperature of the medium
 MAX limit value setting limiter
 Temperature coefficient (tC) and cell constant (CF)
 Limiter test command
 Status and fault information
 Alarm signals from the conductivity electrode if the limit values are exceeded
 Error messages in the event of faults in electronic or mechanical parts
 Temperature in the terminal box of the conductivity electrode
Displays and signals; see page 49 / 54*
The LRG 1x-6x conductivity electrodes have a 4-digit green 7-segment display for showing measured
value and status information as well as fault codes. Several coloured LEDs are used to display alarm
states and the set unit.
Behaviour in the event of alarms *
The alarm state for an exceeded limit value is shown on the display as “Hi.C” alternately with the actual
value of the conductivity.
The alarm is transferred to the URS 60 or URS 61 safety control unit via CAN data telegram. Once the time
delay has elapsed, the alarm signal triggers the safety shutoff in the safety control unit. The URS 60 or
URS 61 safety control unit does not lock automatically for this.
The LEDs 1 and 4 signal the corresponding MAX alarm case see page 51 .
The safety circuit is interrupted without a delay if the following faults occur:
 Fault in the sensors (open circuit, short circuit, faulty component, excessive temperature)
 Communication failure
Behaviour in the event of faults *
The safety functions of the equipment are monitored in the sensors through periodic self-tests. Fault
messages are updated with each self-test and are permanently stored in the fault memory. If there are no
faults, only the list of current faults and their display is deleted.
Alarm and fault signals are indicated by LEDs and shown on the 7-segment display of the conductivity
electrodes and are transferred to the URS 60/URS 61 safety control unit via CAN data telegram.
Electrode alarms and errors faults cannot be acknowledged.
When the alarm or error is cancelled the display also goes blank, and the URS 60/URS 61
safety control unit closes the safety contacts again.

17LRG 1x-6x - Installation & Operating Manual - 819878-00
Simulating the alarm state *
The triggering of an alarm can be simulated by a keystroke on the rotary knob of the LRG 1x-6x, by
pressing the appropriate button on the URS 60/URS 61, or by using the URB 60. The device combination
will then behave as if a normal alarm had been triggered.
* The tables from page 49 clearly show the relationship between the
equipment status, the display and the alarm LEDs.
Parametrising or changing the factory settings
If necessary, you can adapt the conductivity electrode to the system conditions on site. You can set the
parameters or change the factory settings by means of a rotary knob on the terminal box; see page 40.
Function

18 LRG 1x-6x - Installation & Operating Manual - 819878-00
Technical data
Model and mechanical connection
 LRG 16-60, LRG 16-61, LRG 17-60 Thread G1 A, EN ISO 228-1, see Fig. 9, 10, 11
Nominal pressure rating, admissible service pressure and temperature
 LRG 16-60 PN 40 32 bar (g) at 238 °C
 LRG 16-61 PN 40 32 bar (g) at 238 °C
 LRG 17-60 PN 63 60 bar (g) at 275 °C
Materials
 Terminal box 3.2581 G AlSi12, powder-coated
 Sheath 1.4301 X5 CrNi 18-10
 Measuring electrodes 1.4571 X6CrNiMoTi17-12-2
 Electrode insulation PTFE
 Screw-in body:
 Measuring tube, measuring screw
LRG 16-60, LRG 17-60 1.4571, X6CrNiMoTi17-12-2
 Spacer
LRG 16-60, LRG 16-61, LRG 17-60 PEEK
Deliverable installation lengths of the electrodes (cannot be shortened)
 LRG 16-60, LRG 17-60 200, 300, 400, 500, 600, 800, 1000 (mm)
 LRG 16-61 180, 300, 380, 500, 600, 800, 1000 (mm)
Temperature sensor
 Resistance thermometer Pt 1000
 Measuring range for the temperature of the medium 0 to 280 °C
Conductivity range at 25 °C
 LRG 16-60, LRG 17-60 0.5 μS/cm to 6,000 μS/cm, 0.25 - 3000 ppm *
 Preferred measuring range up to 1000 µS/cm
 LRG 16-61 50 μS/cm to 10,000 μS/cm, 25 - 5000 ppm *
 Preferred measuring range from 500 µS/cm
* Conversion µS/cm in ppm (parts per million): 1 μS/cm = 0.5 ppm
Measuring cycle
 1 second

19LRG 1x-6x - Installation & Operating Manual - 819878-00
Technical data
Measuring quality (information for value ranges between the factory calibrating points)
 LRG 1x-60
Triggering internal processing * Measurement deviation Linearity deviation
 Range 1: 0.5 µS - 10 µS 7 % 2 %
 Range 2: 10 µS - 250 µS 3 % 2 %
 Range 3: 250 µS - 2600 µS 3 % 1 %
 Range 4: 2600 µS - 21000 µS 3 % 1 %
 LRG 16-61
Triggering internal processing * Measurement deviation Linearity deviation
 Range 1: 10 µS - 100 µS 2 % 2 %
 Range 2: 100 µS - 2000 µS 2 % 1.5 %
 Range 3: 2000 µS - 50000 µS 2 % 1 %
* Triggering internal processing based on 15 bit with sign before (16 bit).
The aforementioned values are related to uncompensated conductivity.
Time constant “T” (measured using the two-bath process)
Temperature Conductivity
 LRG 16-60, LRG 17-60 9 seconds 14 seconds
 LRG 16-61 11 seconds 19 seconds
Temperature compensation
 The temperature compensation process is linear and is adjustable via parameter tC; see page 44.
Supply voltage
 24 V DC +/-20 %
Power consumption
 max. 7 VA
Current input
 max. 0.35 A
Internal fuse
 T 2 A
Safety cutout at excessive ambient temperature
 The cutout takes place at excessive ambient temperatures Tamb. = 75 °C
Electrode voltage
 < 500 mV (RMS) at no load

20 LRG 1x-6x - Installation & Operating Manual - 819878-00
Technical data
Input/output
 Interface for CAN bus as per ISO 11898, CANopen, insulated
 M12 CAN bus connector, 5-pole, A-coded
 M12 CAN bus socket, 5-pole, A-coded
Indicators and controls
 1 x 4-digit green 7-segment display for showing measured values and status information
 1 x red LED for indicating an alarm state
 3 x green LED for indicated the unit μS/cm / ppm and the OK state
 1 x rotary knob IP65 with button for menu navigation and test function
Protection class
 III Safety Extra Low Voltage (SELV)
IP rating as per EN 60529
 IP 65
Admissible ambient conditions
 Service temperature: 0 °C – 70 °C
 Storage temperature: - 40 °C – 80 °C
 Transport temperature: - 40 °C – 80 °C
 Air humidity: 10 % – 95 %, non-condensing
Weight
 LRG 16-60/LRG 16-61/LRG 17-60 approx 2.1 kg