Model 750 Gas Meter
Installation, Commissioning and Maintenance Manual
Page 5 of 17
© Gyr
17-May-2016
Due to continuous product development the details above are subject to change without notice. Whilst Gyr has taken all reasonable
care in reviewing the information in this document, Gyr makes no representations in respect of, gives no guarantees or undertakings in
relation to, and to the maximum extent permitted by law, excludes all warranties in relation to, the accuracy or completeness of the information
in this document. Prospective users of this information should make their own enquiries as to its accuracy and reliability.
Gas utilities around the world are pursuing cost
reductions through the adoption of "field life extension
programs" in preference to the rigid statutory periods
of field service of the past.
The Model 750 has a designed capacity of 6 m
3
/hour
of air at 125 Pa pressure drop, a standard proven in
Australia and worldwide, to give durable meters which
maintain high levels of accuracy for many years.
LOW FLOW ACCURACY
Landis + Gyr pioneered enhanced very low flow
accuracy through design and attention to detail in the
manufacturing process. The model 750 meter is rated
to a minimum flow rate of 10 litres per hour
(0.01m
3
/hr) which easily records the smallest pilot
flame, low consumption appliances and system leaks.
Leaks and appliance pilots operate continuously and
therefore the dollar loss is significant if a meter is used
which does not accurately record these low flows.
The importance of low flow accuracy is highlighted by
the fact that some appliances such as storage hot
water heaters etc. which provide the base load, have
permanent pilot flames. Gas is consumed then 24
hours per day, however the consumption profile is
such that a pilot load can account for approximately
18 hours usage. The revenue effect of low flow
inaccuracy can be clearly demonstrated as shown
below.
Example based upon only a 5 litre/hr (0.005 m
3
/hr)
loss of registration: Error of only 0.005 m
3
/hr @ 18
hour/day pilot light = 0.09 m
3
/day x 365 days/year =
32.85 m
3
/annum per consumer.
Any claim that a smaller cyclic capacity meter (such
as 1.2 dm
3
/cycle) is “more accurate” as it has a
smaller membrane surface than that of a 2 dm
3
/cycle
of the 750 meter is incorrect. Design construction, low
friction and low pressure drop ensures low flow
accuracy.
ACCURACY RETENTION
The initial accuracy of a meter is essentially easy to
achieve using calibration to pass initial testing
however sustained accuracy over time comes down to
the quality, the design and construction of the meter.
Landis + Gyr has always recognised that the cost of
changing meters in the field, coupled with repair costs,
is significant, and a meter with a shorter field life also
attracts other costs, including uncertain revenue
collection and hidden but real cost associated with a
loss of consumer confidence. The following are items
of significant importance to the design of a diaphragm
meter that ensures long life with sustained accuracy
retention;
CYCLIC CAPACITY
Cyclic capacity is the volume of gas passed during
one (1) complete cycle of the diaphragm system
within the meter.
The 750 Meter is a slow revving (2 dm
3
/cycle), sliding
valve low friction design with large passageways
which result in low pressure absorption and reduced
wear. The pressure absorption across the 750 meter
is 125 Pascals at a flow rate of 7.5 m
3
/hr of natural
gas. This ensures extended field life and gives long
term accuracy retention.
Countries with
high
accuracy requirements and with
statistical re-verification
procedures in place
normally use only domestic G4 meters with a cyclic
capacity of
2 litres per revolution
.
Countries with
low
accuracy requirements and with
no
statistical procedures in place normally use only
domestic G4 meters with a cyclic capacity of
1.2 litres
per revolution
.
A 1.2 litre cyclic capacity meter has to work
60%
harder than a 2 litre cyclic capacity meter at any given
flow rate and hence this increased speed at which the
valves move, exponentially increase their rate of wear.
For example at just 3 m3/hr = 3000 litres per hour;
a 1.2 litre cyclic capacity meter has to rotate
2500 per hour or 41.67 times a minute.
a 2 litre cyclic capacity meter has to rotate
just 1500 times per hour or just 25 times per
minute.
This is 25 / 41.67 * 100 =
60 % More !!
The faster the meter revolves the higher the velocity
of the valves the greater the wear rate.
The rate of wear will always be a function of the
quality of the valves and valve plate, but a 1.2 litre
cyclic capacity meter will wear out the valve quickly
resulting in poor accuracy retention particularly at low
flow rates.
SLIDING VALVES
The 750 meter is fitted with low friction sliding valves
manufactured from Phenolic Resin. This ensures
extended field life, gives long term accuracy retention
and ensures low flow accuracy.
The Model 750 meter is manufactured with both the
valve table and valve covers lapped to a high
standard. This ensures superior low flow accuracy.
ADJUSTABLE TANGENT
A tangent design allows precision fine tuning of both
the meter accuracy and the accuracy trend (the
