BASIC VOLUME CORRECTORS - 101
CGA Gas Measurement Conference,
June 1, 2010 Saskatoon, SK. Canada
Mercury Instruments / Honeywell HFS
In order to buy and sell a gas between parties there must be an agreement between the buyer
and seller of how to accurately measure the gas volume and content. Normal transactions for
natural gas are conducted at a set temperature value and a set pressure value, called “Base
Conditions”. Additionally, the constituents in the gas are measured and the gas given a BTU
value or heating value based on the amount of methane in the gas. The volume is adjusted for
these “factors” and gas can then be transferred between the parties on an equitable basis. In
Canada, the USA and most countries these values are mandated and measurement revolves
around these set values on a contractual basis.
When natural gas was first sold to a customer, the supplier would count the number of gas lamps
in the location and charge a flat fee per lamp. Over time rudimentary gas meters were developed
that made a great improvement and suppliers could charge for the gas actually used. As time
passed and these meters were improved, the price of gas rose and the customer’s gas usage
increased requiring a more accurate measurement method. Even with the increasing quality of
gas meters, they did not calculate volume based on “Base conditions” and some additional
means was needed to accurately adjust the meter’s reading to bring it into an equitable
measurement of the gas actually used. This need created the first attempts at mechanisms and
constructing devices to adjust for these effects of pressure and temperature. These would lead to
instruments called “Volume Correctors”.
The first correctors were clock like mechanical devices with links, levers and various methods of
adapting a conventional meter’s “uncorrected volume” to a “corrected” measurement more
reflective of how much gas was actually used. The links, levers and cams “corrected” for the
actual temperature and pressure effects on the gas bringing the “corrected” read to the base
standards. Today we use electronic devices that are actually computers in a box built for outdoor
environments. These can accurately “correct” the gas volume calculation used for bill creation in
real time, record data points, alarms and historical usage information. They communicate these
accurate measurements to host data collection systems using various communication
Mechanical corrector Volume chart recorder
Ideal gas laws
The density of a gas at a specific pressure can be estimated by using the ideal gas law. Doubling
absolute pressure doubles the density of a gas, and doubling absolute temperature halves the
density. The number of molecules in a given gas volume depends on the pressure and
temperature of the gas. This is why the gas temperature and pressure must be stated in order for
a volume measurement to mean anything. Provincial and Federal regulators use base standards
industry wide to regulate the exchange of natural gas. Volume correctors produce very accurate
measurements of actual gas usage from .002-.005% of the instruments overall pressure range.
Commonly used base standards in North America:
Canada, Pressure 101.325 kpa absolute, temperature 15 degrees C
USA, Pressure 14.73 PSIA, temperature 60 degrees F
Mexico, Pressure 1 kgf/cm2 (equivalent to 98.1 kpa absolute) temperature 20 degrees C
Common EVC installations
Volume correctors are used with diaphragm meters, rotary meters, turbine meters, ultrasonic
meters and other flowing gas meter types. In all applications they perform the same function of
converting their volumes to “Corrected Base Standard” volumes.
Typically, there are two types of EVC (electronic volume corrector) installations, direct mount on
the meter (conventional) or indirect mount / remote mounted.
Direct mounted on turbine meter Direct end mounted on rotary meter
Remote mounted on Ultrasonic meter Direct mounted for coriolis meter
How a volume corrector works
Volume correctors consist of a mix of mechanical and electronic devices. They primarily are
battery powered microprocessors in a box built with very rugged components to work outdoors in
a wide variety of environments which can range from -40F to +170F. They are built to be installed
and operated in Class 1, Division 1 or Class 1 Division 2 hazardous environments. They must
pass all industry and government standards for accuracy and safety requirements in order to be
used for “custody transfer” of natural gas.
First, there must be an input from the meter of some type, either a mechanical drive, electronic
pulse or high frequency input to begin the process. An input “pulse” is generated into the main
board of the corrector from the meter as an “Uncorrected volume” pulse input. It references a
customer’s set value in the instrument for the meter type and size. For instance, a diaphragm
meter has a drive dog or “wriggler” that rotates past a switch in the instrument base. When the
main board see’s this input pulse, it knows 10 Cu Ft of uncorrected gas has passed through the
meter. This input causes the instrument to measure the pressure and calculate a pressure factor
multiplier, it measures the temperature and calculates a temperature factor, it also calculates a
composition value from the values in memory for CO2, N2 and specific gravity content of the gas
and creates a multiplier factor for that. It will then perform the algorithm using these live real time
calculated factors, update the “Corrected Volume” item registers, display and check for any alarm
limit points exceeded. It then time and date stamps the information and records it into its memory.
Typical volume corrector configuration Temperature probes Pressure transducers
Beyond the basic live correction function
At the top of the correction interval record, generally an hourly record, the corrector will store
historical information about the previous hour in internal memory. This is typically, incremental
Corrected and Uncorrected volume, average pressure and average temperature, corrected and
uncorrected meter reads, flow rates etc. This data becomes the “audit trail” data in the instrument.
These records are not changeable in the instrument for security reasons. These records are
downloaded from the instruments by the utility to create billing records for the customer. They can
be downloaded daily, weekly or monthly and the data will remain in their memory 41 days to 165
days as audit trail interval records.
Example audit trail records retrieved from EVC
In addition to the historical audit trail, the instrument has a large quantity of data registers and the
current values they contain. These are called “item files” and are both utility chosen values and
real time reads from the last “wake up cycle”. These for instance, would be the meter reads, flow
rates, gas temperature, customer identifier information and many other values. For example item
number 209 would be the last calculated instantaneous flow rate expressed in CU Ft per hour.
The item files can be downloaded and saved locally or remotely.
Example of item files retrieved from corrector
Other recording reporting functions
Electronic volume correctors retain data and produce the information for many other reports such
• Alarm reports, list of time and date stamped alarms that have occurred in the instrument
• Calibration reports, date and time stamped record of last three calibrations and required
adjustments for pressure transducers and temperature system.
• Event log report, listing of date and time stamped changes made to the configuration of
the instrument including the values before and after the change.
• Custom reporting functions, customer created items
• Information used for graphing pressures, temperature and gas flows
Example of a calibration report using items from EVC
Modern Volume Correctors
Electronic volume correctors are the center component of many modern gas measurement
systems and often do much more than their standard volume correction function. Frequently, they
interface with AMR (automated meter reading) and AMI (automated meter interface) and
communicate customer usage information, alarms and maintenance information to the host
utilities data collection system. Often, they send signals to other hardware at the customer
location for their internal customer energy management system. Very commonly they send usage
and control signals directly to other hardware at the site such as automated meter oilers, gas
odorization equipment. All of these functions have become central to the instruments purpose
and relied on for metering operations.
Sensus Autoadjust Turbine meter w/oiler Instromet ultrasonic meter w/corrector/SCADA
Communicating the data
Electronic volume correctors communicate their data in various ways beginning with the local
display. This is a pushbutton accessible meter reader list of programmed items the utility or
customer may need to see. All data is accessible through serial laptop connections or handheld
devices such as HP pocket PC. Commonly they will be wired to a pulse collection unit which
accumulates these volume pulses and communicates them to a host data collection system such
as Mercury Instruments DC-2009, Itron ERT, Cellnet or Aclara based systems using a radio
EVC’s can have internally mounted serial telephone or wireless modem communications or be
connected to externally mounted communications equipment. They will operate with radios, Leo
and Geo satellite modems or local LAN systems and often work in a mixed system using multiple
types of communications in a utility.
Corrector with external cellular communications Internal wireless modem
Other tasks performed by volume correctors include direct interfaces to other equipment at the
metering station such as automated meter oilers or gas odorizers. These are normally achieved
by the corrector sending a pulse, based upon metered volume, having gone through the meter. In
a typical scenario, the corrector will instigate a pulse to an oiler when the chosen amount of
volume is achieved and the oiler will inject oil into the turbine meter. Another example would be a
signal to add odorant into the gas using odorization equipment based upon volume through the
meter. Some customers also receive this pulse to know when to change fuels if using fuel
switching systems, near curtailment limit or for blending gases.
Commonly, another type of output from the corrector is a 4-20ma output signal. A daughter board
is used to convert serially communicated volumes, pressures, temperatures or flow rates to an
analog 4-20ma signal which can be read in reverse by many energy management systems. The
information is used by the customer to control their operations based on gas flows and pressures
to their equipment.
A critical function of EVC’s is monitoring the equipment at metering stations for alarm conditions.
An alarm can be a simple low battery alarm up to a critical high pressure situation. Some
companies use electronic correctors as end points for their SCADA systems and import data such
as pressure, temperature and flow rates. Often the corrector is equipped with communications
and will call in and report alarms to customizable alarm monitoring and reporting software which
can process the alarms in their organizations.
Often data in the internal memory is used in dispute resolutions with the utilities customers. The
day to day meter usage can be verified from the recorded information as the instrument will have
extensive time tagged hourly records of the uncorrected volume, average pressure and average
temperatures. Using these records allows documented answers to questions such as when, how
much and under what conditions allowing quick inexpensive resolutions to complex problems..
Electronic volume correctors are evolving into more applications every day. A major new
application today is the addition of slimmed down correctors to allow cost competitive versions for
smaller meters and customers with lower volumes. In the past only the largest customer meters
could justify the additional cost of volume correctors. While not offering all the features of a full
blown electronic volume corrector, they add increased accuracy, memory and communications
capability to a never before served level of customer.
Mercury TCI Dresser Roots PTZ Romet ECM
Electronic volume correctors have evolved to be a critical part of equitably exchanging custody
between parties of natural gas, a difficult to measure material. Based on the primary need to
measure and correct the actual standard cubic volume of gas in real time, it has grown to include
many other features and act as the central device in a complex metering station. In the opposite
vain, the technological advances have allowed the costs to be reduced and the reliability of
instruments to greatly increase. Electronic volume correctors are getting smaller, smarter and
offer lower cost solutions providing more flexibility to use them. They are making possible cost
savings through AMR/AMI data collection, alarm monitoring and being able to easily prove
accuracy to customers of the billing data.
For more information, visit, www.Mercuryinstruments.com, www.Dresser.com or