As the installation of flowmeters increases on farms and on irrigation schemes, the need and demand for verification and re-calibration services are also increasing. Inaccurate meters refute the purpose of water metering, and a grower can incur a steep fine from the local Water User Association if he withdraws more than his allocation as a result of an inaccurate meter.
Regulation 131 of 2017 of the Department of Water and Sanitation requires that the accuracy of a water measuring device be “verified at least once every five years or for such period as required by the manufacturer, whichever is the shortest period, by a person or an institution accredited to verify the accuracy of the device and furnish proof of such verification and the accuracy of the water measuring device to the authority within thirty days after the verification“.
Many growers and scheme managers are unsure of verification and re-calibration. Questions that are typically raised include:
- What is the difference between calibration, verification, and re-calibration?
- What are the methods available? And which will give the most accurate results?
- How often do I have to verify and re-calibrate my water meters?
- Who can assist me with it?
- Will the verification and re-calibration happen on-site or in a laboratory?
- How long will it take?
- What will it cost?
What are calibration, verification, and re-calibration?
After flowmeters are manufactured, they need to be calibrated before they can be sold. Most flowmeter manufacturers have their own flowmeter calibration facilities to do this initial calibration.
All instrumentation, whether a flowmeter or another type of instrumentation, loses its accuracy over time due to wear and tear and other factors. To correct this loss in accuracy, instrumentation needs to be regularly tested or verified, and re-calibrated if necessary. The two most common methods of verifying a flowmeter are:
- Using a master meter
- Volumetric calibration.
Using a master meter, which is simply another flowmeter, to verify and re-calibrate your flowmeter is a valid method, but only if the master meter itself has been recently calibrated, or verified and re-calibrated, through a process that can be traced back to some national or international standard.
The South African National Accreditation System (SANAS) is mandated through the Accreditation for Conformity Assessment, Calibration and Good Laboratory Practice Act (Act 19 of 2006) to accredit calibration laboratories. In the United States, the national standards laboratory is the National Institute of Standards and Technology (NIST), with locations in Gaithersburg, Maryland, and Boulder, Colorado. In Europe, the main national standards laboratory is the Netherlands Metrology Institute (NMI), also referred to as the Van Swinden Laboratory.
Important: You, the grower or scheme manager, must make sure that you know where and how the master meter that you are using has been calibrated and how long ago.
Volumetric calibration is an alternative method to calibrate a flowmeter. Volumetric calibration means that you push a known volume through your meter, test the meter readings against the known volume, and make the necessary adjustments. Volumetric calibration is the most accurate method, but it is time-consuming and difficult to do on-site. It is usually done in a laboratory.
What are the most suitable master meters?
Clamp-on ultrasonic water meters are considered in the industry as the most suitable and accurate master meters.
There are two types: the “Doppler” type and the “transit time” type. Isobel van der Stoep recommends the “transit time” type for irrigation water. The Doppler type works best with very dirty water, with suspended solids, and is more suitable for the municipal environment.
The price of these meters varies between R30 000 and R200 000 or more. Suppliers include Flotec, Flometrix, Switches International and Flexim (Peter Jones Electronic Equipment are the agents for Flexim in South Africa).
Isobel van der Stoep recommends that you buy the best master meter that you can afford.
She stresses that clamp-on ultrasonic master meters will only be successful if it is used correctly:
- The pipe onto which the ultrasonic meter is clamped should be straight, without any valves or fittings that can cause disturbances, and with a constant diameter for a length of at least 10 times the pipe diameter upstream of the meter and at least 5 times the pipe diameter downstream of the meter.
- The pipe should not vibrate, because the vibration will interfere with the accuracy of the meter.
- The pipe should be preferably PVC or HDPE (high-density polyethylene) and not steel because steel pipes tend to rust, especially older ones. If there is rust, the ultrasonic meter will struggle to take a reading, because the air that gets trapped behind the rust layer in the pipe interrupts the ultrasonic signal.
She recommends that growers and scheme managers create permanent verification ‘’stations’’, for example, by opening a suitable section of the pipe close to a pump station and covering it with a lid.
How often do I have to verify my water meters?
When you buy a water meter, the manufacturer will recommend how often the meter must be verified, and re-calibrated if necessary.
For example, FloCheck recommends that their ElectroFlo meters be verified every 2-3 years because of the normal wear and tear of the pump impeller and the motor. If the water has a high sand content, it will be necessary to verify and re-calibrate every 1-2 years.
Volumetric water meters must also be re-calibrated when:
- You replace the pump, or if you make any changes to the impeller
- You replace or re-wire the electric motor of the pump
- You change to a variable speed drive (VSD).
Who can help me to verify and re-calibrate my water meters?
We could find three companies in South Africa that offer verification (and re-calibration) services:
Flowmetrix has a test facility at their factory in Durban where they can perform a volumetric calibration on flowmeters for a pipe diameter of up to 800mm.
They also have an on-site flow verification service where they use a clamp-on ultrasonic flowmeter as a master meter to test and re-calibrate the existing meter. After verification, a certificate is issued. The re-calibration service will depend on the type of meter and whether Flowmetrix can get electronic access to the meter.
Please contact Theo Dormehl at (031) 274 2266 or Rollan Pillay at (031) 274 2246 for details and cost.
FloCheck has an on-site service where they use a clamp-on ultrasonic flowmeter as a master meter to test and re-calibrate the existing ElectroFlo meter.
The company uses volumetric calibration to re-calibrate their ultrasonic master meters. Francois du Plessis outlines some of the challenges:
We re-calibrate our master meters every 2-3 years, or whenever we feel it is necessary. Recent re-calibrations for flow tempos less than 10 m³/hour took about three days. You have to test the meter for different pipe diameters and different flow strengths, and for each of these, you must push a known volume of water (we use a 2.5 m³ tank) through the meter.
Please contact Francois du Plessis at 083 626 4605 for details and cost.
N&Z Instrumentation has an on-site verification service. They also use clamp-on ultrasonic flowmeters. See http://www.nz-online.co.za/water-metering-mangement/check-or-verify-flow-meters-using-flow-loggers-and-clamp-on-flow-meters.htm for a description of their service. After verification, a certificate is issued. The re-calibration service will depend on the type of meter and whether N&Z can get electronic access to the meter.
If you buy their Fuji ultrasonic meters as test meters, they will give you training on how to verify water meters free of charge.
N&Z has offices in Johannesburg, Cape Town, and Durban.
Please contact Andries Schreuder at 082 361 9532 for details and cost.
GWK has an on-site verification service. They also use clamp-on ultrasonic flowmeters. After verification, a certificate is issued. The re-calibration service will depend on the type of meter and whether GWK can get electronic access to the meter.
Please contact Gert Engelbrecht at 082 923 3440 for details and cost.