5 things to consider when choosing a flow meter
Business management guru Peter Drucker said “If you can’t measure it, you can’t manage it”. The sentiment certainly applies to irrigation, although the mantra hasn’t always been strictly followed! However, with increasing water scarcity, the irrigation sector is becoming more accountable for its water use, and many districts are implementing metering at the farm service point.
To manage well, you need to measure accurately and measuring water in a gravity irrigation district can be difficult. But most importantly purchasing the wrong meter can be a costly mistake. As the USBR warns in its Water Measurement Manual, “Measuring devices and techniques must be properly selected, installed, used, and maintained.”
To help you select well and avoid some of the pitfalls, here are the top five things you should think about before investing:
1. Can you afford head loss
Loss of energy or “head” occurs as water moves through structures such as pipes, flumes and flow meters, reducing the amount of land that can be irrigated using gravity irrigation. Head loss is proportional to velocity squared, and a meter with a larger diameter or cross-section will result in lower velocities and thus lower head loss. So a larger meter is generally better, provided it can measure low velocities accurately.
If you can’t afford head loss, remember that some meters such as mechanical meters (that rely on moving parts for measurement), differential head meters and orifice meters also increase head loss compared to others such as ultrasonic or magnetic meters.
2. Initial versus total cost
The total cost of a meter includes the cost of the meter, plus the cost to install, the annual operational and maintenance costs and of course, the life of the device. The purchase cost and lifetime are fairly easy to determine.
But when considering installation costs, be aware that many meters require a straight length of pipe or channel upstream and downstream of the meter to ensure a stable and repeatable flow velocity. The civil infrastructure and/or pipes required to achieve this can easily cost more than the meter itself. Meters that can cope with turbulence and swirling flows are therefore cheaper to install.
When considering maintenance costs, consider the accessibility of the installed meter. How easy is it to remove to clean or maintain? Is the meter built for reliability in harsh irrigation conditions? Given the water conditions in your area, how often will you need to visit the site to check or clear obstructions? Most meters can only tolerate a limited amount of foreign matter. If weeds or sediment or other matter is an issue in your irrigation water, the required regular maintenance can have a dramatic impact on total cost. And as the USBR warns, “Maintenance costs can be excessive if meters are used for water with sediment”. In such situations you may want to consider a meter that is tolerant to silt and sediment.
Solar powered meters can help keep costs down by removing the need to travel to site and replace batteries on a regular basis, particularly if your turnouts are located in remote areas.
3. Alarms and alerts
Ongoing, accurate flow measurement means you need to know if there is a fault with the meter or with calibration right away. A meter that is connected to a telemetry network can send you an alarm if anything goes wrong, giving you confidence that the meters are installed, working, and measuring correctly. Some meters can also tell you the amount of sediment so you know when it is time to give them a clean to maintain high flows with low head loss.
4. Tolerance to silt and sediment
Silt and other sediments are common problems for irrigation meters. Deposited silt in particular is a problem because it changes the cross-sectional area of the flow meter and this generally causes the flow meter to over-read. So it’s worth checking with your supplier if sediment is likely to be an issue.
5. Future-proofing your investment
It’s difficult for any of us to read the future, especially as computers and telemetry are increasingly applied to irrigation. So try to buy a meter that lasts that test of time. You may not need telemetry today but what about in 5 or 10 years? You may not need high accuracy today but what if the regulations change?
Look for meters that have an upgrade path, that are modular and allow components to be added or replaced when technologies change or your budget allows. Try to avoid something that closes your options.