When an electromagnetic flow meter is worth it — and when it is not
Short answer: Choose an electromagnetic flow meter only if one of three things is true — the line is larger than 40NB, the water is dirty or chemically aggressive, or you need the reading in a panel or SCADA system. Outside those cases, spending ₹30,000 and upwards makes no sense; a rotameter or digital meter will do the job.
This is the most expensive flow meter we supply, so it is worth understanding before buying. In ETP and STP plants people often skip it and then have to fit one anyway to meet pollution board requirements.
How it works — with no moving parts
Coils around the pipe create a magnetic field. As water passes through that field it generates a small voltage, and that voltage is directly proportional to the flow. The meter measures the voltage and converts it into a flow reading.
This means nothing rotates inside the pipe — no float, no impeller. As a result:
- Nothing wears out, so it lasts for years
- Dirty water, solids and sludge all pass through without a problem
- Pressure drop is essentially zero (a rotameter costs you a little pressure)
Four situations where it is necessary
1. Lines larger than 40NB
On a large line a rotameter becomes physically big, expensive and adds pressure drop. Above 50NB an electromagnetic meter is the practical choice.
2. Dirty water — ETP, STP, raw water
A rotameter tube clouds over within a few months here and the reading disappears, and deposits also build up on the float. An electromagnetic meter simply does not have this problem.
3. Pollution board requirements
ETP and STP discharge flow has to be recorded. That needs a meter with an output that can transmit data — usually together with a telemetry modem.
4. Chemical or acid lines
These need PTFE lining. Standard lining degrades in acid. The PTFE version costs roughly 10 to 30 percent more, but on an acid line the meter will not survive without it.
Size chart and prices
Note that ranges in this category are given in m³/hr. 1 m³/hr equals 1,000 LPH.
| Size | Range (m³/hr) | Equivalent (LPH) | Standard | PTFE |
|---|---|---|---|---|
| 25NB | 0.2 – 10 | 200 – 10,000 | ₹30,450 | ₹34,125 |
| 40NB | 1 – 20 | 1,000 – 20,000 | ₹34,125 | ₹40,425 |
| 50NB | 2 – 35 | 2,000 – 35,000 | ₹38,325 | ₹51,450 |
| 65NB | 4 – 60 | 4,000 – 60,000 | ₹44,625 | ₹56,700 |
| 80NB | 6 – 90 | 6,000 – 90,000 | ₹44,625 | ₹58,800 |
| 100NB | 7 – 140 | 7,000 – 1,40,000 | ₹53,550 | ₹71,400 |
| 150NB | 10 – 300 | 10,000 – 3,00,000 | ₹69,300 | ₹90,300 |
When it is a waste of money
- On a 500 LPH RO plant — a ₹504 F500 rotameter does the same job
- On RO permeate or DM water — the water is not conductive, so the meter will not read at all
- Where you only need to look, not record — a rotameter is enough
- On 15NB or 20NB lines — there is no benefit at that size
Four installation essentials
- The pipe must always be full. A partly full pipe gives a wrong reading. Fit the meter in a low section of pipe or on a rising line, never on a falling line.
- Proper earthing is essential. Without it the reading wanders. This is the single most common installation error.
- Allow air to escape. Trapped air causes errors, so an air release valve (25NB) should be fitted.
- Leave straight pipe — 5 diameters upstream and 3 downstream.
Sending the data further
To get the reading to a control room or a pollution board server, the meter is paired with a telemetry modem (₹13,650) or a 4G modem with RS485 input (₹9,450). For logging on a laptop, an RS485 to USB converter (₹6,300) is enough.
Get the right magnetic meter for your plant
Send us your flow in m³/hr or LPH, the pipe size and the type of liquid — we will confirm the correct model and lining.
💬 WhatsApp par poochhein 📞 9910646957Frequently asked questions
Why does an electromagnetic flow meter not work on RO permeate?
The meter relies on the conductivity of the water. RO permeate and DM water have very low dissolved solids, so they do not conduct well and the meter receives no usable signal. On those lines a rotameter or an FT-650 digital meter should be used instead.
When is PTFE lining necessary?
When the line carries acid, alkali or any aggressive chemical, such as an ETP dosing line or a pickling plant. Standard rubber lining degrades within a few months in those conditions. The PTFE version costs more but there is no alternative there.
The meter reading keeps fluctuating — what should I check?
Check the earthing first, as that accounts for the majority of such cases. Then confirm the pipe is completely full and that no air is trapped in the line. Fitting an air release valve usually resolves it.
How long does an electromagnetic flow meter last?
It has no moving parts, so with correct installation and the right lining it will run for many years. Unlike a rotameter float, there is nothing inside that needs periodic replacement.
Which size should I choose for an ETP or STP?
Work out your discharge flow in m3/hr and choose a size where that flow sits mid-range. For example 20 KLD is roughly 0.85 m3/hr, for which 25NB is adequate. The line size also has to match, so please send us both your flow and pipe size.