How to Select a flowmeter

How to Select a flowmeter

Flowmeter selection must consider a few parameters.

Siemens has an excellent guide on liner and electrode selection, which is attached to this article.

Power Supply

The power supply voltage which can be 24VDC/24VAC or 120VAC. The simplest is to use 24VDC wiring, but if not involved in the panel, you should show preference to a flowmeter with a power supply that can support multiple voltages

Conductivity of Liquid

Generally Wastewater Flowmeters are rated for conductivity of  ≥5uS/cm, but some are ≥20uS/cm,
If you consider that  1 millisiemens/centimeter [mS/cm] = 1000 microsiemens/centimeter [μS/cm, uS/cm],
You can see that either  ≥5uS/cm, but some are ≥20uS/cm is more than sufficient.

Water TypeElectrical Conductivity (mS/cm)
Distilled Water0.5-3
Tap Water50-800
Potable Water (US) 30-1500
Freshwater Streams100-2000
Industrial Wastewater10,000
Seawater 55,000


Viscosity should not have any impact on magnetic flowmeters...but IFM seems to set a limit in every case. It seems the reasonf or this is potential fouling of the electrodes.. 

Below is a range of Viscosity in mPas (1 mPas = 1 CP) 

The unit of measure of kinematic viscosity is Centistokes (cSt). [mm2/s = (centistokes, cSt)]

  • Kinematic (cSt) x Density = Dynamic (cP)
  • Dynamic (cP) / Density = Kinematic (cSt)

A basic difference between the dynamic and kinematic viscosity measurements is density. Density actually provides a way to convert between a kinematic and a dynamic viscosity measurement. The formula for the conversion is:

When talking about wastewater we generally consider a Density of 1 so Cst= CP.

This means that for 1% Sludge we can consider that a flowmeter must be rated for 20Cst(mm2/s)
For a 3% Sludge we need a flowmeter rated for 100Cst(mm2/s)

For sludge without polymer, viscosity was 0.04–0.11 Pa·s, while for dewatered sludge with polymer, it was 0.1–0.2 Pa·s.

This means some flowmeters from our approved vendor list are not suitable for wastewater

Manufacturer Table

Operating Range
AISI 316
DN25 1" NPT
Sludge up to 2%
AISI 316
DN50 2" NPT
Sludge up to 2%
Endress Hauser

AISI 316
DN65 2.5"
Sludge, not impacted by viscosity
DN65 2.5"
When ordered with order code P13 it can go from 0.31-to 40m3/hSludge, not impacted by viscosity

Liner and Electrode Materials - The liner for the meter can vary depending on the application being considered. In applications where moderate amounts of abrasion are likely to occur, one of the following materials may be selected; Polyurethane, Butyl rubber, Neoprene or Polytetrafluoroethylene. In applications where corrosion is likely to occur, one of the following materials may be selected; Ceramic or Polytetrafluoroethylene. Stainless steel electrode material should be used for applications where corrosion is not likely to present a problem. Hastelloy electrode material should be used for applications where corrosion is likely to present a problem. 
Electrodefor most applications for Drycake which is in Wastewater and not drinking water, 316L Electrodes are sufficient and NBR Rubber Linker although FKM is ok.

Stainless Steel 316

Stainless steel facts and features
  1. General purpose electrode 
  2. Not suitable for strong acids and alkalis 
  3. Low cost
  4. Not recommended for salt water and brine


Hastelloy® facts and features
  1. Good all-round corrosion resistance
  2. High resistance to localized corrosion (superior to Stainless Steel)
  3. The preferred material within the process and water industry due to cost benefits
  4. Preferred material for salt water and brine


Hydrolysis is the chemical breakdown of substances by water and depends on the chemistry, solubility, pH, and the oxidation–reduction (Eh or redox) potential of compound.
Hydrolysis resistance is the ability to withstand exposure to extended periods of humidity and heat.

Hard rubber(NBR) is generally the recommended choice for the water/wastewater industry because of its excellent resistance to water absorption, relative low cost, and many drinking water approvals. EPDM has drinking water approvals, but should be avoided in wastewater applications.

NBR Liner (Siemens Standard MAG5100W)

NBR facts and features
  1.  Lowest priced liner
  2. NBR is highly resistant to hydrocarbons
  3. Suitable for process applications and certain chemical applications, where PTFE or PFA is not required. 
  4. NBR is better suited for waste water than PUR(Polyurethane). PUR has good oil, grease, gasoline and aromatic hydrocarbons resistance, but in comparison to NBR it is not recommended for water containing these media due to its low hydrolysis resistance.

 EPDM Liner (Siemens Alternative)

EPDM facts and features 
  1. Many country specific drinking water approvals including NSF61.
  2. Can be used for some chemicals, where PTFE or PFA is not required
  3. Can be used for some food and beverage applications with pipe sizes greater than DN 100/4” 
  4. For wastewater applications, where hydrocarbons can be present, consider NBR as the best liner choice.
  5. EPDM has a much better water resistance than PU(PolyUrethane) due to high hydrolysis stability