Most engineers size a metering pump based on two things: the required flow rate and the discharge pressure. These are the right starting points. But for viscous chemicals, there is a third variable that determines whether the pump actually delivers what the datasheet says it will — stroke rate.
Get the stroke rate wrong for a viscous fluid and the pump will underperform, the dosing accuracy will drift, and in some cases the pump will fail prematurely. Understanding why requires understanding how a metering pump actually moves fluid at the mechanical level.
How a Metering Pump Generates Flow
A metering pump — whether diaphragm or piston type — generates flow through a reciprocating mechanism. On the suction stroke, the diaphragm or piston retracts, creating a low-pressure zone that draws fluid into the pump head through the inlet check valve. On the discharge stroke, it pushes forward, closing the inlet valve and forcing fluid out through the outlet check valve.
The total flow rate is the product of two things: the volume displaced per stroke (determined by stroke length and diaphragm/piston diameter) and the number of strokes per minute (stroke rate). A pump running at 100 strokes per minute with a displacement of 1 ml per stroke delivers 100 ml per minute. Double the stroke rate, double the flow. Reduce the stroke length by half, halve the flow.
This is straightforward for water. For viscous chemicals, it is not.
What Viscosity Does to the Suction Stroke
Viscosity is a fluid’s resistance to flow. A fluid with high viscosity — think polymer solution, concentrated acid at low temperature, or heavy cleaning chemical — resists being pulled into the pump head during the suction stroke.
The suction stroke creates a partial vacuum that draws fluid in. For water, with its low viscosity, the fluid responds quickly and fills the pump chamber before the next stroke cycle begins. For a viscous fluid, the response is slower. The fluid takes longer to flow through the suction line, through the inlet check valve, and into the pump chamber.
If the stroke rate is too high, the pump completes its suction stroke before the chamber is fully filled. The next discharge stroke then pushes out a partially filled chamber rather than a full one. The actual volume delivered per stroke is less than the theoretical displacement — and the shortfall increases as stroke rate increases or viscosity increases.
This is called volumetric efficiency loss, and it is the primary reason why metering pumps underperform with viscous fluids.
The Check Valve Problem
High stroke rate compounds the problem through another mechanism: check valve performance.
Metering pump check valves must open and close reliably on every stroke cycle. At low stroke rates, there is enough time for the ball or disc to fully seat between strokes, providing a clean seal that prevents backflow. At high stroke rates, the valve may not have enough time to fully seat before the next stroke begins. Partial valve closure means some fluid flows back through the inlet on the discharge stroke — reducing net flow and dosing accuracy.
Viscous fluids make this worse because the resistance to check valve movement is higher. A ball check valve designed for water will open and close more slowly in a fluid at 1,000 cps than in water at 1 cps. At high stroke rates with viscous fluids, check valve lag becomes a significant source of dosing error.
The Flow Rate Correction
Because of these effects, published metering pump performance curves are based on water. When the fluid viscosity is significantly higher than water, the actual deliverable flow rate at any given stroke rate is lower than the published figure.
The correction factor is not linear. At 1,000 cps the pump might deliver 80% of its rated water flow. At 6,000 cps, the deliverable flow drops to around 60% of the rated value. At 20,000 cps, performance degrades substantially further and the upper limits of what the pump can reliably handle begin to be approached.
For the Injecta TMP series metering pumps, flow rate is adjustable both by stroke length (0–100%) and stroke rate (43, 86, 131, or 175 strokes per minute depending on model). This dual adjustment allows the operator to compensate for viscosity effects. For a viscous chemical, reducing stroke rate while increasing stroke length maintains the required flow rate while giving the fluid more time to fill the chamber on each suction stroke — improving volumetric efficiency and check valve seating.
Why Increasing Stroke Rate Is the Wrong Fix
The instinctive response when a metering pump is not delivering enough flow is to increase the stroke rate. For water-like fluids, this works. For viscous fluids, it often makes the problem worse.
Increasing stroke rate with a viscous fluid reduces the time available for chamber filling on each suction stroke, reducing volumetric efficiency further. It also increases the frequency of check valve cycling, which compounds the valve lag problem. Net flow may not increase significantly, or may even decrease as the pump begins to cavitate — and the mechanical wear rate on the diaphragm and check valves increases substantially.
The correct approach is the opposite: slow the stroke rate down and increase stroke length to compensate. Fewer, fuller strokes deliver more accurate dosing and less mechanical stress than many partial strokes at high speed.
Practical Guidelines for Viscous Chemical Dosing
When specifying or commissioning a metering pump for a viscous chemical, several practical steps help ensure accurate dosing performance.
Size the pump with headroom. A pump rated for twice the required flow rate at water conditions will be operating closer to its optimum when the viscosity correction factor is applied. Undersized pumps running at maximum stroke rate have no room to compensate for viscosity effects.
Optimise the suction line. Keep the suction line short, direct, and sized generously. Viscous fluid cannot flow fast enough through a long, narrow suction line to fill the pump chamber between strokes. Any restriction on the suction side — undersized pipe, tight bends, a partially closed valve — compounds the viscosity problem significantly.
Use a viscous kit where applicable. The Dosatron range offers an optional viscous kit for applications where chemical viscosity varies with temperature — a common situation in outdoor or unheated environments where concentrate may thicken in cooler conditions. The kit adapts the suction mechanism to handle higher viscosity fluids without loss of dosing accuracy.
Check the temperature. Viscosity is temperature-dependent. Many chemicals that dose reliably at 25°C become problematic at 15°C because their viscosity doubles or triples. If the pump works correctly in commissioning but develops problems during cooler weather, viscosity is the first thing to check.
Verify calibration at operating conditions. Always calibrate the pump at actual operating conditions — with the actual chemical, at the actual temperature, against the actual system back pressure. Published performance data is a starting point, not a guarantee of what the pump will deliver in your specific installation.
The Bottom Line
Stroke rate and viscosity interact in ways that are not immediately obvious from a pump datasheet. A metering pump that performs perfectly on water will underdeliver on a viscous chemical at the same stroke rate settings — and the gap widens as viscosity increases or stroke rate is pushed higher to compensate.
The solution is not to fight viscosity with speed. It is to work with the fluid’s properties: slower stroke rates, fuller chamber volumes, well-designed suction conditions, and a pump sized with enough headroom to absorb the correction factor.
Autoflo Technology supplies metering pumps from the Injecta range for water treatment, industrial, and agricultural dosing applications. For help selecting and configuring a pump for a viscous chemical application, contact us at info@autoflotechnology.com.