A jar tester’s RPM setting does not directly state its velocity gradient, or G value. Converting RPM to G requires the jar geometry, paddle arrangement, water volume, temperature, and the manufacturer’s calibration curve. This guide explains how to read a G-curve, calculate Gt, and match jar-test mixing conditions to plant-scale mixing.
What Is the Velocity Gradient?
The velocity gradient, G, describes the mean energy gradient used to mix a fluid. It is commonly expressed in inverse seconds (s⁻¹), also described as feet per second per foot. In water treatment, it provides a more transferable measure of mixing intensity than RPM alone because RPM only reports the speed of the stirrer motor.
The same RPM can produce very different G values across different jars and paddles. A 100 RPM setting on one unit may not represent the same mixing intensity as 100 RPM on another unit. Jar geometry, paddle dimensions, impeller or paddle type, fill volume, and water temperature all affect the power transferred to the sample.
For the basic operating method, see the jar test guide. The jar test hub provides a shorter entry point for comparing related testing resources.
The Formulas for G and Gt
The standard SI relationship is:
G = √(P / (μ · V))
Where:
| Symbol | Meaning | Required unit |
|---|---|---|
| G | Mean velocity gradient | s⁻¹ |
| P | Power imparted to the fluid | W |
| μ | Dynamic viscosity of water | Pa·s |
| V | Volume of water being mixed | m³ |
For a calculated G value, first express power in watts, viscosity in pascal-seconds, and volume in cubic metres. The resulting G is in s⁻¹.
The mixing-energy index is:
Gt = G × t
Here, t is mixing time in seconds. Because G is in s⁻¹ and t is in seconds, Gt is dimensionless. It helps compare mixing systems that use different G values or different mixing durations.
HydroChemix is a water-treatment chemicals supplier and sourcing coordinator, not an engineering contractor. Chemical selection and dosage should remain based on jar testing and applicable local standards.
How to Read a Phipps & Bird G-Curve
Phipps & Bird publishes velocity gradient calibration charts called G-curves for its 2-litre round and 2-litre square jar geometries. These charts should be used only with the geometry and paddle arrangement they represent.
A typical curve places RPM on one axis and G on the other. To use it:
- Confirm whether the tester has a 2-litre round jar or a 2-litre square jar.
- Confirm the paddle type and configuration.
- Locate the intended RPM on the curve.
- Read the corresponding G value.
- Record both the RPM and the resulting G value.
Do not treat a generic RPM-to-G table as an official universal Phipps & Bird conversion. Use the chart supplied with the unit, request the appropriate chart from Phipps & Bird, or check the tester manual.
If the curve is unavailable, confirm the conversion in-client, in the manufacturer’s documentation, or with the supplier. A calculators resource may also help organize calculations, but it does not replace the equipment-specific curve.
Water temperature matters because dynamic viscosity falls as temperature rises. For the same power input and volume, a warmer sample can therefore produce a higher calculated G than a colder sample. Record the water temperature and match the sample temperature to the relevant process conditions where possible.
Geometry-Dependent RPM Starting Points
The following are HydroChemix jar-test-guide starting points, not universal conversion values:
- Rapid mix: 100–200 RPM, described as approximately 200–300 s⁻¹ G.
- Slow mix: 20–50 RPM, described as approximately 20–50 s⁻¹ G.
These ranges are useful for initial setup, but they depend on the jar and paddle geometry. Always check the tester’s manual or G-curve before assigning a G value to a particular RPM setting.
The same RPM does not produce the same G across different paddles or jars. For example, two testers may both be set to 100 RPM while transferring different amounts of power to the water. Record the RPM, mixing time, and chart-derived or calculated G together so that another operator can reproduce the condition.
Matching a Jar Test to Plant Mixing
Plant rapid-mix systems often operate at G values much higher than a laboratory jar tester can reach. Typical plant rapid-mix G values are often cited around 700–1,000 s⁻¹, with some designs operating higher. Their detention time is commonly around 10–30 seconds.
Rather than assuming the jar tester can reproduce the plant G directly, a practical approach is to compare the Gt values:
G₁·t₁ ≈ G₂·t₂
Worked Matching Example
Suppose the plant condition is:
- Plant G = 1,000 s⁻¹
- Plant mixing time = 15 seconds
- Plant Gt = 1,000 × 15 = 15,000
Now suppose the highest G available from the jar tester is approximately 100 s⁻¹, based on the unit’s G-curve at the RPM you can set. The required jar mixing time would be:
t_jar = 15,000 / 100 = 150 seconds
This is a Gt-compensation example, not a Phipps & Bird reading. The jar G value must come from your own unit’s curve. Matching Gt approximately helps align the mixing energy applied in the jar with the plant condition, but it does not guarantee identical floc formation, turbidity removal, or plant performance.
For plant-scale context, including tapered flocculation stages and mixer selection, see optimize flocculation mixing speed.
Typical Plant G and Gt Bands
The following bands are typical design references for matching context, not guaranteed operating targets:
- Rapid mix: approximately 700–1,000 s⁻¹, with some designs higher
- Rapid-mix detention: approximately 10–30 seconds
- Tapered flocculation stages: approximately 50–80 → 20–40 → 10–20 s⁻¹
- Total flocculation time: approximately 15–30 minutes
- Cumulative flocculation Gt: commonly discussed around 30,000–100,000, with a broader water-treatment band of 30,000–150,000
These figures help show why jar matching is more than a matter of choosing the same RPM as the plant mixer. The jar tester may need a longer mixing time to approach the plant Gt.
The distinctions between coagulation and flocculation are explained in coagulation vs flocculation. This is also useful when deciding whether a mixing condition belongs in the rapid-mix or slow-mix stage.
Practical Jar-Test Recording Checklist
For every mixing condition, record:
- Jar volume
- Jar geometry: round or square
- Paddle type and configuration
- Water temperature
- RPM setting
- Mixing time in seconds
- G source: manufacturer curve, manual, or calculation
- G value in s⁻¹
- Resulting Gt
Keeping these details with the test results makes it possible to distinguish a chemical response from a change in mixing energy. It also allows the same condition to be repeated or scaled more reliably.
When Comparing Chemicals
Once the rapid-mix and flocculation conditions are defined and recorded, chemical comparisons become more meaningful. PAC and PAM may be evaluated under a controlled mixing series, but no fixed ppm recipe should replace the jar test and applicable local standards.
To discuss chemical supply, sample requests, or a comparison program, use the contact page or request a free sample.
FAQ
Can I convert jar-tester RPM to G with a universal formula?
No universal RPM-to-G conversion applies to every jar tester. Use the G-curve for the matching jar geometry and paddle, check the manufacturer’s manual, or calculate G from measured power, viscosity, and volume.
Which G-curve should I use for a 2-litre jar?
Use the curve issued for the actual geometry: 2-litre round or 2-litre square. Paddle type and configuration must also match. If the correct chart is unavailable, request it from Phipps & Bird or confirm the value with the equipment supplier.
Why is Gt useful when the jar tester cannot reach plant G?
Gt compares both mixing intensity and duration. If the jar’s maximum G is lower than the plant G, the jar mixing time can be extended so that the two Gt values are approximately equal. This is a matching method, not a guarantee of identical treatment results.
Does matching Gt replace jar testing and local standards?
No. Gt helps align mixing energy between jar and plant, while jar testing determines chemical behaviour and dosage under the selected conditions. Final treatment decisions should follow jar-test results and applicable local standards.