Last Updated: February 26, 2026
Originally Published: 2015
With industrial pumps, two terms come up again and again - PSI (pounds per square inch) and TDH (total dynamic head). At Pye-Barker Engineered Solutions, we see these measurements daily when helping plants across Georgia and Florida keep their rotary gear pumps, Viking pumps, and centrifugal systems running at peak efficiency. Whether you manage a manufacturing line in Atlanta or a processing facility in Orlando, understanding these concepts - and the curves that bring them to life - can prevent downtime and reduce energy costs.
Two common descriptions of pump pressure are PSI (pounds per square inch) and TDH (total dynamic head). PSI is usually associated with positive displacement pumps and TDH with centrifugal pumps, but the terms are interchangeable in practice once you convert between them.
PSI is usually stated in gauge pressure as PSIG - the pressure you read on a pressure gauge. Negative pressure is usually stated in inches of mercury vacuum (HgV). These are common values used in America, but pressure can be translated into metric and other terms.
Differential pressure (ΔP or delta P) is the difference in pressure across the pump. Differential pressure is used for pump and motor selection because it is the actual pressure the pump sees. DP is the pressure on the suction side of the pump plus the pressure on the discharge side of the pump in relation to the desired discharge pressure. If 50 PSI is the desired discharge pressure and you have +10 PSI on the suction side, the pump needs to develop 40 PSI for the total discharge pressure to equal 50 PSI (+10 + 40). Likewise, if the suction pressure was negative –5 PSI (~10” HgV) such as a suction lift application where the liquid level is below the pump, the pump would have to develop 55 PSI (–5 + 55) to achieve the total discharge pressure of 50 PSI.
PSIA is pressure per square inch absolute. This takes into account atmospheric pressure. 0 PSIG is equal to 14.7 PSIA at sea level.
TDH or total dynamic head is another common term for describing pressure across a pump. Every 2.31 feet of vertical level equals 1 PSI for water and liquids with a specific gravity of 1 (or 8.34 pounds per gallon). For liquids heavier than water, the pressure exerted is greater for the same vertical level. For example, a tank with 23.1 feet of water level would read 10 PSI on a pressure gauge at the bottom of the tank. With a liquid with a specific gravity of 1.2 (10 pounds per gallon) at the same 23.1-foot level, the pressure gauge would read 12 PSI. And a pressure gauge would read less for liquids lighter than water. (Source for the 2.31 ft = 1 PSI conversion: Engineering ToolBox, “Pump Head and Pressure: Conversion, Calculations, and Charts")
If your current readings don’t match expectations, our team can quickly diagnose whether the issue is suction lift, specific gravity, or something else - reach out to our Atlanta or Orlando service locations today for a review.
Rotary and positive displacement pumps behave differently from centrifugal models, and that difference shows up clearly when you compare PSI and TDH. In positive displacement pumps (including rotary gear, lobe, and vane styles we service every week), the flow is nearly constant regardless of system pressure. That produces a relatively flat performance curve. TDH still matters, but operators more often speak in PSI because the pump delivers a fixed volume per revolution and builds pressure only as high as the downstream system allows.
In our experience at plants throughout the Southeast, this flat-curve characteristic makes PD pumps ideal for viscous fluids, metering applications, or systems with varying backpressure. However, it also means you must pay close attention to relief valves and bypass lines. Over-pressurization can happen fast if a valve closes unexpectedly. We convert TDH to PSI on every job ticket, so technicians and engineers speak the same language when selecting a Viking or other pump for a new installation.
The same conversion rules apply. Multiply TDH in feet by specific gravity and divide by 2.31 to get PSI. But because PD pumps do not rely on impeller velocity, you rarely see efficiency “islands” on their curves. Instead, you focus on slip, volumetric efficiency, and power draw at the design pressure.
Need help choosing between a centrifugal and positive displacement solution for your process? Our engineers can walk you through the numbers. Contact us, and we’ll provide a side-by-side comparison tailored to your flow, viscosity, and pressure requirements.
A pump performance curve is the manufacturer’s roadmap for how a specific pump will behave at a given speed. The most common version plots flow rate (usually in gallons per minute) on the horizontal axis and total dynamic head (in feet) on the vertical axis. Additional lines or shaded areas show efficiency, power consumption (in horsepower or kilowatts), and net positive suction head required (NPSHr).
Start at the left side of the chart (zero flow, or “shutoff head”). This is the maximum head the pump can produce when the discharge valve is closed. Move right along the head curve and you see how head drops as flow increases. The best efficiency point (BEP) is usually marked or visible as the peak of the efficiency islands - operating here gives you the lowest energy cost and longest seal and bearing life.
For centrifugal pumps, the curve slopes downward from left to right. Positive displacement pumps show an almost flat line because flow stays steady while pressure varies. To find your real operating point, overlay your system curve (the head required by your piping, elevation, and friction losses). Where the two curves intersect is where the pump will run. If that point sits far left or right of the BEP, you risk cavitation, vibration, or excessive power use.
NPSHr curves are critical on the bottom of the chart. If available NPSH (from your tank elevation and vapor pressure) falls below the required value, cavitation occurs and damage follows quickly. In Florida’s warm climates or Georgia’s variable process temperatures, we often see NPSH problems surface first. Our field teams carry portable gauges and can verify these values on-site the same day.
Reading your pump’s curve before purchase or during troubleshooting can save thousands in energy and repair costs. If your current setup is operating off the best efficiency point, let our Atlanta or Orlando technicians perform a curve analysis and recommend the right impeller trim, speed change, or replacement model.
What Is the Difference Between PSI and TDH? PSI measures pressure directly in pounds per square inch and is common with positive displacement pumps. TDH expresses the same energy in feet of liquid column and is standard for centrifugal pumps. Both describe the work the pump performs. They are mathematically convertible.
How Do I Read a Pump Performance Curve? Locate flow on the bottom axis and head on the vertical axis. Find the best efficiency point, then intersect your system curve to see the actual operating point. Check NPSHr and power lines to avoid cavitation or overload.
What Does a Flat Pump Curve Mean for Positive Displacement Pumps? A flat curve means nearly constant flow across a wide pressure range. This is typical of rotary and PD pumps and makes them excellent for metering or high-viscosity fluids, but it requires proper relief-valve protection.
Why Is Differential Pressure Important for Pump Selection? Differential pressure tells you exactly how much work the pump must do after accounting for suction conditions. Using it prevents undersized motors and ensures reliable performance in suction-lift or pressurized-feed applications.
How Does Specific Gravity Affect Pump Pressure Readings? Heavier liquids exert more pressure for the same height. Multiply the water-based head by the liquid’s specific gravity to get the correct PSI reading on your gauge.
What Is NPSH and Why Does It Appear on Pump Curves? Net positive suction head required (NPSHr) is the minimum pressure needed at the pump inlet to prevent vapor bubbles from forming. Every curve shows it because running below this value causes cavitation and rapid damage.
When Should I Consult an Expert for Pump Pressure Issues? If pressure readings fluctuate unexpectedly, efficiency has dropped, or you hear unusual noise or vibration, call our team. We service pumps throughout Georgia and Florida and can diagnose issues the same day.
Understanding PSI versus TDH and knowing exactly how to read your pump performance curve puts you in control of reliability and operating costs. At Pye-Barker Engineered Solutions, our goal is to make that knowledge actionable for every customer we serve in the Southeast. If you’re ready to verify your current pump’s operating point, select a new model, or simply get a second opinion on pressure readings, contact our team today. Call (404) 363-6000 or request a quote online. We’ll respond quickly and deliver the practical, experience-based solutions your operation needs.
Reviewed by: Eric Lunsford
Title: President
Experience: Over 35 years serving industrial facilities across Georgia and Florida
Reviewer Statement: I personally reviewed this guide (updated February 2026) to confirm that every explanation matches the real-world conditions our technicians see daily in Southeast plants. The conversions, curve-reading steps, and troubleshooting tips are accurate and directly tied to the pumps we service every week.
