Consistent and accurate performance is an absolute necessity to meet U.S. Food and Drug Administration (FDA) and other regulations regarding installation qualification (IQ), operational qualification (OQ) and performance qualification (PQ) of equipment used in the pharmaceutical and biopharmaceutical industries. An example is high-performance liquid chromatography (HPLC) pump systems used to identify analytes. The process demands accurate, reproducible column retention time as an analyte is pumped through the HPLC system.

An HPLC pump system is a complex device with multiple moving parts subject to leaks and wear. This impacts accuracy in terms of flow rates such as 0.05 to 25 milliliters (mL) per minute. Operators of these systems must exercise care to avoid or at least minimize wear on parts that can occur if particulates settle in valves and other moving components. An inventory of parts subject to wear should include valves, pistons and piston seal components. From a maintenance standpoint, wear can be minimized by flushing HPLC pump systems with rinse solvents on a regular basis.

Preventive maintenance is certainly important. In addition, regularly checking and confirming pump flow rate accuracy must be undertaken by periodic performance verification of the HPLC system.

This is accomplished by accurately measuring the time it takes a specified amount of a solvent to pass through the system. Here is where digital liquid flowmeters come into play.

Digital flowmeters for HPLC systems

While digital liquid flowmeters greatly simplify checking HPLC pump system accuracy, it is useful to understand manual alternatives that are still applied. They are broadly defined as gravimetric (requiring a precision analytical scale) or volumetric (requiring a finely graduated cylinder). They also require a stopwatch, excellent eyesight and a high degree of patience from the technician.

For volumetric measurement, technicians begin to collect HPLC eluent in the graduated cylinder and simultaneously start the watch. When sufficient volume is collected, the technician simultaneously stops the flow into the cylinder, stops the watch, then calculates and records the flow rate in milliliters per minute. This tedious but critical process repeats over and over until a standard deviation can be calculated. Moreover, solvent evaporation collected in a graduated cylinder can throw off readings.

Contrast this laborious and error-prone approach with digital liquid flowmeters now on the market. Compact, portable and lightweight, these operate on the volumetric method and function by taking repeated and accurate measurements of the time it takes a solvent to pass through a tube with a capacity, for example, of 140 microliters.

Simple & accurate

Digital liquid flowmeters are placed at the end of HPLC pump systems being tested. This enables solvent to be collected in a bottle for later disposal. The procedure is simple when compared to manual methods.

Here is a sequence of operation that typically takes less than a minute for each reading:

  • When solvent enters the tube, the meniscus passes an optical sensor that activates a timer.
  • When the meniscus reaches a second sensor, the timer stops, a valve opens and the tube drains.
  • Readings are displayed on the meter’s four-digit LCD panel and recorded to a six-digit printer or computer.
  • The process repeats itself.

Measurement accuracy is ±1 percent of the reading over flow rates of 0.05 to 25 mL/minute. Consistent readings mean the HPLC pump system is operating correctly. Otherwise the system requires attention.

Because these flowmeters do not require constant supervision, using them frees up technicians to perform other tasks. Moreover, a portable flowmeter can be transported throughout the lab to conduct tests elsewhere. In one instance, a single flowmeter is being used to service 100 HPLC systems. In another case, a pharmaceutical industry service provider transports the unit between different client locations. The spokesperson noted that a flow analysis can be completed in less than five minutes, including the all-important data output for record keeping. This works out to a 15- to 20-minute savings in time over the gravimetric method.

Suggested digital liquid flowmeter specs

If a digital liquid flowmeter is chosen to check on the performance of an HPLC system, some suggested specifications include:

  • A traceable calibration certificate at standard or user-selected flow rates
  • Linear range of the flowmeter includes the flow rates matching those used in the HPLC pump(s)
  • Accuracy of ±1 percent of the reading over the linear range
  • Reproducibility of ±0.5 percent of the reading over the linear range
  • Compatibility with any model HPLC system
  • Usability with all standard HPLC solvents
  • A carrying case and cleaning kit

A flexible mounting system is a useful feature because it allows the flowmeter to be located in various sections of the plant. Several mounting options may be available but all require that the meter be mounted within ~10 degrees of vertical. Mounting should also allow a suitable collection vessel to be placed below the flowmeter. Options include wall-mounting; freestanding; stack system-mounting to position the flowmeter on the front, left or right side of almost any stacked pump/detector system; and a bottle top adapter kit that allows the flowmeter to be mounted directly onto standard wide-necked reagent bottles.

The calibration certificate mentioned above deserves expansion because, as with HPLC pump systems, digital liquid flowmeters should be recalibrated at least annually or according to government or industry regulations. The flowmeter supplier should be able to offer recalibration services, typically at 1 mL/minute and at other specified flow rates. Users can specify the reports to indicate “as found” data – that is how the unit performed on receipt.

After determining the “before” data, the flowmeter tube is cleaned, the sensors are checked, and repairs and adjustments are made to bring the equipment up to specification. The flowmeter is then calibrated to all the customer’s required flow rates. “After” data are recorded on the calibration certificate to confirm the returned flowmeter performs according to specification.

Maintaining a digital liquid flowmeter

Good laboratory practices call for annual flowmeter recalibration as noted above. But improper use of these precision instruments can soon render them inaccurate, thereby compromising fast, accurate calibration of HPLC pumping system flow rates. Maintenance instructions are usually included in user manuals shipped with the flowmeters. Here are some tips:

  • Clean meters in constant use every two weeks in a process that should take about 15 minutes.
  • Do not introduce two immiscible liquids into the flowmeter without either allowing the first to fully evaporate or by introducing a third solvent in which the others are miscible.
  • A dirty sensing tube may trap bubbles and create falsely high readings. If bubbles are observed, a quick fix is to turn the unit off, wait approximately two seconds to allow it to drain, then switch it on. Avoid this problem with regular use of the cleaning kit. Do not disassemble the unit.
  • Flush the flowmeter with ethanol if it will not be used for a week or more. This prevents algae growth in the tube.

These digital liquid flowmeters have been performance-proven in the field as a way to save time and improve accuracy when checking HPLC system pump rates.

Tovatech Director Dr. Bob Sandor has more than 15 years of experience managing high-tech businesses. He led the startup and growth of five technology companies and has authored more than 40 patents and technical publications. Sandor holds a Bachelor of Science in chemistry from the University of Rochester and a Ph.D. in inorganic chemistry from Brandeis University. Contact him at or 973-821-4400.


Based in Maplewood, New Jersey, Tovatech LLC specializes in digital scales, pharmaceutical laboratory equipment, digital liquid flowmeters and ultrasonic cleaners. The company is staffed by chemists and engineers dedicated to providing the right equipment for the food, drug, chemical, health care and equipment manufacturing and maintenance sectors.