Avoiding Power Disturbances

Choosing the Right UPS Improves Test Accuracy and Completion Rates

In the lab environment, every key piece of equipment must produce accurate and repeatable results. Great care is taken by lab personnel in following detailed written procedures that specify sample preparation, equipment setup, preconditioning and conducting required testing. Most of the time all goes well, ending with accurate results. When inaccurate test data results—due to the complexity of the procedures, processes and equipment—determining exactly what went wrong can be difficult.

Take, for instance, a typical gas chromatograph (GC). One or more high purity gases are supplied to the GC. One of the gases flows into the injector, through the column and into the detector. The sample to be tested is fed into the injector, which has been allowed to stabilize at a temperature of 150°C–250°C for 24 hours. The heat in the injector causes the volatile solutes to vaporize. The vapor is then carried into the column by the gas. The critical temperature of the column is maintained by its location inside a temperature-controlled oven. The solute vapors travel through the column at differing rates depending on their physical properties, temperature and design of the column. The fast-moving vapor exits the column into the heated detector. An electronic signal is generated upon interaction of the solute with the detector. The size of the signal is recorded by a data system and is plotted against elapsed time to produce a chromatogram. The ideal chromatogram has closely spaced peaks, with no overlap of the peaks. The time and size of a peak are important in that they are used to identify and measure the amount of a specific compound in the sample. The size of the resulting peak corresponds to the amount of the compound in the sample. The time it takes for the sample to travel through the column is called the retention time. This value can be compared against the results of known samples to determine what compound is present.

Any number of variables during testing can cause inaccurate results, but most of the variables in the process can be rechecked and verified. As with most lab equipment problems, the most often overlooked is the lab’s utility power source and specifically its voltage regulation.

GC equipment manufacturers go to great lengths to design the power supplies used in the systems to have the widest input range possible. The manufacturers also attempt to design the power supplies with enough internal stored energy to ride through momentary utility voltage disturbances when operating from a nominal 120VAC utility voltage. Should the disruption be longer than the power supplies can withstand without their outputs going out of regulation, the equipment will malfunction. Another area of concern is a low line voltage condition in the lab, usually created by the large amount of equipment being powered in the lab. A low line voltage of less than 105 volts to below 100 volts can result in the equipment power supplies being hypersensitive to the smallest power disturbances.

Even if the power supplies operate from a normal nominal utility voltage, problems may occur due to other equipment. Motors, refrigerators, copy machines, etc., operating from the building’s main electrical panel outside the lab can cause the utility voltage regulation to vary widely. For example, a low or unstable voltage source supplied to the GC could result in the following problems: Critical temperature controllers for the injector, the column and the detector stages could go out of calibration, skewing test results. Computer-controlled solenoid valves regulating gas flow could be adversely affected. Sustained low utility voltage levels may result in the GC’s associated computer or internal microprocessors experiencing memory problems or even crashing. This could result in a wasted sample and lost test results. Finally, without an uninterruptible power system (UPS) battery backup connected to the entire GC system, the integrity of the 24-hour preconditioning and testing cycles could be compromised.

These problems can be avoided if the correct type of UPS is purchased at the time of GC installation. Often the UPS purchased to back up a $50,000 GC is a low-cost office computer–grade UPS, obtained at a local office supply store. One cannot blame the purchaser for looking at the bottom line or having little knowledge of the differing UPS designs available. Unfortunately, the worst possible decision has been made, considering the application—one that may even magnify the problems.

The IEEE defines UPS products in three distinct categories: off-line, line-interactive and online. They provide the following three increasing levels of protection:

1. The off-line UPS gives users battery backup, basic surge protection and no voltage regulation when operating from utility power. This low-cost solution provides “squarewave” AC output power when operating on battery. This output is compatible only with a computer, since the power supply inside is robust enough to accept this distorted power.
2. The line-interactive UPS is similar to the off-line design, except it provides grossly regulated AC power. Voltage regulation is accomplished by electronically changing transformer taps whenever the utility power changes drastically. Paradoxically, this UPS can actually make a greater change in output voltage than the power provided by the utility company (Figure 1). In addition, the attempt to regulate the output voltage in this manner takes a toll on the batteries, which are used frequently as part of the boost-buck automatic voltage regulation (AVR) feature. Typically, this AVR feature is used several times a day to mitigate sag and brownout conditions. The line-interactive UPS design usually provides a sine-wave or semi-sine-wave output when operating on battery.
3. The online UPS maintains a regulated sine-wave AC output voltage (±2%) to the critical equipment 100% of the time, whether being powered by the utility or from internal battery power. First, the incoming AC is passed through the MOV surge-protected rectifier stage, where it is converted to DC, which is heavily filtered by large electrolytic capacitors. This removes line noise, high-voltage transients, harmonic distortion and all frequency-related problems. The input capacitors also act as an energy storage reservoir, giving the online UPS the ability to “ride through” momentary power interruptions without battery drain. As the battery source is also connected to this DC circuitry, it simply takes over as the energy source in the event of a complete utility loss. This makes the transition between utility and battery power seamless, without the 4–25 millisecond interruption in the UPS output associated with the other two UPS designs. The filtered DC is sent to a DC/DC converter or chopper circuit that acts as a DC voltage regulator. The DC voltage is tightly regulated and fed to a second set of filter capacitors. This stage gives the UPS its ability to provide a constant output even during sustained deep brownouts or low line conditions, which require the off-line or line-interactive UPS to go to battery mode. The regulated DC voltage is fed to a 100% continuous duty cycle inverter, where a totally new AC sine-wave output is generated. Some online models on the market will accept a drop-in utility line voltage below 60V, while maintaining a perfect sine-wave output voltage of 120VAC (±2%).

The real-world graph and pictures on the following page show the difference in output voltage regulation and quality between a high-quality line-interactive and an online UPS. As can be seen, when the utility voltage changes (black dotted line), the output voltage of the line-interactive UPS (red solid line) at some points drops under 100VAC and increases to over 130VAC. Over the same utility voltage range, the output of the online UPS (green solid line) steadfastly remains at the 120VAC level without deviation. Even when the online UPS goes to battery mode, the output stays at the 120VAC level without interruption or voltage dropout.

Next, note the picture to the right showing a generator. The three output waveforms shown are actual waveforms taken while the line-interactive and online UPS units were being powered from a very dirty and polluted generator source. Note the line-interactive UPS fed the dirty, polluted power through to the sensitive connected equipment. It did not clean the dirty power. Also note that its output voltage dropped to 102.4VAC, with an output frequency of 64.9Hz. Both the output voltage and frequency are great areas of concern. In addition to affecting the lab equipment’s performance, when subjected to sustained low utility voltage conditions, the lab equipment’s internal power supply has to work harder to power the equipment, putting added stress on the supply, elevating operating temperatures and in some cases overheating. Note the output of the online UPS. It completely removed the power pollution and regenerated new pristine AC power. The output voltage was a tightly regulated 121.4VAC at a frequency of 60.5Hz.

This amazing cleanup of dirty power can be accomplished only through true online UPS technology. You truly get what you pay for with an online UPS. The connected lab equipment continuously receives the optimum power and voltage levels and the added benefit of added overall reliability. An improved test accuracy and completion rate will also be realized.