Is your gas blend bottle half empty or half full?

When a specialty gas is blended and stored in a bottle at a pressure in excess of 2,000 psi, the demand for high measurement accuracy during bottle pumpdown may appear to be unnecessary. The same thought has occurred to this writer. “Can’t all of the contents of the bottle simply be pumped out?” The problem lies in the “all.” How do contaminants left in a bottle relate to an absolute vacuum level?

Assume that the fill pressure is 2,000 psig. At this pressure, 100 ppb corresponds to 0.0002 psia. That pressure unit, converted into a vacuum unit, equals approximately 10 mTorr. Thus, a vacuum that approaches the mTorr range of vacuum in the evacuated bottle is critical to obtaining sub-ppm contamination levels.

At atmospheric pressure, most of the contents in the bottle are in the gas phase and it is relatively simple to remove them using a conventional vacuum pump; i.e., two-stage rotary vane or similar.

If the only goal of a pump-down were to achieve a mTorr vacuum, this article would be quite brief; however, there is more to the story. The challenges in achieving a desirable vacuum level are realized when the following are considered:

• How the pump is connected to the manifold/bottle and
• What gauge is most appropriate for measuring this range of pressure.

Let’s take these issues one at a time and understand how they impact the purity of the final blend.

Pressure/Pumping Problems Traceable to the Connecting Line

In most situations, the effectiveness of the evacuation process is dependent more on how well the vacuum pump is connected to the manifold/bottle than on the speed of the pump. If a relatively long 1/4-in. to 1/2-in.-diameter tube is specified as the pump line, the pressure at the bottle is quite likely much higher than is the pressure at the pump. This configuration can be quite deceptive. By analogy, consider a set-up in which water if flowing from a garden hose nozzle (Figure 1). The pressure at the source may be set at 45 psi, however, the pressure at the nozzle end of the garden hose is different unless the flow of water is closed. As flow through the hose is increased, the pressure at the end is decreased.

The vacuum line scenario is analogous; as gas flow increases, the pressures at each end of the tube changes. If a vacuum gauge is located near the pump, it will always measure a lower pressure than that actually in the bottle. And this “deception” is exacerbated as pump speed increases. For example, assume that a moderately sized, 120 l/min vacuum pump is connected to a manifold by a three-foot long, 1/4-in. ID tube. After a few minutes of pumping, the gauge on the pump reads 0.007 Torr or 7 mTorr. What is the pressure in the bottle? While the exact answer depends on the type of gas being pumped and its temperature, a reasonable calculation would indicate that the bottle is at 450 mTorr. This is more than 50 times the pressure indicated by the gauge, and will not provide the final gas purity needed.

To determine if this is happening in your system, run a quick check by closing a valve between the pump and the gauge. This can be compared to closing the nozzle on the garden hose. If the pressure immediately jumps to a higher level, it means the pressure gradient along the line is trying to fool you while pumping at full speed. The actual bottle pressure is closer to the final, higher pressure than is the pressure measured while pumping.

Selecting the Right Gauge

As we now know, positioning the gauge, or at least checking for the effect of location, can be just as important as the measurement itself. Now let’s focus on the gauge’s capability. As shown above, the vacuum gauge must be sensitive in the mTorr to Torr absolute vacuum range. A dial gauge cannot meet the requirement for this application. It could be compared to using a bathroom scale to weigh a paper clip.

The selection of a suitable gauge is quickly narrowed to two choices:

• A diaphragm-based electronic gauge
• A thermal conductivity gauge

Diaphragm gauges are available in various forms—from sensitive and pricey, to economical but less capable for this measurement. The advantages of a diaphragm gauge in this application include the ability to accurately measure the pressure regardless of the gas type and the long-term stability in the presence of oily or dirty gas environments. The drawbacks that need to be carefully considered include its inability to withstand an overpressure incident and the range of the gauge, which typically can be as low as 50 mTorr at the edge of the requirement.

The thermal conductivity gauge determines pressure indirectly by measuring the rate at which heat is carried away by the gas from a hot wire. Gas is a pretty good thermal insulator, but a vacuum is even better. This type of gauge is based on the principle that the lower the pressure of a gas, the lower is its ability to conduct heat from the wire. This technique works especially well in the vacuum range from 1 mTorr to 1 Torr, and it therefore seems a perfect fit for this application. Its main drawback is the dependence of the pressure indication on the gas type. In other words, an inherently better insulator, such as argon, will tend to give low readings, while a more conductive gas, such as helium will read high. This characteristic is often manageable since the gauge is used to ensure that the pressure is below a certain mTorr value.

At this point, the residual gas is usually water vapor. Over the years, two different versions of thermal conductivity gauges have been developed. These are based on two different methods applied to measure the heat loss from the sensor wire. One way is to attach a thermocouple to the sensor wire and monitor temperature while a separately controlled electrical current heats the wire (Figure 2). Another approach applies the principle that the resistance of the sensor wire changes with temperature. This principle is also utilized in electronic thermometer RTDs (resistance temperature devices).

Using this principle, a circuit is specially designed to provide the heating current and simultaneously measure the wire’s temperature. In its modern form, the wire is actually regulated at a constant temperature (typically 80C) while the power needed to keep it hot indicates the pressure. This implementation of this type of thermal conductivity gauge is called a Pirani gauge (Figure 3).

With either of the thermal conductivity gauges any change in the temperature surrounding the sensor wire would also result in a change in the heat loss, which, in turn, results in an error in the indicated pressure. So care is taken in the design to internally measure and automatically compensate for the ambient temperature.

Since the thermal conductivity gauges use a sensor wire, which is freely suspended in the gas, the overpressure capability depends only on the containment housing. While most thermal conductivity gauges are designed to withstand nominal 100 psig overpressure conditions, they cannot tolerate an exposure to fill pressure. If your requirements demand the use of high overpressure, a special model, which is available, must be used. With a specially designed electrical feed-through, post, and vacuum envelope, the Pirani gauge (Figure 4) can be exposed to pressures up to 5,000 psig without damage or need for recalibration.

Doing It Right

To ensure that sub-ppb contamination levels are achieved in bottling specialty gases, the pump-out pressure at the bottle must be in the mTorr range or below. Determining this condition is made difficult by the pressure gradient that exists between the bottle, manifold, and pump while actively pumping. A momentary valve closure between the pump and gauge can help diagnose and quantify the extent to which this problem may be occurring. The more proximate the gauge is to the bottle, the more easily this problem can be mitigated. The vacuum gauge best suited for this application is the thermal conductivity Pirani gauge with high overpressure capability. Its ideal measurement range, from 1 to 2,000 mTorr coincides perfectly with the bottle evacuation pressure. A suitable vacuum gauge, used correctly, can help ensure that “stuff left in the bottle” will not compromise the purity of your blended specialty gas.

Mark McKee is Sales Manager for Vacuum Research Ltd. (412)261- 7630 or vrl@vacuumresearch.com Dr. Luke Hinkle is Manager for Complex2Simple, Consulting and Training Ph.D. in Physics from Pennsylvania State University in 1989 (508)237-4650 or luketech@earthlink.net.

Specialty Gas Report FIRST QUARTER 2007 //