Bob Steinberg is the president of Sage Metering Inc. (www.sagemetering.com), a manufacturer of thermal gas mass flowmeters for industrial and municipal applications. Mr. Steinberg has started three successful enterprises and has over 20 years of management, sales, and marketing experience in the process industry. In addition, he has had extensive experience managing, supporting, and training industrial rep organizations, specifically in the thermal mass flowmeter industry. Mr. Steinberg can be reached at [email protected] or 866 677-7243.
Q: In regard to gas measurement, what are the key advantages thermal flowmeters offer over other flowmeter types?
A: Perhaps the most important advantage is the ability to measure mass flow rate of gases over an extremely wide range. For example, a customer who orders a meter with a full scale of 100 SCFM will be able to resolve and obtain readings at one SCFM or even lower. Essentially, the technology has a 100-to-1 turndown or better. In fact, the slightest amount of movement of gas passing the sensor causes an output. This can be demonstrated by gently waving a piece of paper near the sensor of a demonstration flowmeter. Even such minute flow rates can be observed. In velocity units, velocities as low as 10 SFPM can be detected. In contrast, other technologies have a limited turndown. Orifice plates and other differential pressure devices have a turndown no better than 5-to-1 and lose sensitivity at the low end due to a square root relationship between flow and pressure (1/2 of the flow rate will have 1/4 of the pressure). Positive displacement meters, vortex meters and turbine meters typically have a turndown of 10-to-1.
An equally important advantage is that it does not require temperature or pressure corrections to output mass flow units (SCFM, SCFH, Lbs./Min., Lbs./Hr., etc.). In contrast, the other devices (with the exception of Coriolis) require ancillary instrumentation, such as temperature and pressure gauges, which bring with them additional installation costs.
Other key advantages include: no moving parts; negligible pressure drop (will not impede the flow nor waste energy); dirt insensitivity (provides sustained performance); and ease of installation.
Q: The accuracy levels of thermal meters have improved in recent years. What level of accuracy can users expect from current-generation thermal meters? What about the next-generation of thermal meters; do you see accuracy as an area where there is still room for improvement?
A: The accuracy levels of thermal flowmeters have improved in recent years due to the use of microprocessor technology. For example, microprocessing eliminates older methods of linearizing the inherently nonlinear signal associated with the heat transfer of a thermal meter.
Microprocessors and new methods of driving the sensor also provide the opportunity to hold the accuracies over wider process temperature variations. In this regard, some of the newest technologies offer dramatic improvements in this arena and can maintain accurate reading even with temperature swings in excess of 200 F.
Thermal meters also offer other improvements that are directly related to accuracy. For example, the newest class of meters can provide up to four totally independent calibrations or settings in the form of four different channels that are user selectable within each meter. A customer therefore could have one channel covering an extremely high flow rate (such as an upset condition in a flare gas application) and another channel covering the very low flow association with normal operation (which could be fumes moving at extremely low velocities). Another adaptation of a multiple-channel meter would be to handle various gases. For example, a natural gas meter could have a second calibration for propane, which might be required as a backup fuel. Multiple calibrations can also be used for switching channels for different pipe sizes or even for extreme temperature situations.
However, there is a limit to the accuracy that can be achieved, simply because thermal meters need to be calibrated with a more accurate NIST-traceable standard. It is common practice to have a standard of at least twice as accurate, and preferably four times as accurate, as the published accuracy of the thermal meter. To achieve this, often up to three different NIST-traceable flowmeters are used in a given calibration to obtain the full range ability of the thermal meter. Thus, practically speaking, published accuracies generally do not exceed one percent of reading.
Q: How does the constant-current method of measurement differ from the constant-temperature method, and what are the advantages/disadvantages of each principle?
A:.The constant-temperature method fundamentally works as follows. There are two sensors constructed of reference-grade platinum. The two RTD’s are clad in a protective 316 SS sheath and are driven by a proprietary sensor drive circuit. One of the sensors is self-heated (flow sensor), and the other sensor (temperature/reference sensor) measures the gas temperature. As gas flows by the heated sensor (flow sensor), the gas molecules carry heat away from the surface of the sensor, and the sensor cools down as it loses energy. The sensor drive circuit replenishes the lost energy by heating the flow sensor up until it is a constant temperature differential above the reference sensor. The electrical power required to maintain a constant temperature differential is directly proportional to the gas mass flow rate.
The constant-temperature method in contrast to the constant-current method of measurement has a much faster response time to flow changes (typically one second versus 15 seconds), improved temperature compensation, greater sensitivity at the low end (can resolve mass velocity as low as 10 SFPM versus 45 SFPM), and lower-power dissipation (typically six watts versus 16 watts).
The main advantage of the constant-current method is that it can be used as a liquid-level switch whereas the constant-temperature method cannot.
Q: Beyond gas measurement, what other applications are thermal meters particularly well suited for?
A: Thermal meters are primarily used to measure the mass flow rate or consumption of gases or absence of gas flow rate for leak detection (the technology can also measure gas temperature). In addition, the constant-current method can also monitor liquid level (level switch).
Q: From a user’s perspective, what are some of the key features to look for in a thermal meter?
A: A thermal meter should have a large, backlit, easy-to-read display of mass flow rate total and temperature. It should provide a large variety of insertion of inline styles for pipes from 3/8" to 36 inches. The inline styles should be provided with flow conditions to optimize accuracy when there is limited straight run. Convenient mounting hardware should be made available for the insertion styles, and a variety of fittings (NPT, flanges) should be made available for the inline styles. A choice of 24V DC, 115V AC and 230V AC power should also be provided. General-purpose and explosion-proof options should be available, as well as integral and remote options.
The meter should have isolated outputs of 4-20 mA flow rate (and optionally temperature) and configurable relay outputs for pulsed outputs of total and/or setting high or low alarms for flow rate. In addition, the ability to remote the meter hundreds of feet away without any error (lead length compensated) is a big advantage, and the newest class of meters offers this. Also, the meter should have extraordinary temperature compensation so that the meter accurately reads gas mass flow rates even if there are wide variations in process temperature. It should have a user-friendly menuing system that is field rangeable either through keypad or laptop, allowing the user to modify full-scale settings, units of measurement, pipe area, signal filtering, decimal point selection, and so forth. The menuing system should also provide on-command diagnostics with the ability to check sensor performance and functionality. And in many applications it is beneficial to have multiple calibrations built into one meter where the user can select the channel of interest.
Finally, the software to run the system should be furnished at no cost.
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