KURZ THERMAL CONVECTIVE MASS FLOW ELEMENTS

Kurz offers three types of thermal mass flow elements:

In-line Mass Flow elements (Kurz series 502, 504FT, 510, 522 UHP. 532 for example).
Single-Point Insertion Mass Flow elements (Kurz series 452, 454FT, 410 for example).
Multi-Point Insertion Mass Flow elements (Kurz K-BAR 24)

THEORY OF THERMAL CONVECTION MASS FLOWMETERS

THERMAL MASS FLOWMETERS

Many versions of thermal convection mass flowmeters have been made, starting in the early 1900’s. Originally labeled "hot wire" anemometers, they were small and fragile. Generally, the sensors were operated in a constant current mode and rarely compensated for temperature changes of the fluid stream. Because of their small size they had a fast velocity response, but were susceptible to dirt and breakage. As industrial users learned of the possible advantages of thermal convection sensors, larger and much more rugged sensors were needed for a wide variety of process applications. Kurz was the first to develop all-welded dual-sting sensors that have the following features:

  • Direct mass flow measurement without the need for pressure or temperature correction
  • High level electronic signal output
  • Exceptional low speed sensitivity
  • High turn-down ration (up to 1,000:1)
  • Nearly constant percent-of-reading accuracy
  • Low cost, easy installation
  • Negligible pressure drop
  • Large temperature and pressure range
  • Solid-State, no moving parts, shock resistant
  • Very good repeatability
  • Fast response to velocity and ambient temperature changes
  • Insensitivity to non-axial velocity components
  • As we all know, the process industry is rapidly shifting from pneumatic to electronic signals and controls. The Kurz thermal mass flow meters easily fit into this computer and microprocessor environment.

THEORY OF OPERATION

There are two basic types of thermal convection mass flow sensor in general industrial use today.

  • Constant Power Anemometer (CPA)
  • Constant Temperature Anemometer (CTA)

Kurz utilizes the Constant Temperature Anemometer (CTA). In this instrument, a single RTD sensor is operated by a solid-state feedback control circuit to maintain a constant temperature difference between the heated sensor and the process fluid temperature which is measured by a second RTD sensor. The amount of electrical power needed to maintain this temperature difference is the measured output variable. As the fluid temperature changes, the CTA control circuit maintains a constant "over-heat" temperature difference between the heated sensor and the ambient fluid temperature. The CTA circuit has a significant advantage over the CPA because it may be compensated for the temperature difference and rate of change of the temperature difference. The CTA is the most recent method of sensor control and has been used almost exclusively for research anemometers and recent entries into the industrial market place. The CTA has several advantages over the CPA and the original "hot-wires" that used constant current. These advantages are:

  • A high level output is obtained. In most cases, a power transfer ratio of 9:1 from zero velocity to 200 SFPS is obtained.
  • Only two sensors are needed, rather than three as used in a CPA
  • CTA’s have a much faster response to velocity. CTA’s have velocity time constant of about 1 second.
  • CTA’s are much less sensitive to the angle of velocity approach because the sensors are circular.

CTA SENSOR CIRCUITY

Most CTA’s use a modified Wheatstone Bridge in which the voltage difference across the bridge is amplified and fed back to the top of the bridge to maintain a constant temperature difference between the heated sensor and the temperature compensation sensor. The heated sensor is the active element in the control circuit. Kurz uses a special platinum RTD
(resistance temperature detector) for both sensors. RTD’s are very repeatable, essentially linear with temperature, and can be self-heated to provide a known "overheat" temperature based on their standard resistance versus temperature table. Thus, the RTD has the unique property of being a heater as well as a very accurate temperature sensor and one less element may be used compared to most CPA’s in which a separate heater is required. This also allows superior flow characteristics. Years ago, Kurz recognized the need to have the mass flow element remote from the signal conditioning because of EMI, radiation, temperature or long cable length. Because of this need, Kurz introduced the loop-powered current output bridge circuit. Kurz also recognized that nearly all of the current used to heat the sensor is directly related to the total current of the bridge. The signal conditioning circuitry provides the dc power (generally +24VDC) and through the means of a dropping resistor at the power supply return, converts the output current to a voltage which is then processed to obtain the calibrated linear 0-5 VDC and/or 4-20 mA signal. Because of the loop powered method extremely long distances can be used between the sensor circuit and the mass flow computer.

Figure 2:

Figure 2 shows the Kurz 2-wire loop-powered sensor circuit with sensor lead length compensation. The bridge ratio is set by RX/RB and RY provides the overheat resistance reference. Kurz products use a three-wire heated sensor and a unique sensor lead resistance compensation circuit to eliminate the sensor lead length resistance effect. This circuit allows the user to shorten or lengthen the sensor lead wires without changing the calibration. This is an important consideration because the heated sensor has low resistance (10-20 Ohms) such that changes in the sensor lead wires, without compensation, can create a significant error in the output.

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