Abstract:
Thermal dispersion probes used as flowrate sensors in process control of a medium heated by a heater. The device includes a reference temperature sensor for producing an electrical signal indicative of the temperature of the medium in which it is immersed, and an active temperature sensor for producing an electrical signal indicative of the temperature of the medium adjacent the heater. The temperature difference between the active sensor and the reference sensor is processed in a processor which varies the heater power to maintain the temperature differential between the active sensor and the reference sensor within a predetermined range, whereby the predetermined range provides an optimal sensitivity for the probes.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a thermal dispersion probe, and more particularly to thermal dispersion probes used as flow rate sensors in process control. 
     BACKGROUND OF THE INVENTION 
     A thermal dispersion probe typically includes two thermowell-protected RTD&#39;s (Resistance Temperature Detectors) which are placed into a medium (air, gas, liquids, slurries or solids) to be monitored. One RTD is preferentially heated while the other RTD senses the temperature of the medium, the temperature differential of the two RTD&#39;s is related to the medium flow rate as well as the properties of the medium. The principle of operation of the probe is based on the rate of dispersion of thermal energy from the heated RTD by the medium. As the flow-rate of the medium increases, more of the heat created by the heater by the heater is carried away resulting in a reduction of the temperature differential between the sensors. Using a well-known mathematical formula, the device uses the temperature differential between the RTD&#39;s to determine the flow rate of a particular medium or, given a constant flow rate, can determine the type of medium being measured. This data is then processed by devices such as a computer to effect control systems. The device may be utilized in virtually any condition as it may be paired with external software controls which can be downloaded into the device. 
     Current designs offer a single heater setting for the entire range of the RTD&#39;s, these designs cannot intelligently allocate the proper amount of thermal energy required in all necessary instances as it is either ‘full on’ or ‘full off’. ‘Full on’ results in wasted energy when the sensor is located in a medium of low specific gravity and additionally results in very slow response times to major changes in the medium movement or composition. Additionally, when physical jumpers are utilized to select heater power for specific sections of the flow spectrum, it unwittingly restricts the spectrum of the sensors&#39; range. Any significant change in medium will require operator intervention. 
     In other words, current flow rate measurements may not be as accurate as necessary if the flow rate is either very high or very low. The heat source in the probe is designed to operate for all rates of flow. If the flow-rate is very high, most of the heat created by the heat source will be removed by the first flowing fluid before the thermistor has a chance to measure it. Therefore, small changes in the flow-rate at this end of the spectrum may not be noticed. Similarly, if the flow-rate is very low, most of the heat generated by the heat source will be measured by the thermistor. Too much heat has the same effect on the results as too little heat in that the smaller changes is flow may go unnoticed. Accordingly, where large fluctuations in flow rate are encountered, accurate measurements over the whole range is difficult. 
     Another shortcoming of present thermal dispersion switches is the lack of appropriate methods to test the switch to ensure it is operating properly. Even those switches that do provide a self-test still require some operator intervention. Therefore, a malfunction of the switch can still go undetected until the next scheduled operator test. 
     It is an object of the present invention to obviate or mitigate at least some of the above disadvantages. 
     SUMMARY OF THE INVENTION 
     In general terms, the present invention provides a thermal dispersion switch in which a heat source is controlled by a switch having a variable duty cycle to provide a variable heating effect. 
     The variable heat source that is designed to self-adjust for all level and interface applications and all rates of flow. By self-regulating the energy used by the heater in this invention, the microcomputer within the device optimizes the heater settings for all the different fluids and gases found in flow and level applications. When the heater is provided with only the appropriate amount of energy needed to yield the required differential, the sensitivity and response rate of the switch is optimized and maintained without operator intervention. In addition by reducing the amount of energy drawn form the power source in low flow, and level applications, the switch uses less power and is more environmentally sensitive. 
     In accordance with this invention there is provided A thermal dispersion probe for measuring the flowrate of a medium comprising; heater for heating said medium at a predetermined power; temperature sensor for producing a temperature signal indicative of a temperature difference between an active sensor and a reference sensor; processor for varying said heater power to maintain the temperature differential between the active sensor and the reference sensor within a predetermined range, whereby the predetermined range provides an optimal sensitivity for the probe. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described by way of example only, with reference to the accompanying drawings in which: 
     FIG. 1 is a schematic diagram of a thermal dispersion probe; and 
     FIG. 2 is a graph representing the behavior of a resistive device that changes resistance with temperature. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     For convenience in the following description, like numerals refer to like structures in the drawings. 
     Referring to FIG. 1, a thermal dispersion probe  30  includes a constant voltage source  3 , dual constant current sources  6  as well as its operating voltage derived from a power supply  14 . 
     A heat source  1  is powered be the constant voltage source  3  and controlled by a high-speed solid state switch  2 . Temperature-sensing devices, such as an RTD, are located in the circuit, and include an active temperature sensing element  4  and a reference temperature sensor  5 . A current is fed into both the active temperature sensing element  4  and the reference temperature device  5  from the dual constant current source  6  which results in a voltage across the active temperature sensing element  4  and the reference temperature sensor  5 . 
     The voltage difference between the two elements,  4  and  5 , is amplified by an instrumentation amplifier  7 . The output voltage from the instrumentation amplifier  7  will be referred to as the thermal signal  15 . 
     The voltage across the reference temperature device  5  is amplified by the instrumentation amplifier  8  to produce a voltage that represents the medium&#39;s temperature. The output voltage from the instrumentation amplifier  8  will be referred to as the temperature signal  16 . 
     A low pass filter  18  is connected between the heating element  1  and the switch  2 . The filter  18  acts to average its input signal  19  and provide an output voltage. The output voltage from the low pass filter  18  will be referred to as the heater signal  17 . 
     Each of the signals  15 ,  16 ,  17  is supplied as inputs to a Multiplexer and Analog-to-Digital System  9  which selects alternately one of the signals as an input and converts it from an analog signal to a digital signal. 
     The A/D System  9  provides a digital input to a microcomputer system  10  which is connected to the high speed solid state switch  2  in order to control its operation using a pulse width modulated signal. Other outputs of the microprocessor  10  are also sent to a user interface  11 , the remote interface  12 , and the current loop interface  13 . 
     The user interface  11  consists of a display for sending information to a user and a keypad for receiving information from a user. The information input by the user is used by the microcomputer  10  to determine the desired operation of the unit. Many applications, for example in hazardous environments, prohibit the user from using the user interface  11 . The remote interface  12  is provided for a user to monitor or control the unit from a remote location. The remote interface  12  consists of physical such as RS-232 or RS-485 and a data interface such as Modbus. For any applications requiring an analog output, a current loop interface  13  is provided. The current loop interface  13  sinks a current between four and twenty milliamps to represent the thermal signal  15 . The relationship between the thermal signal  15  and the output of the current loop interface  13  is determined by variables entered by the user via one of the user interfaces  11  or  12 . 
     In operation, the thermal energy generated by the heat source  1  is transferred through the medium being monitored to the active temperature-sensing element  4 . The thermal energy being produced by the heat source  1  and how much of that energy is absorbed by the active temperature sensor  4  versus being absorbed by the surrounding medium. The amount of energy that is absorbed by the medium depends on both the nature of the medium itself and the flow-rate of that medium. For example, quiescent isothermal water will absorb more thermal energy from the heater  1  than quiescent isothermal air would, and fast moving isothermal water would absorb more than slow moving isothermal water. Likewise, a fast flowing medium will disperse a relatively large amount of heat away from the active temperature-sensing element  4  whereas a slow moving medium will disperse relatively little heat away from the temperature-sensing element  4 . Therefore, the smaller the thermal signal  15  the faster the flow-rate of the medium. Conversely, and the larger the thermal signal  15  the slower the flow-rate of the medium. The thermal signal  15  can be used to indicate the flow rate of a known medium or if the flow rate is constant it can be used to indicate the type of medium. 
     The reference temperature device  5  is relatively unaffected by the thermal energy produced by the heat source  1  so the voltage across it is an indication of the medium&#39;s temperature. One example as to how this may be accomplished is to place the reference temperature device at a sufficient distance from the heater. Another example is to place the reference temperature device upstream from the heater. These are just two of a multitude of possible solutions. The voltage across the reference temperature device is amplified by the instrumentation amplifier  8  which yields the temperature signal  16 . The difference in signals  15 ,  16  received from the reference temperature device  5  and the active temperature-sensing element  4  is used by the Microcomputer  10  to determine the flow-rate of the medium. 
     The microcomputer system  10  sends a pulse width modulated signal  20  to the high-speed solid state switch to control the amount of thermal energy produced by the heat source  1 . Essentially, since the frequency remains constant, the signal  20  controls how long the switch stays on by varying the duty cycle. When the switch  2  is on, the heat source  1  is activated and produces heat. When the switch  2  is off, the heat source  1  is not activated and is not generating any heat. Therefore by controlling the length of time the switch  2  stays on per cycle, the microcomputer  10  is effectively creating a variable heat source  1 . The appropriate settings of such a heat source depend on a number of conditions. 
     If, for example, the flow-rate of the medium is very slow, then there will be a large proportion of the heat generated by the heat source  1  transferred to the temperature-sensing element  4 . Referring to FIG. 2, the behaviour of a resistive device that changes resistance with temperature is represented generally by 50. If the heat source  1  is generated a lot of heat then the temperature-sensing element  4  may be operating outside of its linear region as indicaated by reference number  53 . Therefore, significance changes in the flow-rate will not be accurately represented by the temperature-sensing element since large changes in temperature will result in smaller than usual changes resistance. Similarly, if the flow-rate of the medium is very fast, very little of the heat generated by the heat source  1  will be transferred to the temperature-sensing element  4 . There will be no way of knowing if any of the heat generated by the heat source reaches the temperature sensing element  4  and the flow-rate continues to increase. Similarly, if the flow-rate of the medium is very fast, very little of the heat generated by the heat source  1  will be transferred to the temperature-sensing element  4 . There will be no way of knowing if any of the heat generated by the heat source reaches the teare sensing element  4  and the flow-rate continues to increase. 
     To establish the operation of the switch in the linear range, the microcomputer system  10  uses the difference between thermal signal  15  and the temperature signal  16  to determine whether or not the heat source  1  needs to generate more or less heat. As the difference between the two signals increases it signifies that the flow-rate of the medium is slowing down. When the difference passes certain predetermined threshold, the Microcomputer  10  realizes that the heat generated by the heat source 1  needs to reduced and reduces the duty cycle of the pulse width modulated signal  20 . This action in turn reduces the length of time the switch  2  stays on per cycle, which reduces the heat generated by the heat source  1 . The active temperature sensing element remains in the linear region  52  and changes in the flow-rate are measured accurately. If, howevr, the thermal signal  15  drops below a certain level, the Microcomputer  10  realizes that the heat source  1  needs to generate more heat and increases the duty cycle of the pulse width modulated signal  20 . The switch  2  will remain on longer than it previously had, increasing the heat generated by the heat source  1 . The additional heat can now read the active temperature sensing element  4  to allow for an accurate reading rather than simply being swept away by the rapidly moving medium. 
     Since the high speed solid state switch  2  is effectively an open or closed circuit the voltage present at the input  19  to the low pass filter  18  will be either zero or the voltage output from the constant voltage source  3 . However the switch is controlled by the microcomputer system  10  using a pulse width modulated signal  20  at a fixed frequency. This frequency is much higher than the cut off frequency of the low pass filter  18 . The resulting output, the heater signal  17 , from the low pass filter  18  is a voltage that represents a ratio of how long the switch is on to how long the switch is off. This signal is used to monitor the heater element as part of the units self-test. 
     The microcomputer  10  is designed to periodically test the heat source  1 , the active temperature-sensing element  4 , and the reference temperature sensor. The tests are accomplished by testing the thermal signal  15 , the temperature signal  16 , and the heater signal  17 . All three of the signals are sent to the Multiplexer and Analog-to-Digital converter system  9  that converts the selected signal to a digital format and feeds it to the Microcomputer  10 . If the value of the temperature signal  16  is a full scale or zero reading, then there is an error with the reference temperature sensor  5 . If the temperature signal  16  is valid and the thermal signal  15  is full scale or zero, then there is an error with the active temperature-sensing element  4 . Since the heater signal is a voltage that represents a ratio of how long the switch  2  is on to how long the it is off, the Microcomputer  10  knows what this ratio should be since it controls the switch  2  via the pulse width modulated signal  20 . Therefore, if the heater signal  17  has an incorrect value, there is an error with the heat source  1  or the switch  2 . 
     the subject configuration thus allows the dispersion switch to more accurately analyze a particular range of flow-rates. If the flow-rates get too close to either end of the range, the range can be shifted so that the flow-rate calculations are not compromised. The microcomputer  10  also provides a self-testing feature that requires no input from an operator. The self-test is performed at regular intervals throughout the lifetime of the switch. 
     Although the invention has been described with reference to certain specific embodiments, various modifications thereof will be apparent to those skilled in the art without departing from the spirit and scope of the invention as outlined in the claims appended hereto.