Process for Calibrating a Calorimetric Flow Indicator

The invention relates to a process for calibrating a calorimetric flow indicator to a medium to be measured. The flow indicator comprises a sensor element having a temperature-dependent resistance and which heats up in a short period of time by means of electrical current heat and whose resistance variation (induced by cooling), which represents a measure for the flow velocity of a medium flowing by the sensor element, is measured. In the process, for a specified medium, the resistance value Ro at flow 0 and the resistance value R.sub.max at maximum flow define a preset range of measurement R.sub.max --Ro.

Media whose flow velocities are to be measured with a calorimetric flow indicator have varying heat capacities or varying thermal conductivities. Therefore, commercial calorimetric flow indicators have, for a specified medium, e.g. water or oil, a characteristic curve. An electric circuit is designed in such a manner that in the best case at flow 0, the lowest value on a measuring scale of a calorimetric flow indicator is assumed, and at maximum flow, the highest value on the measuring scale is assumed, and thus the range of measurement on the measuring scale is assumed. Thus all possible flow velocities are indicated within the measuring scale. A switching point, for example for indicating a flow or temperature threshold value of the measured medium in order to give a signal indicating that the threshold value has been exceeded, can be set within the range of measurement. Owing to these circumstances, a calorimetric flow indicator can be used optimally only for that medium for which is has been designed by the manufacturer. Yet the manufacturer often does not know for which medium the customer wants to use the flow indicator. If the device is adjusted and set by the manufacturer to, e.g. water, the customer, while measuring a different medium, may have an indication of a zero measured value even though there is already a flow.

It has therefore been proposed to provide such calorimetric flow indicators with means that allow a specific adaptation to the medium to be measured by adjusting the switching point that indicates a flow threshold value. However, this has the drawback that frequently the full measuring scale of the calorimetric flow indicator is not used up to the maximum flow velocity for different media to be measured, and thus, for example, only a portion of the indicator lights may light up at the maximum flow velocity in an LED display. At the same time, a portion of the measurement is "given away" or lost when the accuracy of the indicator is correspondingly reduced. Thus, it can happen that the switching point, at which a signal is supposed to be given that a threshold value of the flow velocity has been exceeded, is not within that portion of the measuring scale that is even scanned for the specific medium up to the maximum flow velocity.

In another known device a second potentiometer is provided that is rotated by hand until, at maximum flow velocity, the entire range of indication has been exploited, and thus, for example, in the case of an LED display, all indicating lamps light up. It assumes, however, not only that the start value, but also the final value, has been specified by hand. This is tedious manipulation that is to be avoided with the present invention.

The object of the invention is, therefore, to propose a process for calibrating a calorimetric flow indicator that delivers an optimal indication of the measured value for different media and also allows an arbitrary definition of a switching point (threshold value) within the measuring scale.

The problems with the processes of the aforementioned kind are solved, in essence, by:

--measuring a resistance value Rx of a sensor element of a calorimetric flow indicator at flow 0 of a medium that is to be measured that deviates from a specified medium by means of an electric circuit and

--measuring a resistance value Ry of the sensor element at maximum operating flow of the medium that is to be measured, and

--automatically bringing the measuring range Ry--Rx, defined by the measured resistance values Rx and Ry into coincidence with a measuring range R.sub.max --RO preset by the manufacturer for the specified medium by means of displacement and extension.

Thus, even though the calorimetric flow indicator is designed by the manufacturer for only one specific medium, such as water or oil, for which it functions utilizing its entire measuring scale, it is in this manner achieved that it can also be used for other media in the same optimal manner without requiring any special manipulations and adjustments by means of a potentiometer.

This process can be preferably implemented by an electric circuit that has a program-controlled microcontroller to store and process data representing the resistance values Ro, R.sub.max, R.sub.x and Ry. With such a circuit, the process according to the invention can be implemented quickly and reliably.

The resistance values Rx and Ry are preferably measured as voltage values by sending a constant measuring current through the sensor element at the respective flow velocities.

To obtain a signal which indicates that a predefined flow velocity and/or temperature has been exceeded or fallen below, a circuit can be used that emits such a signal when a switching point corresponding to a threshold value set within a range of measurement has been exceeded.

For this purpose, the value of the flow velocity or the temperature of the medium is periodically polled, preferably by means of the program-controlled microcontroller. The measurement result is processed into a trend display (in order to deliver to the user corresponding intermediate information) and compared with the preset threshold values or the preset threshold value, and a signal is emitted when the threshold value is exceeded. Thus, when monitoring a coolant cycle, e.g., it can be determined when circulation of the coolant falls below a necessary flow velocity or a critical temperature of the coolant is exceeded, and a corresponding alarm can be given.

Another more reliable indication of the measured value and monitoring of flow is obtained when a circuit with temperature compensation for the measured medium is used.

The accompanying FIGURE is a block diagram of an electric circuit used in the process according to the present invention.

To implement the invention a user proceeds, for example, as follows, reference being made to the accompanying block diagram.

First, the user fills a line in which flow is to be monitored with the medium concerned. A calorimetric flow indicator is provided in the wall of the line. At flow 0, a so-called set key 1 is depressed, the set key 1 being connected to a microcontroller 2 of a circuit 200. The microcontroller 2 is connected to a sensor element in such a manner that a resistance value Rx that is characteristic for flow 0 is automatically determined and is stored in the microcontroller. The user then produces a full operating flow and again depresses the set key 1, with the effect that now a resistance value Ry that characterizes the maximum operating flow is acquired in the microcontroller 2 and stored. The difference Ry--Rx defines the range of measurement, whose position and span is acquired by the microcontroller 2. Depending on the medium, this range of measurement can correspond, for example, to a flow velocity range of 0 to 0.5 m/s, 0 to 1.35 m/s, 0 to 2 m/s or 0 to 4 m/s. With the aid of the microcontroller 2, the range of measurement Ry--Rx is now displaced and/or expanded so that the calorimetric flow indicator exhibits virtually the same characteristics as with the medium originally preset by the manufacturer for the calorimetric flow indicator, with the result that now the indication of the measured value at flow 0 of the medium chosen by the user is at the beginning of the measuring scale and the indication of the measured value at a maximum flow of the medium chosen by the user is at the end of the measuring scale also, so that the entire measuring scale is utilized over the entire range of measurement. A switching point for signaling a danger state can now be arbitrarily set over the entire measuring scale.

A resistance value is measured according to the block diagram by switching on a heating stage 3 for a brief period of time, switching on a constant current source 4 during a cooling phase to determine the resistance value or by changing the resistance value of the sensor element (sensor Pt 100), which is assigned to the medium chosen by the user to be measured, and by evaluating the corresponding measured values.

With the aid of a program stored in the microcontroller 2 the values for the flow velocity and/or temperature are periodically polled by way or an analog MUX (multiplexer) 5. The data obtained are converted from analog to digital by 12 Bit A/D-converter 6. The digital data are then fed to the microcontroller 2 and processed by the microcontroller 2. To this end, the microcontroller 2 is intended for and suitable for delivering a trend display 7 of drops in flow velocity and/or increases in temperature, and triggering one or both of the relays 8 and 9 (connected with LED 1 and LED 2, respectively) when a threshold value for the flow velocity and/or temperature is exceeded. With the aid of the relays, a system, for example in which a coolant cycle is monitored, can be turned off in order to avoid extensive damage to the system.