GB2191292A - Measuring equipment - Google Patents

Abstract

The forward voltage drop at constant current is used as a measure of the temperature of sensing diodes embedded in concrete. In a calibration process, the response of the sensing diodes to temperature is measured and memorized at two known temperatures. A constant current is passed from an FET 1 having a filtering capacitor 2 and a pair of protection diodes 3 to a sensing diode 4. Another constant current is passed through a standard resistor 5 from a second FET 6. A series of incremental resistors 7 are switchable into circuit parallel with the resistor 5 via switches 8. The difference between the voltage across the diode 4 and that across the resistors 5,7 is amplified in a variable gain amplifier 9 and the output signal is passed via an analog to digital converter 10 to a central processor unit 12. The diode 4 is subjected to two known temperatures and two corresponding voltages are developed across it by the constant current. Memory 12 is used to memorize the results subject to the full range of multiplication increments of the amplifier 9. The best fit of the differences is selected to calculate the slope and offset of a linear voltage-temperature characteristic which is used in the subsequent measurement. A number of diodes are thus readily calibrated and may be used as disposable temperature sensors. <IMAGE>

Description

SPECIFICATION Measuring Equipment The present invention relates particularly though not exclusively to measuring equipment suitable for measuring temperatures in concrete during curing.

The curing of concrete to a load bearing condition is a complex process. Within limits, the higher the temperature of the concrete during curing, the faster the curing and to a lesser extent the stronger the resultant concrete. The integral, i.e. area under, a temperature/time graph gives one measure of maturity of concrete. The integral of other functions of the temperature against time gives other measures of maturity. Such measures are known as the "maturity factor". The most suitable measure is subject to debate.

An assessment of the maturity factor is important not only for quality control reasons, but also to enable a manufacturer of pre-cast concrete or an engineer supervising in-situ concrete pouring to know when he can remove (or strike) shuttering. In the case of pre-stressed concrete members, this is particularly important because premature striking of the shuttering and release of external tensioning of the pre-stressing members causes the latter to apply their tension to the concrete before it is sufficiently mature to withstand the tension without degradation.

Traditionally, concete curing temperatures have been measured externally, and for instance clockwork temperature plotting equipment for such measurement is known. However, this equipment is believed to give an inadequate indication of temperature within the bulk of the concrete.

This results from a number of factors. Firstly, concrete releases heat during curing, but it is not a good conductor of heat so that surface temperature is unlikely to be exactly representative of internal temperature. Secondly, surface temperature is likely to be influenced to a substantial extent by ambient conditions. Thirdly, external heat is often applied and research has shown that this does not result in even temperatures within the bulk of the concrete.

Accordingly, the need has been perceived for more accurate means for measuring temperature in concrete during curing. Since the requirement is for measurement within the bulk of the concrete, there is a practical necessity for cheap, sacrifical temperature sensing elements to be used.

Suitable thermocouples and so-called platinum thermometers are commercially available, but are prohibitively expensive.

Accordingly, it has been decided to use conventional diodes as temperature sensoring elements.

A conventional diode will pass negligible current when a reverse voltage is applied to it and a comparatively unrestricted current when a forward voltage of greater than a certain value is applied to it. For an intermediate voltage, the diode will pass a current dependent on the applied voltage and in particular if a constant current is applied, a specific voltage is developed across the diode. This voltage varies substantially linearly with temperature, and so diodes can be used as temperature sensors-as can other similar semiconductors such as a transistor connected in circuit between its collector and base.

A problem with commercially available diodes, whose use is desirable due to cheapness, is that they are not selected according to their temperature characteristics, which can vary considerably between diodes whilst remaining constant and predictable for individual diodes. Not only the slope of a diode's voltage/temperature graph (for constant current) but also its value or offset at 0 C vary.

The object of the present invention is to provide measuring equipment which can be used to measure temperature in concrete with sacrificial diodes.

According to the invention, there is provided a measuring equipment for use with a parameter sensing-element utilizing a constant current, the equipment comprising: means for applying a constant current to the sensing element; means for calibrating the equipment to the sensing element; the calibrating means being adapted to memorize the response of the sensing element for one predetermined value of the parameter and to compute the sensing element's characteristics from the memorized response and the response for another predetermined value of the parameter; and means for memorizing the sensing element's characteristics and calculating an output indicative of the value of the parameter being measured from the sensor's response thereto.

Although it is envisaged that the equipment may be used for measuring other parameters, the usual parameter is expected to be temperature. The equipment will usually be adpated to measure temperature at a plurality of sensing elements and therefore be able to memorize the response of a plurality of sensing elements to the one predetermined temperature for calculation of the plurality of elements' characteristics. However, the equipment may be arranged to memorize the responses of each element as they are moved one at a time between the predetermined temperatures with the elements' characteristics being computed and memorized one at a time.

The equipment advantageously includes a standard resistor and a series of incremental resistors connectible in series and/or parallel therewith (preferably the latter) through which another standard current is passed to provide an incrementally variable voltage thereacross for balancing with the voltage developed across the sensing element by its constant current. Thus the voltage across the element can be assessed in terms of the value of balance resistance required.

However, in the preferred embodiment, the balance resistance is arranged to compensate the sensing element's voltage/temperature relationship offset; whilst an operational amplifier is employed to amplify the voltage required for balance in accordance with the slope of the relationship.

To help understanding of the invention, a specific embodiment thereof will now be described by way of example and with reference to the accompanying drawings in which: Figure 1 is a voltage/current graph of a diode characteristic; Figure 2 is a voltage/temperature graph of a diode characteristic; Figure 3 is a circuit diagram of a balance circuit for measuring equipment of the invention; Figure 4 is plot similar to Fig. 2 showing a process of trial used in calibration of the equipment; Figure 5 is a block diagram of the equipment; Figure 6 is a diagrammatic view of a concrete beam during curing; Figure 7 is a flow chart for the equipment's program during calibration; and Figure 8 is a flow chart for temperature logging.

Referring first to Fig. 1, this graph shows the voltage/current relationship of a conventional diode. For reverse voltage applied across the diode, -ve voltage, ngeligible -ve current flows in the diode. For +ve voltages a +ve current flows which is unrestricted by the diode once an avalanche voltage Va is reached. Between 0 and Va the voltage/current relationship is substantially linear and varies with temperature. For a point x, in this region, the voltage/temperature relationship is given by Fig. 2. This is a linear relationship with an appreciable value at 00C-in the region of 2 volts-when a constant current of 900,uA is passed through a standard 4001 diode.It will be apparent that once this offset voltage and the slope of graph is known, a measurement of temperature of the diode can be made by passing the standard current through it and measuring the voltage.

Fig. 3 shows circuitry for measuring the voltage. A constant 900fly current is passed from an FET 1 provided with a filtering capacitor 2 and a pair of protection diodes 3 to the sensing diode 4. A further constant current is passed through a standard resistor 5 from a second FET 6. A series of incremental resistors 7 are switchable into circuit parallel with the resistor 5 via switches 8. The voltage across the diode 4 and the resistors 5, 7 is subtractively ampiified-that is their difference is amplified in a variable gain amplifier 9 and the output signal is passed via an analogue to digital converter 10 to a central processor unit 11.

For calibration of the diode 4, it is subjected to two temperatures T1, T2 (in Fig. 2) with the result that two corresponding voltages V1, V2 are developed across it by the constant current.

On calibration, the offser voltage V0 and the slope of the graph are neither of them known. Thus an initial assumption as to the value of either must be made in using the equipment for calibration. In the equipment, a considerable amount of memory is available in a memory 12 and the value of the offset voltage is assumed. The amplifier 9 is stepped through its full range of multiplication increments and the corresponding results memorized for a first predetermined temperature. The actual value of this temperature is input to the CPU 11 and memorized. The diode is then subjected to a second predetermined temperature and its actual value is input to the CPU. At this stage the output of the D/A converter 10 is in digital form representative of temperature.The amplifier 9 is again stepped through its range of increments (with the originally assumed offset voltage) and the output subtracted from the memorized results from this process at the first temperature. For each amplifier increment, the difference between the previous result and the present result is compared with the difference in temperature at the two actual calibration temperatures. For one particular value of gain a best fit of the differences will be found.

This value of gain will correspond to the slope of the voltage/temperature graph in Fig. 2.

This process of trial is illustrated in Fig. 4 with reference to slopes amplifications on either side of the best fit correct position. The correct relationship for the diode 4 is: V=Vo+gT.

If V0 is assumed to be V'e equivalent to passing the constant current ic from the FET 6 through the resistance 5, and the slope of the relationship is tried as g,; then the "temperature" information fed to the CPU will be given by V-V', "T" = gn and the difference in "T" between T1 and T2 will be given by G"T"="T,"-"T",

Representations of "T" for too large g, and too small gn are given in the upper and lower sections of Fig. 4.It will be noted that the equation V2-V1 b"T" = go is independent of V'O. Thus the value of "T" most closely fitting the actual value of T2-T1 will be given by the incremental gain applied to the amplifier 9 which is appropriate to the diode 4.

If V'O is changed, i.e. if a combination of the resistors 7 is switched into parallel with the resistor 5, the value of VT2 fed to the CPU and the value VTA which would have been fed to it at the same value of gain are changed by the same amount. Thus once the correct value of gain is known, the correct value of V0 can be found by switching in the resistors 7 until "T2,, --T2.

These operations are carried out automatically by the CPU under direction of a program stored in a ROM 13.

In the complete equipment shown in Fig. 5 there are up to thirty separate channels for thirty diodes 4. Each channel has its own FET providing its constant current. A further two channels are provided one for a diode 14 within the casing 15 of the CPU to monitor its temperature and another for a fixed resistor 16 to provide a cross-check on the balance circuitry. The fixed resistor should on each calibration be determined to require a zero gain and the same balance resistor.

The housing 15 for the equipment is insulated and provided with an internal resistive heating element 16 so that the equipment can be used in a hostile, cold environment without the circuitry becoming too cold to be reliable. An internal power supply 17 as well as a socket 18 for external power are provided. The housing supports a key pad 19 and display 20 for control thereof and has an output socket for use with a printer 21 or in conjunction with other data handling equipment. As explained below the equipment is adapted to activate audio and/or visual alarms 22 as required. It also has the facility for controlling steam (or other) heating equipment via a control output socket 23.

Use of the equipment will now be described. Via a key switch 24, the equipment is switched to a calibration mode. The thirty sensing diodes are bundled together at the end of 1 metre connection wires and connected via a connection box (not shown) to their constant current circuits shown as block 25 in Fig. 5. It should be noted that all the FET's controlling the currents are heat sunk commonly for stability. On initiation of the calibration process via key pad commands and under control of the program sorted in the ROM 13, the bundled diodes are placed in a high temperature medium, conveniently a vacuum flask of hot water. The water's temperature is measured with a conventional mercury thermometer and the temperature input to the CPU and memorized in the memory 12. The incremental resistors 7 shown in the resistor block 27 are switched out.Under control of selection switches in the block 28, themselves controlled by the CPU, the diodes' voltage is successively fed to the amplifier 9. When the voltage at individual diodes is constant, the amplifier's gain is stepped through 256 steps via the adjustable feedback resistance network 29, and the resultant values memorized for each diode at each gain setting.

The diodes are then transferred to a cold medium whose temperature is measured and input.

The resultant diode values are compared with the memorized values in the manner described above whereby for each diode a calibration gain and offset is computed and memorized. This information can be printed.

The flow chart for calibration is shown in Fig. 7.

The diodes can then be marked with their channel number and gain & offset constants for use. The equipment is now switched with key switch 24 to a use mode in which the values memorized during calibration are deleted from the memory leaving only the calibration constants memorized.

The diodes are now taped to reinforcing members at desired positions in the concrete member to be cast. They are connected in sequence to the equipment via a cable 30 having spaced, numbered connection points 31.

The equipment is now operated to measure the temperatures at the diodes by sequentially connecting the diodes to the amplifier 9 via the switch block 28 with appropriate values of gain set by the resistance network 29 and offset by the resistor block 27. Each diode is scanned a plurality of times and the average measured temperature is memorized.

Initially every diode's temperature is memorized every minute.

The flow chart for temperature measurement and memorization-or logging-is shown in Fig.

8.

After pouring of the concrete into the shuttering 32-see Fig. 6-the temperatures measured will settle to steady values, and the equipment can be adjusted to log temperatures at ten minute intervals.

The process of curing can take tens of hours. During this time, a maturity figure is continuously updated for each sensing diode. This figure may be the integral of temperature above - 100C against time. Alternatively, the figure can be computed in accordance with other models of the integral of a function of temperature against time. Once the maturity reaches a predetermined value, the equipment gives an indication of the fact. The shuttering can be struck when maturity is reached at all sensors.

Should a sensing diode register an abnormal temperature or rate of change of temperature, an alarm is given in order that the reason may be assessed as a diode malfunction or a potential fault in the concrete.

The equipment may be used to initiate and interrupt the supply of heat-for instance via passage of steam through a shuttering pipe 33. Since heat should not be applied too soon, the equipment may be set to initiate heating after a certain elapsed time. Since curing concrete releases heat, the equipment may be set to interrupt heating when the temperature at selected diodes reaches a selected value. The heating may also be controlled in accordance with the rate of temperature rise or temperature gradient within the concrete. Too large a value of either of these being undesirable.

Claims (15)

1. Measuring equipment for use with a parameter sensing-element, the equipment comprising: means for applying a constant current to the sensing element; means for calibrating the equipment to the sensing element; the calibrating means being adapted to memorize the response of the sensing element for one predetermined value of the parameter and to compute the characteristics of the sensing element from the memorized response and the response for another predetermined value of the parameter; and means for memorizing the characteristics of the sensing element and calculating an output indicative of the value of the parameter being measured from the response of the sensing element thereto.

2. Measuring equipment as claimed in claim 1, for use with a plurality of sensing element, wherein the calibration means is adapted for calibration of the plurality of sensing elements simultaneously in parallel, with the sensing elements being all subjected to one parameter state and then all subjected to the other parameter state.

3. Measuring equipment as claimed in claim 1 for use with a plurality of sensing elements, wherein the calibration means is adapted for calibration of the plurality of sensing elements sequentially, with each sensing element being successively subjected to one parameter state and then the other parameter state.

4. Measuring equipment as claimed in any preceding claim, including a standard resistor, a series of incremental resistors connectible in series and/or parallel therewith, a respective first constant current generator for passing constant current through the or each sensing element, and another constant current generator for passing another constant current through the standard resistor and selected one(s) of the incremental resistors; the equipment being so adapted that the other constant current is passed through the standard resistor and the incremental resistors to provide an incrementally variable voltage thereacross for balancing with the voltage developed across the or each sensing element by constant current from its constant current generator, whereby the parameter is measured with reference to the balance resistance required.

5. Measuring equipment as claimed in any one of claims 1 to 3, including a standard resistor, a series of incremental resistors connectible in series and/or parallel therewith, a respective first constant current generator for passing a constant current through the or each sensing element, another constant current generator for passing another constant current through the standard resistor and selected one(s) of the incremental resistors and an operational amplifier for amplifying voltage differences between the voltage across the or each sensing element and the voltage across the standard resistor and selected incremental resistor(s), the equipment being adapted for selection, on measurement following calibration, amongst the incremental resistors for compensation of the voltage/parameter relationship offset of the or each sensing element and for selection, on measurement following calibration, of gain of the operational amplifier for voltage amplification in accordance with the slope of the voltage/parameter relationship of the or each sensing element.

6. Measuring equipment as claimed in any preceding claim for use with a plurality of temperature sensing diodes, the equipment comprising a further diode arranged with a respective first current generator for measuring the temperature of the equipment and a control resistor arranged with a further first current generator for monitoring the equipment.

7. Measuring equipment as claimed in claim 6, the equipment having an thermally insulated housing and including an equipment heating resistor.

8. Measuring equipment as claimed in any preceding claim, the equipment being adapted for measurement of maturity of concrete and arranged for measurement of concrete temperature at short intervals during an initial period and at longer intervals subsequently.

9. Measuring equipment as claimed in claim 8, including means for control of application of heat to curing concrete whose temperature is being measured.

10. Measuring equipment for use with temperature sensing diodes substantially as hereinbefore described with reference to the accompanying drawings.

11. A method of calibrating an element having an electrical response to the application of a constant current, which response is indicative of the value of a physical parameter to which the element is subject, comprising subjecting the element to a first value of the physical parameter, measuring the electrical response, of the element to this first condition, memorizing the measured response, subjecting the element to a second value of the physical parameter, measuring the electrical response of the element to this second condition, memorizing the measured response, and computing the response constants of the element.

12. A calibration method as claimed in claim 11 for calibrating a temperature sensing diode having a straight line voltage/temperature characteristic, wherein during response of the diode to a first temperature its voltage response to the constant current is subtractively amplified with a standard voltage through a range of amplification gains corresponding to a range of voltage/temperature characteristics, during response to a second temperature the subtractive amplification is repeated through the range of gains and during the computation the difference in results of subtractive amplification for the two temperatures is compared with the actual difference in the two temperatures to determine the gain corresponding to the slope of the voltage/temperature characteristic and the size of standard voltage required for subtractive amplification at the determined gain is itself determined.

13. A method of calibrating a temperature sensing diode substantially as hereinbefore described with reference to the accompanying drawings.

14. A method of measuring the value of a physical parameter, comprising calibrating, according to the method as claimed in claim 11, 12 or 13, an element having an electrical response to the application of constant current, which response is indicative of the value of a physical parameter to which the element is subject, subjecting the element to an unknown value of the physical parameter, measuring the electrical response of the element and computing a value for the physical parameter utilizing memorized calibration characteristics.

15. A method of assessing the maturity of curing of concrete by means of sensing diodes embedded therein, comprising connecting the sensing diodes as sensing elements to equipment as claimed in any of claims 1 to 10, subsequent to calibration thereof by the method as claimed in claim 11, claim 12 or claim 13, measuring by the method as claimed in claim 14, at predetermined time intervals the temperatures of the sensing diodes, and integrating the temperatures against time.