GB2136135A - Calibration Warning Apparatus - Google Patents

Abstract

Apparatus for use with a measuring or reference device provides a calibration warning in response to extended use of the device. It is primarily intended for use with a thermocouple operating at high or constant temperatures. It comprises an analogue indicating device 6 coupled to an analogue driving circuit, 1, 3, 4, 5 the circuit being responsive to the output of the sensor such that the reading provided by the indicating device 6 is a function both of the sensor output and elapsed time. Thermocouple 2 feeds indicator 6 via buffer amplifier 1, linear amplifier 3 exponential amplifier 4, linear amplifier 5 & ballast resistor 7. Indicator 6 may be a mercury indicator. Amplifier 3 may become operable at a predetermined input level. <IMAGE>

Description

SPECIFICATION Calibration Warning Apparatus This invention relates to apparatus for use with a measuring or reference device, the apparatus providing a calibration warning in response to extended use of the device. The invention is primarily intended for use with a thermocouple operating at high or constant temperatures.

It has been found that when a thermocouple is installed in, for example, a furnace, its electrical output relative to temperature will drift gradually from an initial calibrated condition depending on the length of time the element is exposed to elevated temperatures and the magnitude of those temperatures. In the past, replacement of thermocouples has been carried out at regular time intervals loosely based on previous experience or, in the case of a furnace, when a batch of material has been ruined due to the thermocouple drifting outside heat treatment temperature limits.It is known to obtain a thermocouple calibration warning from a computer connected to a number of thermocouples by programming the computer to build up a digital calibration count for each thermocouple based on the number of heating cycles to which the thermocouple has been subjected and the maximum temperature reached in each cycle. A disadvantage of this known system is that, although it can provide a defined method of predicting a predetermined calibration drift, it requires the use of a computer and interfacing circuits. It is an object of the present invention to provide calibration warning apparatus that is relatively simple and more suitable for use with a single thermocouple.

According to one aspect of this invention a calibration warning apparatus for use with an electrical sensor comprises an analogue indicating device coupled to an analogue driving circuit, the circuit being responsive to the output of the sensor such that the reading provided by the indicating device is a function both of the sensor output and elapsed time. According to another aspect of the invention a calibration warning apparatus for use with a sensor or electrical reference device comprises an indicating device coupled to an analogue driving circuit arranged to provide an indicator reading which is a function both of the temperature at which the sensor or reference device is operated and the total time the sensor or reference device has been used.

The invention will now be described by way of example with reference to the accompanying drawings in which Figure 1 is a circuit diagram of a warning module which may be separate and intended for connection to the thermocouple throughout the operational iife thereof or may be permanently incorporated for example, in a connector at the thermocouple end, and Figure 2 is a circuit diagram of a warning module for use with a thermocouple which is operated at substantially constant temperature.Referring to Figure 1, the module has a high input impedance unity gain buffer 1 coupled to a single high temperature thermocouple 2, and an amplifier chain 3, 4, 5 having both linear and exponential gain or attenuation characteristics for generating an output current 1o which is dependent on the thermocoupled voltage VT according to the relationship 10=aebVT This current is fed to a known elapsed time indicator 6, such as the type having a drop of electrolyte separating two columns of mercury in a glass tube. Such indicators are relatively inexpensive and compact.As direct current passes from one column of mercury to the other through the electrolyte, mercury ions migrate from one column to the other, so moving the electrolyte drop along the tube in proportion to the total electrical charge transmitted since the indicator was installed.

The gain relationship 10=aebVT is obtained by firstly amplifying the output voltage of the buffer 1 by a factor of b in a linear amplifier 3, feeding the amplified voltage bVT to an amplifier 4 having an exponential characteristic to yield an output ebVT, and then amplifying the signal again to provide an output current 10=aebVT through a ballast resistance 7 and the elapsed time indicator 6. The ballast resistor 7 serves to reduce to an insignificant level, any minor variation in the impedance of the elapsed time indicator.

It will be seen that the output current lo is increased exponentially with VT, so that the rate at which the indicator reading changes is a function of the rate of drift in the thermocouple calibration, such drift being higher at relatively high temperatures.

The constants a and b can be altered by changing the value of the resistance 7 or the gain of the amplifiers to suit the drift characteristics of the thermocouple type, so that the indicator 6 reaches the end of its scale when the thermocouple is judged to be due for replacement or re-calibration.

Other devices for determining and/or responding to the total quantity of charge passing through the resistor 7 in a given time may be used in place of or in addition to the indicator 6. A device of this type, or the indicator 6 could be such or be so modified that visual and/or audible warnings are given when a predetermined value of the charge passing through the resistor 7 is reached.

C-Mos operational amplifiers are preferred to ensure that current usage from the batteries is such as to yield a useful life of at least one year.

Examples of suitable devices are an ICL 7611 DCPA and a TCL 251 both of which have the required high input impedance ( > 10 ohms) and adjustable quiescent currents settable down to 10 uA.

The present invention is also applicable to socalled "profiling" thermocouples which are used predominantly in the semiconductor industry where accurate temperature measurement is essential and which generally operate at a single predetermined temperature. For such thermocouples, one of the linear amplification stages and the experimentation stage could be eliminated at the expense of obtaining a reasonably accurate warning only within a narrow temperature band of the set temperature typically at +25 C. The warning module is, in this application connected to one of the thermocouples comprising the profiling thermocouple.

In the above, reference has been made to an exponential equation but it will be understood that other equations for example, a polynominal, of the general form lo=f(tE) or lo=f(T, tE) can be applied; where tE is the time elapsed and T is the temperature at which the thermocouple operated during that time.

Figure 2 is a circuit diagram of a warning module for use with a thermocouple which is operated at a substantially constant temperature.

Circuit component reference designations are taken from the "Radio Spares Catalogue" and specific values are given by way of example. The module of Figure 2 includes an elapsed time indicator 6A which may, as in the case of the warning module of Figure 1, be a mercury indicator.

The circuit of Figure 2 is, in essence, a presettable clock and is caused to operate whenever input from a thermocouple 2A (reference TLCICP) exceeds, say 2 mV. The thermocouple is connected as shown in circuit with a power source P, an amplifier 3 (a low current C-Mos amplifier) a transistor 4A (reference 2 N3705) a programmable current source 7A (reference LM 334Z) and setting current resistors R1 and R2 as indicated.

The amplifier 3A is operated in the "openloop" mode and has, therefore, a high gain characteristic. When a voltage of greater than 2 mV is applied to the input terminals of the amplifier 3A, the amplifier is caused to saturate and turn the transistor 4A on. The suppiy is then fed to the mercury indicator 6A and the current is limited by the programmable current source 7A and appropriately adjusted by the setting current resistors R1 and R2.

As indicated previously the indicator 6 may be so modified that visual and/or audible warnings are given. To this end a subsidiary circuit may be incorporated which triggers a readable device.

The readable device may be an LED, an LCD or an electrochromic display. Alternatively the subsidiary circuit may initiate an audible alarm.

Claims (13)

1. A calibration warning apparatus for use with an electrical sensor comprising an analogue indicating device coupled to an analogue driving circuit, the circuit being responsive to the output of the sensor such that the reading provided by the indicating device is a function both of the sensor output and elapsed time.

2. Apparatus according to Claim 1 wherein the electrical sensor is a thermocouple.

3. Apparatus according to Claim 2 including a buffer coupled to the thermocouple and an amplifier for generating an output dependent on the thermocoupled voltage.

4. Apparatus according to Claim 3 wherein the amplifier has linear and exponential gain characteristics for generating an output current lo which is dependent on the thermocoupled voltage VT according the relationship: 10=aebVT where a and b are constants.

5. Apparatus according to any preceding claim wherein the indicating device is a mercury indicator.

6. Apparatus according to Claim 1 including a pre-settable clock circuit operable when input from the electrical sensor exceeds a predetermined value.

7. Apparatus according to Claim 6 wherein the electrical sensor is a thermocouple.

8. Apparatus according to Claim 6 or Claim 7 wherein the circuit includes a high gain amplifier coupled to a transmitter operable upon saturation of the amplifier and means for limiting the current applied to the indicating device.

9. Apparatus according to Claim 8 wherein the means for limiting the current is a programmable current source and associated setting resistors.

1 0. A calibration warning apparatus for use with a sensor or electrical reference device comprising an indicating device coupled to an analogue driving circuit arranged to provide an indicator reading which is a function both of the temperature at which the sensor or reference device is operated and the total time the sensor or reference device has been used.

11. Apparatus according to Claim 10 wherein the indicator reading is in the form of a visual display.

12. Apparatus according to Claim 11 including a subsidiary circuit incorporating an LED, LCD or an electrochromatic display.

13. Apparatus according to Claim 1 2 wherein the subsidiary circuit includes an audible alarm unit.