Patent GB-1460707-A (GB)
Title: Measuring devices
IPC: G01K1/12
Filed: 1974-01-17
Abstract: 1460707 Measuring voltage; measuring temperature INTERTECHNIQUE 17 Jan 1974 [19 Jan 1973] 02332/74 Headings G1D G1N and G1U] In measuring physical quantities in a hostile environment, a sensor is passed therethrough, this being electrically connected to a measuring circuit fixed adjacent a large heat sink, both of which are surrounded by thermal insulation inside a housing, the sensor being outside the housing and the measuring circuit provides a signal representing the physical quantity being measured and stores successive values thereof. -The invention is utilized to measure temperature and/or pressure in a sterilization process for pharmaceuticals or canned foods. Circuit and transducer mounting, Figs. 1 (not shown) and 2. The circuitry is carried on a flexible printed  circuit board 32 fixed to the outside of an insulating tube 29, which is held inside a large, cylindrical, copper heatsink 25 by means of a screw cap 26. Three mercury cells 33 are held inside the tube 29 between a spring 30 and contact 31 forming connections to the board 32. Leads from the board 32 are fed to a control plug 39 and a connector 20, the latter being for connection to the sensors e.g. thermistors or pressure-sensitive devices. The heat sink 25 is encased in a thick layer of thermal insulation 37 which has removable portions 36, 38 for access to the plug 39 and cap 26. The whole arrangement is mounted in a stainless steel or epoxy resin enclosure 24 which is sealed by a cap 21 and gasket 22. The thermistors (11, 14), Fig. 1 (not shown) are mounted on a tubular support (10) which is arranged to carry a can similar to those carrying the products being sterilized. One sensor is arranged inside this can which is sealed to the support at a flange (12) and the support (10) is mounted on the circuit enclosure 24 by means of the connector 20. Measurement circuits - Fig. 3 The two thermistors 41A, 41B are connected into divider chains with current stabilizing resistors 40A, 43A, 40B, 43B and reference resistors 42A, 42B. The two channels are alternately connected to the measuring circuitry, under the control of timing signals derived from a clock generator 60. In each channel, the measurement is thus: a signal R goes low closing a switch 53 (e.g. an FET) to discharge capacitor 52. R then goes high and CE also goes high closing switch 47. If channel A is being measured, switch 44A is closed connecting the voltage across thermistor 41 A to the integrator 51, 52 which thus charges at a rate determined by the value of 41A. At the end of the signal CE, a further signal CR goes high closing switch 48 and enabling AND gate 55. Since switch 46A is closed, the reference resistor 42A is now connected to the integrator, but with a negative voltage. The integrator, thus discharges at a rate determined by the reference resistor 42A. The length of time which the integrator takes to discharge to zero is a measure of the relative magnitudes of the thermistor and reference resistor. While the integrator output is present during the discharge period, AND gate 55 enables AND gate 56 which allows clockpulses into a counter 57 whose final count indicates the temperature. At the end of the measurement period, a timing signal F m4  passes via gates 83, 84 to the clock input of a memory 58 comprising paralleled shift registers. After this reading, a timing signal opens switches 44A, 46A and closes 44B, 46B and the process is repeated for the channel B thermistor 41B. Timing signals - Fig. 3 A 200 kHz clock signal 60 is divided by series connected circuits 61, 63, 64 and 65, 66, 67. The divider comprising the counter 16 and gate 65 is selectable to divide by any number 1-16 depending on the links 35. These links are provided in the control plug 39 Fig. 2 and are selected to produce the desired channel alternation and measurement times consistent with the capacity of the memory 58 and the transit time of the device through the sterilization process. The signals are combined in shift register 68 and logic 70-76 to provide the timing signals R, CE, CR as well as the channel alternation signals A, B and a channel identification signal to the control plug 39. Measurement readout - Fig. 5 (not shown) After passage through the sterilization, the control plug 39 is removed, this inhibits the clock generator and internal memory reading. An external connector (90) replaces the plug 39 and an external clock source (93) reads out the memory 58 to a binary-analogue converter (95) whose output is switched between two displays, recorders or printers (96, 97) according to the channel identification. The battery voltage is also monitored and displayed (91, 92) to check whether it has a reliable supply voltage.