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Method and apparatus for making fine-wire thermocouples

This content has been downloaded from IOPscience. Please scroll down to see the full text. 1988 J. Phys. E: Sci. Instrum. 21 52 (http://iopscience.iop.org/0022-3735/21/1/006) View the table of contents for this issue, or go to the journal homepage for more

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I . I’hys. E: Sci. In.;trum. 21 (IOSS) 51-54. l’rintcd i n thc CJK

Method and apparatus for making fine-wire thermocouples T McBurney and D C Johnson Institute of Horticultural Research, Wellesbourne. Warwick CV35 9EF, UK Received 6 May 1987

Abstract. A simple method was developed for making subminiature thermocouples for use as dewpoint psychrometers. The apparatus was constructed for entwining pairs of wires as thin as 12.5 p m and welding them together with an electric spark in an inert gas. Dewpoint psychrometers consisting of chromel/constantan thermocouples, made by this means. were used for continuously monitoring water stress in growing plants.

1. Introduction A sensor for monitoring drought stress in growing crop plants is needed for elucidating causes of yield losses and for improving irrigation practice. A promising technique is to use fine-wire thermocouples as dewpoint psychrometers for measuring water potentials (vapour pressures) in a chamber of air at vapour equilbrium with the plants’ tissues. In developing the technique for field use we needed a simple means of manufacturing thermocouples for use in our own psychrometers. A method was developed by Zanstra (1976) in which pairs of wires were entwined and then welded together with an electric ,. ,

Figure 2. ( a ) Manufacturing method. Spool-wrapped wires were threaded through a guide and clamped between binding posts mounted on two rotating spindles. The binding posts were aligned by independently rotating the spindles using a knurled knob and gear mechanism. The distance between the spindles was adjusted to alter the twist pitch by moving the distal spindle on a rack and pinion. Accurate positioning was facilitated by referring to a scale scribed onto the rack. The position was clamped by tightening a wing nut. To entwine the wires the distal spindle (a) was rotated by driving it with a 12 V precision motor and worm gear (controls for speed and direction were built into the base). The entwined wires were then clamped (b) and trimmed under a low power microscope using scissors. (b)The distal spindle was then pivoted about the base providing clearance for aligning an electrode (c) mounted in a micromanipulator (d). (c) Schematic diagram of the electrode which consisted of a short length of tungsten wire (e) (diameter = 2 mm) clamped in a needle holder (f) which was adapted to allow a flow of inert gas (argon or nitrogen) over the entwined wires to exclude oxygen during welding. The gap between the electrode and the entwined wires was slowly narrowed causing a spark to jump which welded the thermocouple junction. Power for the spark was generated and controlled by a circuit similar to that used by Zanstra (1976). Up to eight 47 p F capacitors were separately switched into the circuit. Voltage was controlled with a 25-turn IO R variable resistor fitted with a revolution counter for precise setting.

Figure 1. Apparatus for entwining fine wires. ( a ) Front elevation diagram: ( h )oblique-angle photograph. a, base: b. wire guide loops: C. binding post (pinch-clamp): d. spindles: e. rack and pinion: f. clamping nut. 0~~12-373.i/SS/Ol0()51+ 03 $02.50 @

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spark in an inert atmosphere. However it required considerable skill and most attempts with wires of 25 p m diameter or less, suitable for use as plant psychrometers, failed because of wire breakages. The method seemed potentially useful but further development was needed to (i) improve control of wire tension and twist pitch, (ii) reduce handling of the wires to a minimum and (iii) reduce operator fatigue. We therefore designed an

Method and apparatus for making fine-wire thermocouples

Figure 3. Photograph of thermocouple manufactured from 25 ,um diameter wires of chromel and constantan: h, head junction: r, copper referencejunctions. Inset shows schematic diagram of thermocouple mounted in a chamber (c) machined out of a cylindrical metal rod (m).

improved apparatus for making the thermocouples which is described below. Thermocouples made with our apparatus were tested by incorporating them into home-made psychrometers and making measurements of water potentials in growing plants and in known salt solutions.

2. Apparatus and method The apparatus (figures 1 and 2) essentially consisted of an adjustable wire-twisting device and positionable electrode and circuit for producing an electric spark in an inert atmosphere. The method (figure 2) involved clamping wires between two spindles, rotating one spindle to entwine the wires and, where the wires crossed, cutting the wires to form a V-shaped junction. The electrode, surrounded by a stream of nitrogen or argon, was then manoeuvred close to the junction and a spark was caused to jump from it onto the junction producing a spherical weld. Novel features of the method were (i) minimum handling of the wires (once mounted on the spindles no further handling of the wires was necessary), (ii) minimum stretching and breaking of the wires achieved by tensioning them with spring and (iii) easing of operator work by driving the spindle with a precision motor. Also: four binding posts were provided on each spindle which enabled four-terminal thermocouples to be made simultaneously, as used in some psychrometers (Neumann and Thurtell (1972), as well as the more commonly used twoterminal thermocouples. Thermocouples for psychrometers were made from 25 p m wires of chromel and constantan (figure 3). The distance between the spindles was set at 76 mm. The motorised spindle was rotated 5 times at 10 RPM. For the welding spark, settings for capacitance and voltage were 47pF and 18.25 V respectively. Copper leads insulated with enamel were welded to the thermocouple wires forming reference junctions (figure 3). The enamel needed to be flamed off or abraded with fine emery paper at the point where the weld was made. Alternatively. reference junctions could be made by soldering or crimping the thermocouple wires onto thin copper strips. Type T thermocouples (copper/constantan) made from 25 p m wires for rapid response temperature measurements were also manufactured. Settings were the same as those for the chromel/constantan wires except that the capacitance was 141 p F and the voltage was 26.5 V. Similar procedures were adopted for making thermocouples from 12.5;um wires. Welded junctions were cleaned in warm soapy water in an ultrasonic bath and rinsed in distilled water. The highest hourly rate of production by a practised operator was 30 thermocouples.

3. Measuring water potentials The dewpoint psychrometer essentially consisted of a thermocouple mounted in a chamber (figure 3, inset) which was sealed against the cut surface of the plant stem. The dewpoint depression was measured at the (head) thermocouple junction and water potential calculated from it. The dewpoint depression was measured by alternately passing a current to cool the head junction and then measuring the temperature difference between the head junction and the reference junctions. Condensing water gives off latent heat and this provided the basis of an automatic procedure for 'homing in' on the dewpoint (Campbell et a1 1973). Calibrations were made against filter paper strips soaked in known potassium chloride solutions. Typically the calibration was 7.5 ,uV MPa-' over the range 0 to -5 M P a which agreed with measurements made by Campbell et a1 (1973) or predicted by theory.

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Figure 4. Plots of measurements made with the home-made psychrometer (vstem,0 ) and with the pressure-chamber technique (yleaf. 0 ) :(a) for one plant subjected to different temperatures ( T ,0), (b)measurements plotted against each other for several plants. The broken line indicates equipotential.

53

T McBurney and D C Johnson Young Brussels sprout plants were grown in pots of soil and placed in an illuminated chamber where they were subjected to different levels of water stress by altering the temperature. Measurements were made with a psychrometer attached to the plant stem. The psychrometer was insulated against thermal gradients which can cause errors in readings (Wiebe et a1 1977). For comparison. water potentials were destructively measured in foil-covered leaves by the independent pressure chamber method (Scholander et a1 1965) in which a leaf is detached from the plant and a gas pressure applied to it in a chamber until water exudes from the cut end of the stalk protruding through a gland to the atmosphere. Measurements made with the psychrometer responded rapidly to changes in the evaporative demand (figure 4(a)).The agreement with the pressure chamber technique was excellent (figures 4(a) and (b)). The results are similar to those obtained with a commercial psychrometer (McBurney and Costigan 1982). 4. Conclusions The apparatus enabled manufacture of fine-wire thermocouples suitable for use as dewpoint psychrometers for measuring plant water potential. It was adaptable to different materials and wire diameters. Even wires as thin as 12.5 p m could be welded. The main feature was its ease of use. good results being achieved after only minimal practice.

Acknowledgments We are indebted to P E Zanstra for discussions at the outset of this work, to P Springer for engineering guidance and to R Sampson for the photographs. References Campbell E C, Campbell G S and Barlow W K 1973 A dewpoint hygrometer for water potential measurement Agric. Meteor. 12 113-21 McBurney T and Costigan C 1982 Measurement of stem water potential of young plants with hygrometer attached to the stem J. Exp. Botany 33 426-3 1 Neumann H H and Thurtell G W 1972 A Peltier-cooled thermocouple dewpoint hygrometer for in situ measurements of water potentials Psychrometry in Water Relations Research ed. R W Brown and B P Van Haveren (Logan. Utah: Utah Agricultural Experimental Station, Utah State University) pp 103-12 Scholander P F, Hammel H T, Bradstreet E 0 and Hemmingsen E A 1965 Sap pressure in vascular plants Science 148 334-46 Wiebe H H, Brown R W and Barker J 1977 Temperature gradient effects on in situ hygrometer measurements of water potential Agron. J. 69 933-9 Zanstra P E 1976 Welding uniform-sized thermocouple junctions from thin wires J . Phjw. E: Sei. Instrum. 9 526-8

J . Phys. E: Sci. Instrum. 21 (1988) 5 4 5 8 . Printed in the U K

A simple implementation of the ’current integration‘ method T E Cranshaw Nuclear Physics Division, Harwell Laboratory, UKAEA? Harwell, Oxon OX1 1 ORA, UK Received 5 February 1987, in final form 1 June 1987

Abstract. The ’current integration’ method is often used to detect small changes in the intensity of radiation when photon rates are too high for the use of conventional nucleonic counting equipment. When the measurement is made repeatedly in a cyclic fashion, as for example in Mossbauer spectroscopy, some simplifications can be made. We describe a circuit using only a few integrated circuit components which successfully implements the method, and give the conditions under which the simplifications are valid.

1. Introduction In some experiments, the signal to be measured is the change of intensity of radiation as a function of some parameter, B. When the change is small, it is obviously advantageous to set a series of values of B cyclically, and average the results after many cycles. In this way, spurious effects, for example, from drifts unconnected with B can be greatly reduced. When the intensity of the radiation is low, it can be measured by counting photons using a nucleonic amplifier and single-channel pulse-height analyser, and, from a statistical point of view, this procedure makes the best possible use of the radiation available. However, it sometimes happens that the intensity of the radiation can be made quite strong so that conventional counting circuits are overwhelmed. A typical example of such experiments occurs in Mossbauer spectroscopy, which we will consider in the rest of this paper. In this case, B is a velocity which modulates the energy of the 7 rays from a radioactive source. The limitation of high counting rate was first encountered in experiments with the 19’Au isotope (Viegers and Trooster 1974). The solution of the problem found by these authors was to integrate the current from the detector in a capacitor with a time constant, T I , of the order of the ‘channel dwell time‘, i.e. the time of residence at a particular value of B in each cycle, and digitise the resulting voltage with a voltage-to-frequency converter. The output of the converter is then counted in the usual way. From this description it can be seen that in the current integration method, the integrator and the voltage-to-frequency converter act rather like a scaler, giving one pulse out for S photons into the detector. It is not exactly like a scaler, because by eliminating the nucleonic circuits we have lost the ability to reject photons of unwanted radiation, and noise in the detector will contribute to the output. Nevertheless, it is convenient to consider the system as effectively introducing a scaler, dividing the input rate by S. It is then easy to see the requirements laid upon the value of S. in order that the dividing process itself should not conribute to the fluctuations. Let N be the mean number of photons in a channel dwell time. z. Then the number of pulses given out by the scaler is

n =N / S . 0021-i735~8rl~IOl~JO~~ + 0 5 $02.50 0 1988 IOP Publishing Ltd