MAKING THERMOCOUPLES FOR AERIAL AMMETERS
Ray Robinson VK2NO
I was working on an WSNo.11(Aust), which is the Australian version of
the Wireless Set Number 11, and attempting to load the transmitter into an
aerial. There was very little aerial current. I tried various sized resistors,
and finally tried a short circuit. It loaded beautifully into this, but as
I was tuning up, the response was so quick, that the aerial ammeter hit full
scale, and then flopped back to zero. I had burnt it out!
![](WSNo11s.jpg)
This type of meter is a plug in design, and is used in several radios,
the WSNo.11, the WSNo.101, and the FS6. The meter comes in two values,
both are the same physical size, and can be easily unplugged and a different
one plugged in. The 1 Amp size is used in the WSNo.11 HP (High Power) and
the FS6. The 350 milliamp size is used in the WSNo.11 LP (Low Power), the
WSNo.101 and the Aerial Coupling Unit Type C (for the WSNo.11). The WSNo.101
is merely the Australian version of the WSNo.1, and the FS6 (Field Set 6)
is the high power version of it. So I retrieved my box of spare aerial meters,
and checked them. I had 28 meters in the box. All were faulty.
I decided that I should fix one or two.
FAULTS
I sorted the meters into the 1 Amp and 350 mA sizes. I further sorted
them into the different manufacturers. Some were labeled AWA, the manufacturer
of the radios. AWA is the Amalgamated Wireless (Australasia)
Ltd., the biggest Australian radio maker of that period. It was soon apparent
that those meters were identical to the other meter manufacturers, so I
concluded that they were not AWA manufactured at all, just made to an
order and labeled AWA. This reduced it to four manufacturers. They were VANE,
EMMCO, PATON, and an unnamed Model 505 (in a stepped sided case).
Picture: Different Meters (left to right, VANE, EMMCO, PATON, Model
505, top row 1 Amp, bottom row 350 mA)
I had assumed that they would all have burnt out thermocouples, but
that was only partially true. Each type also had a unique fault, which
affected the actual meter movement. I tested all the meter movements, to
find the good ones. The VANE type was the best. Only one EMMCO had
an open circuit movement. The PATON type used a die cast support for the
scale and pivot, and some had suffered pot metal disease, the scales were
buckled, and the meter pivots had distorted or broken off. The Model 505
type, suffered from corroded and open circuit hairsprings.
Picture: Pot Metal Disease
Table of the fault types and the number of faulty meters.
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TYPE FAULT
350 mA
1 Amp
-------------------------------------------------------------------------------------------------
VANE O/C thermocouple
3
1
-------------------------------------------------------------------------------------------------
EMMCO O/C thermocouple
3
3
O/C movement
1
0
-------------------------------------------------------------------------------------------------
PATON O/C thermocouple
3
3
pot metal disease
2
4
-------------------------------------------------------------------------------------------------
Model 505 O/C thermocouple
1
1
corroded hairsprings
2
1
-------------------------------------------------------------------------------------------------
MOVEMENT
Firstly, I cleared the bench, and removed all dirt and iron filings,
a magnet being useful for this. I then put down a white cloth, and set up
a bright light and used a head mounted magnifying lens. I disassembled the
meters, and sorted them into the good meter movements. Most had loose glass,
some had broken or missing adjuster screws, and two had broken cases. This
had let dirt, iron filings, and moisture enter and affect the movement.
I cleaned the cases and glass, glued the glass back in, used good cases,
and swapped adjuster screws to the good cases. I used clear nail polish to
glue the glass back in. Then I connected a bench power supply, with a 4K7
series resistor, with leads with alligator clips, to test each movement. I
varied the voltage, to swing the needle up and down, looking for smooth swings,
without sticking or jamming. I put the good movements aside, cleaned the
sticking ones, and put the faulty ones in a box for spares.
I tried several methods to remove the dirt and iron fillings from the
meter movement gap. Blowing through the gap removed some, but left sticky
or magnetic material in place. Pulling some thin strips of paper through
removed some more. Using stainless steel (non magnetic) very thin tweezers
allowed me to grab some iron filings. I discovered that a "Dissecting Kit"
was useful, as it contained 3 tweezers, 3 scalpels, 2 scissors, one
spike, and spare blades, in a soft zip closed satchel. This is available
from Macquarie University for A$24 or 12 UKP. The best method proved to
be laser printer labels, cut into thin strips, and poked into the meter
gap. I used each strip once only, and threw it away or cut the end off,
and kept on poking around until the gap was clear, and there were no particles
on the sticky strip. This grabbed all foreign matter easily and quickly,
and was less damaging than poking around or blowing. I just had to be careful
not to touch the hairspring and stick to it. I now had 18 good movements,
all they needed was a thermocouple. I should mention that I am not an instrument
fitter and have no training in this field at all.
THERMOCOUPLE
I read up on thermocouples. These thermocouple meters were made in 1940,
so it should be simple to duplicate, and I decided I would attempt to make
one. It was more difficult than I thought.
Deciding on the correct type of thermocouple, and what materials to
use, required some guesswork. All the information I could find, addressed
the use of thermocouples to measure temperature. The classic approach, is
to use two thermocouples, one to measure the desired temperature, and the
other as a reference to reduce the effect of variations in ambient temperature.
I was only interested in the temperature change, absolute temperature was
not required, so I only needed one thermocouple. Thermocouples function by
using two different metals touching or joined to each other. If wire is used,
and the ends of the two wires are welded together, a voltage at the other
ends of the wires is produced, and is dependent on the temperature of the
junction. Several standard combinations of different metals are used, depending
on the application, the temperature range, the cost, and the hostile atmosphere.
In this application, ambient temperature, air, and non magnetic properties
were required. Finding a supplier of the wire proved to be a problem. I
eventually discovered OMEGA ENGINEERING, Atlanta, Georgia, <http://omega.com>
in the United States would supply small quantities, various wire diameters,
various wire metals, with web based mail order, and at reasonable prices.
I ordered two 50 foot reels of ALUMEL and CHROMEL, diameter 0.13 mm (0.005
in), to make an "ANSI standard K" type junction. They were $14 for each
reel, but there was an additional $55 shipping charge! I paid by Credit
Card and the wire arrived a week later.
HEATER
To continue the classic approach, the thermocouple is placed where you
wish to measure the temperature, which is often in a boiler, or in a hot
gas, or in a chemical solution. This was no use to me, I wanted to heat the
thermocouple with aerial current, so I needed a heater resistor, in contact
with the thermocouple. I could find no information on this application.
I knew wire wound resistors were made of nichrome wire, and I had a box full
of them, so one of these should be suitable. I measured the remains of several
burnt out thermocouples in the meters, of which half the thermocouple was
usually intact, and also the length of the heater and the diameter of the
wire. They were between 1 to 2 ohms in resistance, about 7 mm long, and about
0.125 mm in diameter. This should enable me to select some wire for the
heating part of the thermocouple. I destroyed a wire wound potentiometer
to provide the heater for the 1 Amp thermocouple. It was a 1K ohm potentiometer
of about 10 watts dissipation. I destroyed a 5K resistor, 20 watt dissipation
for the 350 mA thermocouple heater. Now I just had to join them together.
Picture: Resistance Wire
WELDER
The thermocouples appeared to be welded together. There were some thermocouple
welders available on the OMEGA web site for US$1800, but this was too expensive
for me. I decided to make one. I decided that an ARC welder would melt the
wires, but I would have to hold them in place. I designed a jig that had
two terminals to hold the heater wire. There was another terminal for each
of the thermocouple wires, and a tube to guide them and hold in place. The
terminals holding the wires would be connected to a meter and a current source,
to test the thermocouple in place. The two arc welding electrodes, mounted
vertically, one above and one below, would meet at the contact point,
create an arc, and melt the wires together. I constructed the jig from junk
box parts, with a knob to push the welding electrodes together, and springs
to retract them. I worked out the leverage, so that the top electrode moved
at three times the distance and speed as the bottom electrode. I dismembered
two carbon zinc AA penlight batteries, and used the carbon rods and the
crimped on metal cap. I used a ceramic insulator as the guides for the electrodes.
Initially I used, a SCOPE soldering iron transformer to provide the current
source, as it provides 3.3 volts AC at a current capable of heating the
carbon element to red heat and melting solder. This proved insufficient.
I used a bench power supply to provide 30 volts DC at 2 amps. This
also proved insufficient.
Picture: Jig
Picture: Prototype Welder
It appeared as though arc welding was not suitable. Perhaps spot welding
would be better. I looked at some designs on the web and most seemed oriented
to spot welding sheets of metal or battery tabs together. I selected a
simple design, using two regulator Integrated Circuits (IC), a Silicon Controlled
Rectifier (SCR), with a convention transformer power supply. I used an old
Golf Buggy battery charger that had burnt out, but the transformer was
still good. I put aside four computer grade capacitors, giving 170,000 uF
in total, each the size of a can of soft drink. I used an LM317 regulator
IC to adjust the voltage to between 5 to 19 volts DC. I connected up the
SCR to the carbon electrodes, with one capacitor and tried it out. The carbon
electrodes produced a small spark, but did not get hot. I decided to change
the electrodes to copper, so that all the energy was dissipated in one
place, rather than in the length of the electrode. I used an old copper
soldering iron tip. This now produced a satisfactorily loud spark. However,
the wires I attempted to weld, were vaporised! There was too much energy,
even when turning down the voltage regulator. I reduced the capacitor size,
and eventually settled for three normal sized electrolytics, each 4700 uF
at 25 volts. The SCR I used was too large, but as it worked, I did not change
it. Eight volts seemed to work the best, and 12 volts melted the wires.
However, I could not see the junction, it was difficult to arrange the wires,
I often only got two welded together, and I could not get them out easily,
or trim them to size. I discarded the jig.
Picture: SCR
Picture: Welder
Maybe a simpler approach was required. I screwed a large copper lug
to the top of the plastic box, held the wires in place with sticky labels,
and applied the upper electrode by hand. The heater wire and the two thermocouple
wires, were gently pulled through some fine grade sandpaper, to clean them
first. I could feel a slight "slump" or "give" when a successful weld
occurred. If I pressed too hard, there was not enough contact resistance,
and no heat and weld happened. If I had the voltage too high, a spark
came out the sides, indicating the wires had been vaporised. If the wires
or electrodes were dirty, then there was no weld. With a few trials, I settled
on a method that works well. After the weld, I cut the wires with the scalpel,
and picked the finished thermocouple up with tweezers. I gave each wire
a tug to test the welds. The welds and the wires are quite strong. The wires
can be bent without breaking, but the weld does not like being bent.
Picture: Thermocouple and Tweezers (ignore light reflections)
MOUNTING
The thermocouples were basically in two physical forms. Most meters
used a straight heater wire, with the two thermocouple wires attached in
the centre, and coming away at 45 degrees, so the shape looks like the
letter "K". The ANSI standard also uses the letter K, as a type of junction,
but I am referring to the shape here. The EMMCO meters had a straight heater
wire, with the thermocouple wires attached in the centre, but coming away
on both sides at 90 degrees, thus looking like a cross. The thermocouples
are mounted on the bakelite meter case rear panel, and soldered in place.
Ordinary 60/40 (tin/lead) solder will not work, as it does not wet or stick
to the nichrome heater wire. I purchased some flux core "silver" solder,
from the local hardware store, and adjusted my variable temperature soldering
iron to full, as approximately 480 degrees F is required. The physical construction
of the meters, has the two plug in pins attached to the rear of the meter
case, and two copper metal bars almost meeting in the middle. The heater
wire is soldered between these. The solid bars and the big pins, act as heat
sinks, to remove the heat from the heater wire, so that the needle can quickly
return to zero, when the heater current is removed. They also suck away the
heat from the soldering iron, making it a little tricky to solder. The ends
of the thermocouple wires are soldered to two small posts, and the meter
movement is connected to these. There is provision of a third and fourth
post, so that a series resistor can be added, to reduce the meter deflection
(for calibration). Some of the 350 mA meters had a length of the heater wire
soldered to these posts, the length being set for the amount of resistance
required. The PALEC and EMMCO meters also had heat sinks on the thermocouple
wire pins. I accidentally dropped a few thermocouples, and they disappeared
into the flotsam on the floor, or into the hair on my arms or legs, as the
hair was about the same size. Wearing light coloured long trousers and a
long sleeved shirt or dust coat would be an advantage.
Picture: Burnt out PALEC Thermocouple
Picture: Burnt out EMMCO Thermocouple
TESTING
I used the other half of my bench power supply, set to 10 volts, but
with the current limit set to zero. I used leads with alligator clips, to
test each thermocouple. These were connected to the meter rear pins. I
had other leads with alligator clips, connected to the meter movement. The
meter movement was lying on the bench in a plastic jam jar lid. This supported
the movement, and prevented it from being damaged or picking up dirt and
metallic contaminates. I used the power supply current limit knob
to swing the current up to 350 mA (or 1 Amp as appropriate). If the needle
went the wrong way, I reversed the leads. Most thermocouples were good.
Some failed straight away. I tried to use some failed thermocouples, but
they were impossible to solder after being welded, and difficult to reweld.
It was easier to throw them away and make another.
Picture: Testing the New Thermocouple
I also tried another method of heating the thermocouples. I decided
that they did not necessarily require the heater actually welded to the
junction, just the heat from it. So I wrapped the heater wire twice around
a junction that a heater wire had broken off. It was easy to do with tweezers
as the heater wire is quite strong. This worked, but it was sensitive
to the heater current polarity. I was using DC to heat the wire. By wrapping
the heater around the junction, it usually made electrical contact with
both the thermocouple wires, and the electrical voltage drop along the heater
wire, was conducted to the meter movement. This made the meter reading inaccurate,
as it was dependent on the applied DC current polarity, one direction increasing
the reading, the other direction reducing the reading. This may not have
been a problem when measuring AC aerial current, but I decided not to use
this method. I also noticed the same effect on some successfully welded thermocouples,
but on close inspection, the heater was not exactly at the junction weld,
but a little back from it, and touching the two wires. These were also discarded.
CALIBRATION
The 1 Amp meters were the easiest to do. They were usually within 10%
of the FSD (Full Scale Deflection), some as close as 1%. The 350 mA meters
were harder to calibrate, most were within 20% but a few were reading
100% low (a reading of 350 mA FSD for 700 mA through them). I had to put
in a new thermocouple to get them closer to the correct value, as my manufacturing
technique had some specification variation in the finished thermocouples.
I found none that read high (over the FSD), so fitting an internal series
resistor would not have helped. I concluded that I needed more temperature
from the heater. I was using a smaller diameter heater wire for the 350
mA meters, but the same thermocouple wires as the 1 Amp meters. I was not
prepared to buy another two reels of smaller diameter thermocouple wire,
so I accepted the inaccuracy. I rationalised this by considering Aerial
Current meters are a tuning aid, to achieve maximum aerial current, and
the actual aerial current is not that important, provided it is the most
you can achieve. Some transmitters only have a series light globe as a tuning
indicator.
CONCLUSION
It is possible to repair thermocouple ammeters. The more sensitive ones
are more difficult. I read with interest, an article about repairing aerial
ammeters with a current transformer and diodes. This is also a good approach
and easier to do. As an interesting aside, I could blow on the thermocouple,
and watch the meter reading drop with the cooling, and then recover. Also
when working in reduced light, I noticed the heater wire was visibly red
hot, at about 1.5 Amps. It would burn out at about 1.7 Amps.
Welder Circuit
Copyright Ray Robinson
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