[Boatanchors] Bird Termaline Question - Engineering Update (long)

[Brian]

Hello Doug,

For other readers coming to this thread late, I have left my previous response to Doug’s plea.

Depending on how non-reactive you want the eventual load to be, you can accept a load resistor of slightly different dimensions from your measured 5” L x 9/16” dia. The diameter is going to be critical, in that you have no room to accept increased diameter, but you can accept lesser diameter, perhaps down to 0.58”. And if you intend using the load horizontally, you could use a solid carbon resistor – because horizontally, there will be no coolant convecting inside the resistor – solid or hollow. Also, because there will be no coolant flow inside the resistor, you could insert a 175 Ohm resistor inside the 70 Ohm resistor to get back to 50 Ohm overall. But, that 175 Ohm resistor needs to be able dissipate about 1/3 the load (43 W). 

When a dissipation rating is given for a resistor, it is usually specified in still air, usually at 20 C, horizontal and the maximum external body temperature will be specified – it could be 275 C. As radiated power is proportional to the 4th power of the temperature differential and the convected power approximately proportional to just the temperature differential, most of the dissipation will be by radiation and a little by convection.  But you will have this resistor immersed in oil – a good thermal conductor – and the oil will transfer the heat to the external walls and thence to the fins, thus increasing the radiative and convective area. There will be a time lag between heat leaving the resistor and its being radiated / convected from the outer fins. So, the load will be able to withstand temporary overloads.

My suggestion, particularly as you have nothing to lose, is to find a tubular-bodied 175 or 180 Ohm resistor that will fit inside the current load resistor; it may be air-rated at 20 W, but in oil it will handle quite a bit more. Reassemble your Bird Termaline with the added resistor in parallel with the 70 Ohm tubular jobbie, fill the case with the correct oil, and measure the cold impedance. Then apply steady DC loads of 50 W, then 100 W, then 150 W (load Voltages of 50 V, 71 V and 87 V respectively) all the time clocking the ambient temperature and the final steady-state temperature on top on the Bird Termaline. If your mains power is relatively crap-free, you could drive the Bird Termaline with these as RMS AC Voltage values using a Variac; does your Voltmeter read true RMS? If not, stick with DC. Another possibility is to use an inline mains power meter in series with your Variac – don’t forget to subtract the no-load magnetising power the Variac requires – this may be different for each output Voltage. The beauty of a good inline mains power meter is that it couldn’t care less about waveform – all it’s concerned about is the in-phase product of V and I. Between each power run, disconnect the power, let the resistor cool and measure the IMPEDANCE. Hot, I expect the resistor’s impedance to creep a bit; but if the impedance back at room temperature has not departed from where it was BEFORE you started your power runs, you’re on a winner. My guess is that Bird’s design spec might say maximum temperature at the fins should not exceed 125 C, but that temperature will be related to the flash point of the oil.

The steady state load carrying capacity of the Bird Termaline is almost totally unrelated to the quantity of oil. The oil is there as a thermal conductor; so long as the dissipating elements and the outer negative-log conical section are fully immersed under all operating conditions, and the oil is in contact with all the fins on the inside of the case, you’ll be OK. In this ‘experimental’ phase, I would be inclined to overfill the Bird Termaline and leave the filler plug out. Then during your maximum heat dissipation run, the case and the oil will expand, and a little oil may be ejected. Wait till you reach steady state. What oil is left inside is what you need to stay with. 

Now say you reach a power input level where the external temperature reaches 140 C and stays there, and the cooled-out impedance is still 50 Ohm, you’re on a winner. But, say that ‘maximum’ power level is less than you want to test for, here’s another wee tip: put the Bird Termaline in a water bath. Now carry out your increasing power runs, but this time your maximum, steady-state water bath temperature cannot exceed 100 C at sea level. But you may now have a working non-reactive load and a great deal of knowledge about just how far you can go. You may even be pleasantly surprised.

This may seem like a lot of effort. However, as a radio amateur, your time is not anywhere near as expensive as $150 + postage. And think of what you will have learned. If you share your results, think what the whole reading group on this list will have learned.

I have done all this experimenting and fitting another parallel resistor inside a Bird Termaline – with great success. Then I designed a negative-log curved conic section that I got turned up to accept the load resistor that I scavenged from a Heath Cantenna. I put that load back inside the Heath can. I painted the outside of the can matt black to increase its radiative effectiveness. I have run that load steady-state at 1 kW with the can outer temperature not exceeding 110 C. The cool-out impedance is flat at 50 Ohm to 175 MHz, the maximum I can get out of my MFJ-259 VSWR meter. So, I speak from scientific, engineering and experimental experience perspectives.

Let us know how you go, Doug.

73 de Brian, VK2GCE.

On Saturday, February 18, 2017 5:53 AM, Doug said:

Here's an update on the Termaline 8135 issue.



I took the load apart and it is as Brian (below) described.  The actual resistor (made by Dale according to one source) is a 9/16" diameter Pyrex glass cylinder with a wall thickness of about 1/8".  Length is right at 5 inches.  The resistive feature is evaporated onto the glass tube as a coating.  When I measured the bare tube itself, indeed it was 70 ohms showing it had been heated beyond its recovery range.  




Checking for a few days for a replacement (to avoid the Bird $150 plus shipping charge), I found two vendors who had previously sold some surplus Dale replacements but no longer had them.  




The construction of the Termaline 8135 is quite impressive, lab quality precision part wise.  The N connector is attached to a thin silver 9/16" female socket to which the glass tube inserts.  The bottom of the glass tube is grasped by the end of a long perforated case cone (the ground side) that is bolted to the body of the aluminum entry plate that the N connector bolts on.  So I am hesitant to try to stuff something crude in that collet, however a similar size 50 ohm resistor could be made to adapt by remaking the N connector entry point.  




The best suggestion I have gotten is to use one of the inexpensive high power microwave resistors now on eBay for a pittance.  Before I do this, I wanted to ask, per Brian's suggestion, if someone might not have a possible substitute for the original Bird tube.



I'm also curious if anyone knows if the volume of that Termaline load, which holds 1 pint of Transformer Oil, is the volume of oil whose maximum sustainable capacity for heat is 150 watts ?




Thanks in advance for information on this and any leads on a possible resistor substitute.



Doug W5JV






-------------------------------- original message -----------------------



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From: Brian <brianclarke01 at optusnet.com.au>
Sent: Wednesday, February 8, 2017 1:10 AM
To: Doug Hensley; test-equipment at mailman.qth.net; boatanchors mailman.qth.net
Subject: Re: [Boatanchors] Bird Termaline Question - Update (long) 

Hello Doug,

The load resistor does not pop or blow like a fuse, as you so quaintly put 
it. The resistive coating on the outside of the ceramic cylindrical former 
expands at a rate greater than the ceramic - different TempCos - exacerbated 
by continued, ignorant overloading. The resistive coating may then crack and 
peel off; also the elevated temperature may stretch the terminal clamps - 
leading to higher resistance. The resistor can withstand 275 C, but the oil 
may start boiling below that temperature, leaving gas bubbles at the 
resistive surface, where the specific heat of the gas is much lower than 
that of the oil and so the temperature at the hot spots may well exceed 275 
C. Many oils vaporise around 170 to 200 C. And the gas may decompose, 
forming all sorts of nasty chemicals, some of which may damage the conic 
sections.

I suspect that so many faulty dummy loads come on the market because the 
present owners cannot afford to put them right. But, as there is no obvious 
outer cosmetic difference between a good one and a crap one, they can pass 
them off to unsuspecting novices.

Measure the length and diameter of the load resistor - now that you have 
dismantled your 8135. Tell us the size - some of us may have exactly what 
you want. The Bird offering, at $150, is moderately expensive. Surplus Sales 
of Nebraska has many non-inductive tubular 50 Ohm resistors - $275 for a 200 
W one a few years ago; however, their 60 W one (priced at $42 a few years 
ago) in oil may well handle 150 W, the rating of the 8135. Search on the 
internet for a replacement; visit ham events and field days; visit your 
local radio club.

Wash out the inside and the conic sections of your Termaline using a good 
oil-solvent soap to remove the last vestiges of the degraded oil - perhaps 
two washes and a water rinse. Then fill the case with water, pour the water 
into a measuring jug (then sun dry the casing and conic section), and you 
will know how much oil you require - probably around a pint. Visit your 
local electricity distribution authority, or better still, go to the 
transformer yard and ask for a pint of their synthetic transformer oil - 
probably no charge.

Bird has no need to comment on one of their products that has been severely 
abused. If the nomenclature plate riveted on the outside says 50 Ohm, that's 
what it was when it left the Bird factory. There is no way a 75 Ohm resistor 
could have crept in there. The fact that your impedance measurements show 
reactance, means that the resistor as it is now, is not working with the 
specific conic sections. (See Jacques Fortin's response in this stream)

The Heath Cantenna has the marvellous performance of 50 Ohm from DC to DC. 
Any higher frequency and it's reactive; it was clearly designed by an 
accountant. But don't throw it away - you may find the resistor inside is 
exactly what you seek for your 8135. The main use for a Heath Cantenna is as 
a doorstop. Really.

73 de Brian, VK2GCE.