A Vintage Moisture Meter - The Protimeter

[AlanBeckett]

I mentioned this gadget in a previous Thread - http://golbornevintageradio.co.uk/forum/...p?tid=4048
The only reason that I have it is that it's based on the AVOMinor.

   

The gadget itself looks like this:-

       

It's not difficult to see where its origins. It even still says AVOMINOR on the front.
It comes with a Instruction Card and Propaganda.

   
  ProtimeterLeaflet.pdf (Size: 3.14 MB / Downloads: 163)

Clearly it simply measures the Resistance between the two Spikes of the Probe.

I'll pause there 'cause this is the third time I've started this Thread and each time I've lost it due to a Power cut.
To be continued shortly.

Alan

[AlanBeckett]

To continue .....

I couldn't immediately remember why I'd put it to one side, apart from vague memories of needing obsolete Batteries - old age I expect.
I assumed that it required a couple of the usual AVOMinor-type batteries but they wouldn't fit so I did a search. It slowly dawned on me that it needs a couple of these - http://www.smallbattery.company.org.uk/s...EREADY,413
Make sure you're sitting down before you access the Link and look at the Price.
Just to make sure I connected my trusty Farnell PSUs - plural - in place of the Batteries and it did what it said on the Card. It's difficult to see what's going on in there but it's possible to see a 175k Resistor. Sure enough putting an AVO 8 across the Spikes gives 340 MicroAmps - near enough.

       

Just as a check I put a series of Resistors plus an R-Box across the Spikes with the following results:-

39k = 80%
120k = 60%
1M = 22%
2M = Edge of 'Dry'
3M9 = 18%

In the usual manner of Constant Voltage Resistance scales it's compressed at the top.
So, what to do about the Batteries?
After a bit of Research and Drawing it seems possible to make a couple using Lithium Button Cells - CR1225s - and a suitable Enclosure.
Something like this:-

   

Plastic probably although there really isn't much material to spare in the 15mm direction.

Suggestions?

Alan

[Tedzed1]

Found this thread on an american site ref the batteries

http://www.antiqueradios.com/forums/viewtopic.php?f=8&t=232194&start=0

[AlanBeckett]

Yes Paul I found a couple like that when I was searching, and it would indeed be possible to wire in three PP3 externally, but there's no hope of getting them inside.

Alan

[Yorkie]

Interesting instrument Al - I noted that the instruction sheet was written not too long ago in 1977 by Professor T A Oxley of Protimeter Marlow, Bucks, so out of curiousity I googled the Prof and Protimeter and noted that the second edition of a book that he co-authored was published in Nov 2013 by E G Gobert, E G and T A Oxley entitled 'Dampness in Buildings - diagnosis, treatment, Instruments':

http://www.amazon.co.uk/Dampness-Buildin...0750620595

Also Protimeter seems to still be in business employing 25 people in Marlow, but is now G.E.Protimeter and as we might expect, the instruments are rather more sophisticated:

http://www.ge-mcs.com/en/

As to the lithium cells, I noted at the Antique Radios link that Paul provided, one of the posts states:

I see outfits offering the Exell 413 battery they spec it at 110mAh, (milliAmp hours). For rough comparison the lithium coin cells are rated from:

CR2016 = 90mAh
CR2025 = 163mAh
CR2032 = 240mAh

So ten 2032s would seem to be the best bet capacity wise, but of course, they won't fit!

The problem with the 1225s that you suggest, which will fit (12mm diam x 2.5mm thick). is that they're only 50mAh

http://en.wikipedia.org/wiki/List_of_battery_sizes

Incidentally, the numbers denote the diameter and thickness of the cells - nothing more, so for example 2016 is 20mm diam x 1.6mm thick, 2025 is 20mm x 2.5mm thick, and 2032 is 20mm x 3.2mm thick, and 1225 = 12mm diam x 2.5mm thick. (Maybe you already knew that).

Have fun!

[AlanBeckett]

Thanks for that David. I hadn't thought to look up the esteemed Prof. I assumed he'd passed on long since

I don't think Battery Capacity is really a problem as it takes less than 0.5mA when shorted and much less when in use. Nevertheless, decent Capacity is no disadvantage.

Really I could do with fatter Cells with a smaller Diameter but that's not the way of the world at the moment.

Joe suggested that I could turn the Cells through 90 degress like this:-

   

If the Enclosure was fabricated from glued Sheet that looks a practical proposition. It could be three or four Stacks if that's more convenient. Also, and it may only require 9 Cells.

I'm pondering.

Alan

[Refugee]

Would it work with two stacks of 1.5V 10mm diameter cells side by side?
I have been looking into this for the 15V battery in an AVO.

[Joe]

Here's how I did a 15V AVO battery: http://golbornevintageradio.co.uk/forum/...p?tid=1317&highlight=Avo It's still working.

- Joe

[AlanBeckett]

I've done some pondering and pottering.

As has been pointed out the Lithium Cells leave very little material in a 15mm Enclosure, although they could be stacked the other way round. Also, the Voltage characteristics of them leaves a lot to be desired in this effectively Constant Voltage application. Finally, Capacity isn't really a problem as I don't expect it get much use.

In the end I settled for LR69 Alkaline Cells. I would have preferred the Silver Oxide version - SR69 - but the price is prohibitive.

   

They're 9.5mm Diameter by 2.1mm Thick, so there's more margin on the Diameter and 20 are a more sensible fit in the length available.

The Diameter is near enough 3/8" so I drilled a hole in a bit of 19mm Nylon and fed them in. It's an interesting task getting them all in the right way round as they're very slippery. I only dropped 2 but managed to find them.

   

So, what to make a rectangular enclosure out of? In the end I settled for a chunk of 15mm Cast Acrylic. I chopped a couple of 25mm pieces of with my trusty Denbigh Donkey Saw, shortened them to 60mm and cleaned up a bit. They fit OK.

       

After that it was just a matter of setting them up in the 4-Jaw and drilling though, slowly.

I think they look quite pretty

       

Now I have to think about the ends. One end can be glued but the other end I want to be removable. I haven't decided how yet.

Alan

[Refugee]

The cells in the second photo look dirty around the seal between the positive and negative contacts. I would give them a jolly good check over before using them. You comment that they feel slippery is another indication that they might be leaky.

I would have drilled a pilot all the way through and the main hole slightly short and run a tap through the small hole at the now "blind" end for one contact with the cells and the terminal in the set.
The other end just needs a spring from a cheap battery holder fixed with one screw tapped deep into the acrylic to prevent cracking over time so that it can be rotated for battery replacement with a couple of washers to prevent it cutting into the acrylic while it is being rotated.