Shunt value for voltmeter installation

[Steve Von Gruben]

Hello, all, I am in the process of installing an Electronics International volt amp meter in my Twin Comanche and need to know the value of the shunt in order for E/I to supply the correct instrument. Has anyone installed one of these units in place of the amp meter gauge and if so, what model number is the unit? It should be something like Va 1-xx where xx is the shunt value. Thanks in advance for your help.

Steve Von Gruben

[Charles Schefer]

Steve,

Funny you should ask this question as I have been looking at the exact same thing. I'm actually going a different direction - I am keeping my stock ammeter and having it overhauled to make sure it's accurate and then I am getting a Davtron Clock that also has a volts display. I'm doing this because I want a new clock too and it's the best way for me to go. That said I looked at the EI volt meter and liked it very much. I believe you need the "EI VA1A VOLT & AMP W/ EXTERNAL SHUNT 50A PRIMARY 2-1/4" PMA" model of the EI volt meter. You don't need to by a shunt because it will work with your existing shunt that should already be in the aircraft. The EI shunt is called a "-50" I believe but if you read the specs it's actually a 100Amp shunt... and so is your stock shunt. So unless you need a new shunt for some reason you should be able to buy that model of gauge without a shunt and just use your stock one.

- Charles

[Steve Von Gruben]

Charles, Thank you for your reply. I agree with you, I don't need a shunt and should be able to install the unit you describe. However, E/I thinks differently and wants to provide a specific unit based on the existing shunt value. I still haven't resolved this as Aircraft Spruce has sent me 2 units that are not correct. I keep ordering the unit you describe but keep getting a unit with an internal shunt. Everyone has been very helpful at both businesses but we are still in the process of sorting this out. I was hoping someone had been through this before and could provide a p/n of the unit they ordered.

Steve

[Charles Schefer]

Steve, as you probably know you definitely don't want the model with the internal shunt. I think EI makes just 3 versions of this Amp/Volt meter... one with an internal shunt (that you don't want), one that relies on a shunt up to 100Amps (that they call a 50 for some reason) and one that relies on a 300 Amp shunt. The existing factory Piper ammeter has a scale of +/- 100 Amps and to the best of my knowledge it's a 100 Amp shunt (the existing factory). So I think the model you need is the one for a 100 Amp external shunt. The listing I previously gave in my last post was that version as listed on Aircraft Spruce.

Incidentally the 300 Amp shunt version only reads in Amps (but not tenths of an Amp) whereas the 100 Amp version reads to the precision of a tenth of an Amp.

Good luck,

- Charles

[Charles Schefer]

I've learned some more on this subject in the past 24hrs. Looks like I was partly correct and partly wrong in my previous posts. I have the stock ammeter in my plane (). Right now my ammeter is at Air Parts of Lockhaven because I just wanted it checked for accuracy and recalibrated as necessary. They are also going to clean it up a little to make it look like new.

Yesterday Air Parts told me to calibrate my factory gauge would be nearly $300!!! Turns out (I am relieved to say) that they mixed my gauge up with another and it's only $180. Anyway, the initial incorrect quote of $292 caused me to start looking at the Electronics International VA-1A gauge again. I talked to EI and they told me if using an existing shunt the shunt value must be known and the gauge ordered to that shunt value. The gauge ordered would be a VA-1A-XX where XX = the shunt value.

I learned that my factory gauge is a 50 millivolt (mv) gauge and I relayed that to the folks at EI. They further told me that their VA-1A is also 50 mv but we still need to know the shunt value. The question is... at what shunt (amps) value is 50 mv produced back to the gauge??? I made a number of calls yesterday to try and find that answer. It's not in the factory manuals, and Webco also could not provide the answer.

This morning I spoke to "Jim" at Air Parts of Lockhaven. Jim retired from Air Parts years ago and after 3 years of retirement he agreed to come back part time but only works from 9am - Noon each day (East Coast Time). Good thing tho because he has a ton of knowledge around old Piper gauges...

Jim looked at my gauge and for starters determined that I had been quoted $292 incorrectly as referenced above (the first good news) and secondly, he told me that beyond a shadow of a doubt my factory shunt is a 50 mv / 100 amp shunt. He said he knows by looking at the gauge in detail because that is what it is designed to be matched to and also he said he just knows from having calibrated and checking Piper Comanche shunts in the past.

So as it turns out I was correct in my previous post by saying the factory shunt is a 100 amp shunt but perhaps I was just lucky in my assumption. Technically the factory shunt (according to Jim at Air Parts) is a 100 amp / 50 mv shunt. However, I was wrong in saying just order an EI VA-1A-50 gauge. Aircraft spruce only lists two variants for an external shunt... the -50 and the -300. You definitely don't want the -300 but in talking with EI they said even ordering from Spruce you give them the shunt value and they custom order the gauge from EI and it is then drop shipped. So if ordering a VA-1A you need to order it for a 100 amp / 50 mv shunt which in EI parlance would make it a VA-1A-100.

I hope this is helpful and my apologies for the error of my previous post.

One other detail.... I asked many I spoke to the question; "what about my existing factory shunt?, do they ever go bad or need to be replaced?". The general answer I got from many (including at EI and Air Parts) is; "No". A shunt is really just a calibrated very low resistance piece of metal with a known resistance value. In ideal conditions that should really never change. However this morning, Jim at Air Parts gave me a slightly different answer. He told me that he used to believe that the answer was a resounding No - a shunt never changes but he says he has on occasion seen a shunt go bad. He said if there is corrosion that adds resistance and changes the shunt value.

I asked Jim if given his experience if I should also send my shunt in with the gauge for calibration / checking. In my case my plane has been hangared it's whole life and the shunt literally looks brand new sitting there under the breaker box. Jim said his general advice is; if you are having problems with your ammeter readings then send the shunt in too but if not then just send the gauge. In my case we believe the shunt is fine.

Anyways... just wanted to pass this new info on. I hope my previous posts did not cause anyone to order the wrong thing.

- Charles

[Steve Von Gruben]

Thank you Charles, that is a wealth of information that hopefully gets saved for the next person that has this issue. Well done.

Steve