Hi Bill,

 

(Bill Rumburg, that is; there are two Bills in the thread)

 

I don’t disagree with anything you said. But the fact that the VMS-1000 DPU could be difficult to keep going in the long
term – liable to become toast as you say – makes me reluctant to go for sensors that are specific to it. If my scheme works then I have pressure sensors that might (not guaranteed, but might) work with another display system. At least they will be industry
standard and I’m also going to use industry standard connectors, not the spade terminals.

 

If the fuel flow sensor failed, then I would probably ignore it, at least for a while. But without OP and FP displays,
you have to take action immediately.

 

Yours,

 

Nick

 

 

----- Original Message -----

From:
nick [at] beaglepup.info (Nick Long)

To:
lml [at] lancaironline.net

Sent:
Friday, April 25, 2014 7:55 AM

Subject:
[LML] Re: VMS1000 pressure gauges on LNC2

 

Hi Bill,

 

This is my understanding.

 

There are 4 terminals on the transducer or sensor, marked 1 to 4.

4 – 5 V supply – usually red wire

2 – 0 V Ground – usually black wire

3 – V1 out – usually green wire

1 – V2 out – usually white wire

 

The signal out is the voltage difference between V2 and V1. These voltages are about 2 V, but the difference
between then varies from 0 mV to 50 mV in normal operation.

 

The transducer seems to be a passive bridge network, or almost. V2 varies with the pressure, and V1 is a reference
that doesn’t change with pressure. The important value is V2-V1. This is what the VM1000 measures and uses to calculate the pressure.

 

With the device powered up and with no pressure, , then V2 and V2 should be the same, to within a mV.

This is fairly simple to test, either in the aircraft or on the bench. You could either use a voltmeter to
measure them each accurately, or put a voltmeter on a small range scale between them.

 

If it passes that test then the next one is to see how the differential voltage V2-V1 varies with pressure.
This is a more difficult test but you may not need to do it. By the time you have got this far, it may well be clear where the problem lies.

 

HTH,

 

Nick

 

 

From:
billhowie [at] me.com (WILLIAM HOWIE)

Sent:
Thursday, April 24, 2014 4:18 PM

To:
nick [at] beaglepup.info (Nick Long)

Subject:
Re: [LML] Re: VMS1000 pressure gauges on LNC2

 

Nick:

I have a IVP, and I had a indication of no oil pressure LAST WEEK

I PUT A PRESSURE GAUGE ON THE OUTLET OF THE TRANSDUCER AND I HAVE OIL PRESSURE.

THE PROBLEM THEN IS WITH THE TRANSDUCER OR WITH THE CONNECTION FOR THE DPU. I CAN'T FIND MY TESTING PROCEDURE FOR THE TRANSDUCERS, I was hoping you could help me with this.

Could you tell me you're testing procedure on the vision Microsystems transducer.

The pins are numbered one through four but I don't know where to apply the 5 V and where to read the output.

Thanks a lot Bill Howie

Sent from my iPad

On Apr 23, 2014, at 10:46 AM, "Nick Long" <nick [at] beaglepup.info

> wrote:

Hi Bill,

 

Thanks for the info. I think you have done everyone a service and it’s good to know you have some drop in
replacements available at half the price I have seen them elsewhere.

 

If you don’t mind, though, I’m going to pursue my ideas a bit further. I quite like the idea of using off
the shelf sensors and using the same one for OP and FP. There is also the background thinking that the VM1000 is a bit old now and that rather than building a solution around that, I could devise something with general applicability.

 

Over the holiday weekend I did some more investigation. I built a rig in my workshop to pressurise the old
sensors from my air line system* and measure the resistance changes and the voltage outputs. The two sensors gave the right voltage changes (50 mV and 100 mV for 100 psi) but with horrendous offsets. The zero psi values were way out. So they were working,
but not actually any use.

 

Then I checked the VM1000 panel by injecting voltages on the FP and OP inputs. That did exactly what was expected,
but now I also know what common mode voltages it will cope with.

 

Anyway, I have ordered some modern sensors with 0 to 5 V outputs and I’m going to scale the outputs down to
suit the VM1000. I will let you all know how I get on. If I’m right, then I will have something that should work with any other monitoring systems that are out there.

 

Yours,

 

Nick

 

*not as easy as it sounds because the sensors are NPT threads and my system is completely and utterly the
way it should be – British Standard Pipe thread.

 

 

From:
lancair403 [at] verizon.net (William Rumburg)

Sent:
Monday, April 21, 2014 4:43 PM

To:
lml [at] lancaironline.net

Subject:
[LML] Re: VMS1000 pressure gauges on LNC2

 

Nick -

 

The VMS1000 oil and fuel pressures were designed for the Honeywell MediaMate, which produces 50mV DC full-scale output
when excited with 5VDC. The MediaMate, however, is inferior, antiquated technology and outrageously overpriced.

A modern, high-quality replacement is the Measurement Specialties US321. It also produces 50mV DC when excited with 5VDC;
hence, it's an electrically-DIRECT replacement for the MediaMate
http://www.meas-spec.com/downloads/US300.pdf 

A problem, however, is that the VMS1000's combination of 5VDC excitation and 50mV DC full-scale output is, apparently,
rarely used in industry; hence, I couldn't locate any US321's -in the needed pressure range- for sale despite an exhaustive search. I contacted Measurement Specialties last August and learned that they would manufacture them at a cost of $140 each, with a
minimum order of five. So I ordered five @50 psi (fuel pressure for fuel-injected engines) and five @100 psi (oil pressure). I ordered them with four foot pigtails to allow direct routing to the DPU, with no intermediate connector. Attached is a photo of them
installed on my engine. If anyone is interested, I'm offering the extra's for my price of $140 each.

 

Bill Rumburg

N403WR  (Sonic bOOm)

Hello Everyone,

 

A few weeks ago we had a discussion about this, and I then went away and checked my installation. After measuring
the voltages I concluded that the display unit is ok, but both the Oil Pressure and Fuel Pressure sensors are not behaving.

 

So then I looked into replacements. There was an interesting post last September from Bill Rumburg about using
sensors from Measurement Specialities; the originals are Honeywell Mediamates.

 

Bill was saying the OP and FP transducers are different in their voltage vs pressure response by a factor
of 2 and he was exploring some alternative devices. But here is another idea. Why not just find a sensor that gives the correct output for FP? The same sensor will give 2x the reading if used for OP, but that can be scaled down by 2 with a simple precision
resistor network.

 

If that is correct, and I’ve followed the previous discussion correctly, then Digi Key have the very item
we need:

http://www.digikey.com/product-detail/en/M3421-000006-100PG/MSP6892-ND/726352

 

What do you think, people?

 

Yours,

 

Nick

 

---

--

For archives and unsub
http://mail.lancaironline.net:81/lists/lml/List.html