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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
Sent: Tuesday, April 29, 2014 1:13 PM
Subject: [LML] Re: VMS1000 pressure gauges on LNC2
Nick -
The VMS-1000 was a state-of-the-art engine
indication/fuel management system back in the mid-90's and is still quite
respectable. However, it's DPU is comprised of individual electronic devices which are now obsolete; hence, if any one of them
is damaged or fails, you may as well consider the entire DPU toast. The
possiblity of damage to an individual component is my reason for not
experimenting with voltage dividers, etc. in an attempt to make non-standard
sensors work. Fortunately, sensors are -for the most part- still available. One
insurmountable problem, however, would be replacement of the fuel flow sensor.
FloScan went from an 18K to a 6K unit and fuel flow accuracy requires that
sensors "K" factor be programmed into the DPU, which is no longer possible for
the VMS.
The MediaMate's terminals are numbered 1 thru 4 and
function as you've listed; however, terminals 2 and 4 are more correctly
identified as - and + 5 VDC excitaton, with terminals 1 and 3 as + and - mV
signal. I've also failed to mention that the MediaMate's plug-on 'Hirschmann'
terminals are ill-conceived and problem-prone, especially for a mV signal;
whereas, the MSP US 300 series all have a permanent, factory-installed
connection.
Bill
----- Original Message -----
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
Sent: Thursday, April 24, 2014 4:18 PM
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
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.
Sent: Monday, April 21, 2014 4:43 PM
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:
What do you think, people?
Yours,
Nick
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