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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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