Bench testing VDO oil pressure sender (5bar) issue: values higher on power on - stabilise after 15mins testing?

[vas]

morning all,

typical VDO oil pressure sender found on car/marine engines.
sitting on my table at home.
Rigged via a 1/4in tee to a NIKE bike pump on the one side and a WIKA 4bar 63mm glycol filled (doesn't show on the pic below for some reason!) gauge on the other.

VDO wired to my arduino board, finalising tuning values to fit it back on the boat (monitoring yanmar 2GMF generator engine).

So, all stone cold sitting from last night, turn test NMEA2000 rig using a bench power supply.
arduino on (arduino powers sender)
Garmin GMI10 on (shows values)
pump a bit to 50psi, arduino (and GMI) shows almost 60psi,
setup leaks slowly (v.convenient) so within the next 5-6mins I notice values dropping but with a shift.
Go and make an espresso, check email, back again say 15mins, pump it again to 50psi, arduino and GMI show 50psi!
Leave it on for 3-4-10h, values remain accurate.





what gives?
OK, I can live with that, but wouldn't mind an explanation. Pretty sure it's not my arduino messing up, nor bench supply or wiring. Will test that by leaving arduino on unplugging the vdo for a couple of hours to cool and connecting it again to check.
Must be that the sender is warming up, resistance changing and stabilising somewhere.
All that is tuned in the 20something C in my office, how far off is it going to be bolted at a 60+C engine block I wonder.
If it's an internal resistance thing that means the current it receives the first few minutes stabilise the thing, fine, if not I may have to check and shift values further to be closer to real. Temp rig the wika on a warm run at 3Krpm that the generator works and tune to that but takes even more time making things cumbersome and slightly pointless. Until recently I valued VDO highly, the more I play with all that the less I rate them tbh.
Now I know all these VDO senders/gauges are only good for giving an approximate indication of what's going on, interested to hear if others had such experiences.
OK, or I should get a 100-300euro worth of a Danfoss or WIKA, or other brand sender but not really worth it, trying to get the best out of what I have here.

cheers

V.

 

[starfire]

Hang a multimeter on resistance across the sender without the arduino.

Might just be the sender settling ?

 

[rogerthebodger]

How are you measuring the resistance of the sender. Is it part of a bridge .

One of the issues is ensuring the temperature stability of the comparative resistors in the conversion or in the Ato D converter.

 

[vas]

How are you measuring the resistance of the sender. Is it part of a bridge .

One of the issues is ensuring the temperature stability of the comparative resistors in the conversion or in the Ato D converter.

hm,

there may be a slight issue there...
just checked I'm feeding it 5VDC from a buck converter onboard my arduino box and measuring output on A2 analogue pin after a voltage divider on my board.
Want to have a decent stabilised voltage going in the thing, so not keen on getting the straight NMEA2K bus fluctuating voltage to it.
Being a 12VDC device, I guess 5VDC is not really enough to heat the whole thing up internally quickly and takes some time to settle that would be a convenient explanation.
I'll have to live with that as I'm not really keen on altering massively the current setup.

cheers

V.

 

[vas]

Hang a multimeter on resistance across the sender without the arduino.

Might just be the sender settling ?

that's how I did my initial measurement to add the values in an XL sheet and figure some code to do it, failed miserably, so following tests were with powering the device.

 

[andsarkit]

When you compress the air with the pump it will heat up and then slowly cool down and so the pressure will slowly decrease. After a while the temperature will stabilise and so the pressure will be constant. (PV=nRT)
Are you sure you have a leak? Can you check with a soapy solution around the joints?

 

[vas]

thanks, surely have a leak as the sensor itself only goes in just under 2turns in the fitting on the setup on the pic above (wrong thread) plastered it with teflon tape to make it keep pressure for 4-5mins...
Further we're talking about 3 pumpings on the small plastic pump, doubt it's a dramatic change of temp involved, not to mention I'd expect to affect the wika as well.

I'm happy to give that to 5VDC vs 12VDC input to the sender and be done with it tbh.
checking a random sender (not exactly mine, could be) VDO Pressure sender 0-5 Bar - M14
it mentions input V as 6-24VDC, so I'm slightly off

V.

 

[Boater Sam]

I have never thought these senders to be accurate, does it really matter?

 

[vas]

I have never thought these senders to be accurate, does it really matter?

you're ofcourse correct, everyone says they are not v.accurate and I did point that on the first post.
If we weren't in lockdown for the last (almost) three months, I wouldn't bother, but running out of things to do and had the arduino box out of the boat for the last couple of weeks changing some code, so why not improve it if I can.
TBH, finding why it happens is more important as I anyway have setup 3 alarms for low oil pressure, low seawater cooling system and high temperature so if one of the three happens, generator turns off automatically and a warning N2K message sent across all displays onboard with the cause.

cheers

V.

 

[rogerthebodger]

thanks, surely have a leak as the sensor itself only goes in just under 2turns in the fitting on the setup on the pic above (wrong thread) plastered it with teflon tape to make it keep pressure for 4-5mins...
Further we're talking about 3 pumpings on the small plastic pump, doubt it's a dramatic change of temp involved, not to mention I'd expect to affect the wika as well.

I'm happy to give that to 5VDC vs 12VDC input to the sender and be done with it tbh.
checking a random sender (not exactly mine, could be) VDO Pressure sender 0-5 Bar - M14
it mentions input V as 6-24VDC, so I'm slightly off

V.

Do consider that the normal connection is that the gauge G terminal is connected to the sensor . The G terminal will supply a stabilised voltage to the sender and as the resistance changes as the presser changes the current will change altering the position of the needle on the gauge.

If your supply voltage is below the stabilised voltage the stabilisation will not work just like any old type 3 pin voltage regulator

The gauge is reading current not voltage.

If you wish to measure voltage you will need a stabilised voltage and a resistance like the gauge resistance to give a potential divider. This reisiter needs to be an accurate 1% with good temperature stability.

You could also use the variable resistor in the sender as part of a bridge for extra temperature stability as as bridge will tent to self compensate for temperature variations.

Using a bridge circuit allows you to compensate for and variation in absolute resistance variation so long as the linearity is ok.

 

[starfire]

Do consider that the normal connection is that the gauge G terminal is connected to the sensor . The G terminal will supply a stabilised voltage to the sender and as the resistance changes as the presser changes the current will change altering the position of the needle on the gauge.

If your supply voltage is below the stabilised voltage the stabilisation will not work just like any old type 3 pin voltage regulator

The gauge is reading current not voltage.

If you wish to measure voltage you will need a stabilised voltage and a resistance like the gauge resistance to give a potential divider. This reisiter needs to be an accurate 1% with good temperature stability.

You could also use the variable resistor in the sender as part of a bridge for extra temperature stability as as bridge will tent to self compensate for temperature variations.

Using a bridge circuit allows you to compensate for and variation in absolute resistance variation so long as the linearity is ok.

Assuming the bridge is a similar temp to the sender, unlikely in this case

 

[rogerthebodger]

Assuming the bridge is a similar temp to the sender, unlikely in this case

Yes but that means in the case of an engine oil pressure sender that the rest of the bridge components will be fitted to the engine

If also assumes that the temperature/resistance factor to be the same in all legs of the bridge.

If they are away from the engine say on the instrument panel the temperatures will be significantly different.

I would recommend that VAS adds the gauge into the circuit and connect the voltage input to his arduino to the G connector on the gauge