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Old 12-05-2012, 06:48 PM   #1
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Default Oil Gauge Sender Questions

Hey guys, I'm considering getting this dual function VEI Oil Pressure/Temp gauge:

VEI Dual Gauge: Oil Pressure + Oil Temperature

as I'm down to one remaining gauge space in my 3 gauge panel, but still need to cover these two more functions. While the price of the gauge itself is reasonable, it does not include the sender units. Pricing them on their site adds another $27.55 and $16.20 for the pressure and temperature sender units respectively. That seems a bit too rich to me, but then I was not sure if they are basically generic units, or if there is something particular about them.

Does anyone know if they are a specially calibrated unit designed to match up well with their gauge?

If not, does anyone have a link or reference to a more reasonable alternative to the high priced units they offer that will work just as well?

Thanks a bunch

Last edited by Joe Perez; 12-05-2012 at 06:54 PM. Reason: Linkified your link
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Old 12-05-2012, 07:02 PM   #2
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In the instructions, they describe the pressure sensor as "a standard 240-33 ohm sender", and give no specific description of the temp sensor.

That said, $27.55 and $16.20 is an excellent price for a pressure and temperature sensor, and I'm not sure where you expect to pay significantly less for those.

My normal go-to place for gauges is egauges.com, and their 240-33 pressure sensors are all in the range of $29-$54: Pressure Senders: 0 - 80 PSI / 5 Bar - 240-33 Ohms (U.S. Ohm Range) - egauges.com
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Old 12-05-2012, 09:19 PM   #3
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Thanks Joe, I guess maybe I'm just remembering pricing from too far back in the past. I thought I had recently seen someone else mention sourcing senders for way less recently, but (naturally) I couldn't find it again when I searched for it. Anyway, I guess if that is currently the going rate for them, I best recalibrate my wallet.
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Old 12-05-2012, 09:33 PM   #4
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Any good options for volt-type senders (0-5V)?
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Old 12-05-2012, 09:52 PM   #5
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Originally Posted by czubaka View Post
Any good options for volt-type senders (0-5V)?
I'm not entirely certain what you mean.

Nearly all pressure and temperature sensors are passive in nature- they are essentially variable resistors wherein the resistance changes in proportion to the phenomenon being measured.

If you apply a pullup voltage to them through a fixed resistance, then you will have created a voltage divider, and can measure a varying positive voltage at the junction between the two. This will not yield a 0-5v output, but it's as close as I can imagine coming.

Dare I ask what the application is for which you need this?
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Old 12-05-2012, 09:54 PM   #6
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I got a minor in EE. Apparently none of it stuck.

It's for a Reverant enanced MS2. My understanding is the stock oil pressure sensor doesn't fall within this range. Amirite?

I should mention it's on a 2001.
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Old 12-05-2012, 10:06 PM   #7
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It's for a Reverant enanced MS2. My understanding is the stock oil pressure sensor doesn't fall within this range. Amirite?
Ah, I see.

The stock oil pressure sensor on all Miatas '95 and later is just a simple on/off switch. (Ever notice that the needle doesn't move with RPM?)

As for the Rev MS2, just pick any sensor with a positive resistance coefficient, apply a pullup to 5v, and you can scale the ADC in software to match. Since these sensors typically have fairly low resistance, use a fairly low pullup value- maybe 220 ohms or so. That'll keep the measurement range reasonable and increase resolution.
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Old 12-05-2012, 10:11 PM   #8
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You just let the smoke out.

Positive resistance coefficient - resistance rises with pressure? As for applying a pullup to 5v, uh, is that a software thing?

I don't have the unit yet, so I have nothing to play with. Learning curve is steep right now.
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Old 12-05-2012, 10:50 PM   #9
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Ok, so poking around google images, I see a 5V pull up is NOT a software thing. Now I have to figure out how to build the circuit.

Yes, I'm dumb...
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Old 12-06-2012, 06:49 PM   #10
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Joe, is this what you're talking about?


The farthest I got in EE was some signals & filters and some basic amplifier design.
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Old 12-07-2012, 03:58 AM   #11
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The Enhanced MS2 needs a transducer-type pressure sensor, not a resistance type.

This means that the sensor needs to have 3 wires:

1) 12V or 5V supply
2) Ground
3) Signal output

Its only the signal output that needs to be connected to the MS2. The other two are pretty much self-explanatory.
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Old 12-07-2012, 02:24 PM   #12
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It takes an analog voltage as the input, but that voltage can't come from a resistance sensor with a pull-up resistor?
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Old 12-07-2012, 02:28 PM   #13
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The Enhanced MS2 needs a transducer-type pressure sensor, not a resistance type.
WHAT MANNER OF WITCHCRAFT BE THIS?

(And where does one purchase such a sensor?)
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Old 12-07-2012, 02:53 PM   #14
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Quote:
Originally Posted by Joe Perez View Post
WHAT MANNER OF WITCHCRAFT BE THIS?

(And where does one purchase such a sensor?)
Here is on that I have for my prosport gauge. Once I get another sandwich plate for an oil cooler, I will be hooking it up to my MS3 so that it can read oil pressure.
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Old 12-07-2012, 03:39 PM   #15
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Quote:
Originally Posted by Joe Perez View Post
WHAT MANNER OF WITCHCRAFT BE THIS?

(And where does one purchase such a sensor?)
I have two MSI 0-100psi sensors for oil and fuel pressure. They take 12V and provide a 0-5V (actually 1-5V IIRC) signal.

I got them through Motorsport Innovations several years back.
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Old 12-07-2012, 03:40 PM   #16
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Quote:
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It takes an analog voltage as the input, but that voltage can't come from a resistance sensor with a pull-up resistor?
Only if the resistance sensor has a linear resistance curve.
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Old 12-07-2012, 04:27 PM   #17
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Only if the resistance sensor has a linear resistance curve.
Gotcha. The V(T) curve for common resistance sensors with typical source resistors can probably be described reasonably well with a third order polynomial, maybe fourth. What that means is that you'd need 3 or 4 constants instead of the 2 that you're using now.

If you're interested in implementing that in your code then I can help determine the needed coefficients. In fact, I'm sure I've already done it for GM temp sensors, but it should be simple for any resistive sensor given a few sample resistances spanning the useful range. I can whip up an Excel sheet that allows you type in the sample resistances and the source resistor then it'll output the constant coefficients. Your code would continue to work with linear output devices, you'd just set one or more of the coefficients to 0.
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Old 12-08-2012, 10:48 AM   #18
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You're talking about oil temperature, while I'm talking about pressure.

If you are referring to the Steinhart-Hart equation, then its very inefficient to calculate the output from the sensor every single time using the equation, hence why we use lookup tables with known sensors.

The Enhanced MS2 right now supports the 90-97, 99-05 and the GM coolant temperature sensors as oil temperature sensors.

To use these oil temperature sensors, no pullup or bias resistor is needed; the circuit is already built on the expansion board. All you need to do is connect on pin of the sensor to pin #31 on the top connector, and the other pin to a signal ground.
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Old 12-08-2012, 12:45 PM   #19
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I should have been more clear. I was talking about using any resistance type sensor as part of a voltage divider.

No matter what kind of sensor, the 10 bit ADC result needs to be converted to something meaningful (obviously, I know you already know that.) If the output of the sensor is linear, then 1 multiply and 1 add is enough. If the output is not linear then it is sometimes possible to apply the DaveC equation to convert the 10 bit integer into a useful number. That requires 3 multiplies and 3 adds in the case of a typical temp sensor, but I admit that I don't know what the resistance curve looks like for a typical pressure sensor. Is 3 multiplies and 3 adds too expensive?
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Old 12-12-2012, 11:37 PM   #20
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Op here. Went ahead and just sprung for the senders from VEI.




not that anyone cares
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