Anybody know, how to calibrate depth on Suunto ?

[Scuba Scott]

Depth? Or pressure? The computer measures pressure...

This.

How did the op determine the depth is wrong? Your body and computer only care about pressure and pressure and depth are only loosely related. Your computer will guess at the depth based on the pressure and display it for you as something you can relate to more easily. Pressure vs depth varies not only based on salt or fresh water but also the amount of salinity and even water temperature. A pressure sensor can only be tested for accuracy in a calibrated pressure vessel of some sort. You can get a idea in water.

You are probably close enough. Even if you could calibrate it I doubt you would have an accurate reference and could actually make it less accurate.

 

[AlainD]

Hello,
I changed defective pressure sensor on my Suunto Cobra, it work fine now again, but in 20m Suunto shows only 18,8m .
Thanks
MilK

Milk, how did you replace it ?
I read somewhere this is the part
https://mouser.com/ProductDetail/TE-Connectivity/325212001-00/?qs=oFx6pF86PmDlKnOzeHFNMA==
but haven't seen info how to open and re-soder a new one, or if correct part. any info would be great. thanks.

 

[DIVEWATCHDOCTOR]

I know this is an old thread but, since I used to be an electronic engineer in a previous life, I feel compelled to give my two cents:

1. I suspect Milk got a hold of a Cobra pressure sensor, went ahead and replaced the one in his computer to find out, afterwards, that the new pressure sensor and the electronic circuit in its computer had not been paired. That would explain the "depth" difference between what the Cobra reads and the "actual depth" (BurnanMuntasses had a point two years ago about how "actual depth" was measured).

2. The pressure sensor suggested by AlainD a few days ago is not meant to be used in a dive computer because, among other issues, the Wheatstone bridge inside the depth sensor requires 20V to operate and the Cobra (as most DC) uses a 3V, lithium battery. However, I am pretty sure that Suunto's (as well as other DC manufacturers) pressure sensors are similar versions of the Mouser but slightly more sensitive for them to work with the internal 3V power supply of the vast majority of DC.

3. I do not exactly know how each DC manufacturer does the final pairing (calibration if you will) between the pressure sensor and the electronic circuit but two years ago Dmaziuk provided a very plausible hypothesis in his post on this same thread. The end result, in my opinion, is that Milk is (or was) fighting a battle he will not be able to win.

4. To provide further evidence to Dmaziuk's theory we have played Dr Frankestein and interchanged several times the electronic circuit on several Citizen Aqualand dive watches (same model of course) trying to make a good watch out of two defective ones and we have succeeded in having a perfectly running Citizen Aqualand, except that invariably there was a depth offset (up to 10-15 feet) between the depth indicated by the "reborn" Aqualand and what our very accurate pressure tester gage read.

 

[Rred]

Since the OP changed the sensor, they've already been inside the pressure seal of the instrument. Whenever something in the way of an electronic "meter" has to be calibrated, there are generally two ways to do it. Manually, you put a screwdriver or hex wrench into a trim pot and turn it to zero out the variation from the new sensor parts. If you're doing calibration for high precision on an assembly line, it can also be done by using a laser to ablate surface mounted resistors, and literally physically trim the resistors that balance part of the circuit. You just keep moving the laser until the circuit reads the correct value. Hopefully, Suunto isn't being that precise and they're using a cheaper conventional trim pot, which can be spotted by eye.

Which way you turn it, what value you are looking for, I couldn't tell you. I'd say to scribe the position, rotate perhaps ten degrees, see how your reading has changed. Then do some math to figure out how many feet translate into degrees of rotation, in which direction.

It is also possible that calibration is done in software: The sensor may be placed in circuit, pressure built up to xx bars, and a signal given to mark that. Then the pressure is changed, a second signal given, and the system is now calibrated via a software lookup table.

If you are lucky, there's an obvious trim pot (potentiometer) in there. Might have a blob of wax or paint on the edge of it, to prevent it from rotating out of position accidentally.

 

[DIVEWATCHDOCTOR]

Rred has just provided a more detailed (and updated) description about the two or three existing methods to pair the output of a sensor and its corresponding electronic circuit. But the bottom line is the same: if you replace a sensor in a dive computer (or try to use the guts of one with the sensor of another, as we have done with the Aqualands) be prepared to calibrate or pair it. I have inspected hundreds of dive computers and dive watches electronic circuits and have never detected the manual trim pot that Rred makes reference to. I am afraid that even their smaller versions would be too big for the extremely compact miniaturized electronic circuit of modern DC. I remember seeing trim pots in some oldies such as the Aladin brick and the early Dive Rite Nitek series. So I would believe nowdays calibration is achieved either by laser ablating a surface mounted resistor or a software lookup table.

 

[Schwob]

@mkotva
I cannot help you with your inquiry. If / when you have the time to share a little more, i would like to learn:
Were you able to pressurize to different equivalent depth as well? What did the DC indicate at which actual depth (pressure expressed in depth)?
What I am curious about is:
Is the difference you determined just linear deviation (connecting the dots you get a straight line through zero but at increasing depth it varies by a fixed percentage?
Or is it a variance by a fixed offset no matter what the depth?
Or of course a combination?

Not knowing about the details in the sensor and it's behavioral properties and most likely variances, I would plan for calibrating offset and slope so to speak.

I have no idea if this would be done with SW or two trim pots or two e-pots (sw controlled pots) but I personally would not stick manual trimpots into a sealed housing. I would be surprised if this was done anywhere else but in the SW (seems cheapest and easiest to maintain and I see no real need for an actual HW calibration here) But those are the views of a non SW and non HW guy...

If done in SW, yes it could be done with a lookup table. In this case I'd think it also could be done by conversion equation, a variable for the offset and one for gain/ slope correction. I have no idea why, but SW people seem to like tables even where just two variables to look up could do the trick. Maybe it processes faster than doing the math.

Anyway sorry, no help from here. Just expressing curiosity in case you care to share more. You seem to express that you really do not care to, but maybe that can be chalked up to language.

 

[RonR]

OK, maybe I will try to provide some insight, if not answers.

Plenty of Dive computer manufactures have used TE (formerly Intersema) sensors, going back many years. They have many sensors that look identical and have the same footprint. They can be hard to tell apart, usually there are some part numbers on the ceramic underside, but at least my eyes need a microscope to read them. I doubt the sensor linked from Mouser above is correct.

The voltage is not much of a clue, battery voltages are converted to many values for specific components. But it’s more efficient to stay close to the battery supply voltage. This sensor was used in many computers for quite a few years, but I have no idea if it is the one used by the particular Suunto referenced. https://www.digikey.com/catalog/en/partgroup/ms5541c/11443

Most of these sensors (including the ones we use and that I know other manufacturers use) come calibrated and have a digital output. Calibration and temperature compensation is all internal to the sensor. It’s not hard to see that having a pre-calibrated sensor helps with production.

In most instances calibration is not an option, and you would need to know precisely the correct part.

Ron

 

[rsingler]

---

---

A ScubaBoard Staff Message...

Guys, let's keep in mind that the OP's native language may not be English. He asked a perfectly reasonable question, and I'm sure was not trying to be snarky. We've see some interesting technical replies, and some snarky comments. Let's hope he comes back to tell us how he did what he did, for our mutual education.

Play nice! Try not to assume the worst before you shoot from the hip.

---

 

[SHORT]

The depth sensor in my Vyper air is MS5212-HM
Image

Process of removing it is preheat at 250C for 2-3 minutes and elevate to 350C to remove the sensor.

 

[BoundForElsewhere]

i was going to suggest swimming in a vertical configuration figure-8. kind of like calibrating the compass.