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OSTC2 Temperature reading
Posted by: robmason ()
Date: June 10, 2019 06:04PM

Greetings,

I've just completed a full weeks diving in West Scotland. Comparing my OSTC logs to others on the same trip shows a significant temperature difference (2 degrees) between my OSTC2 and other dive computers (Shearwater Petrel and Suunto's all showing 9 degrees - mine showing 11 degrees). Is the temperature sensor on the OSTC2 reliable???

Thanks

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Re: OSTC2 Temperature reading
Posted by: Ralph ()
Date: June 10, 2019 09:14PM

Did your OSTC had about the same depth readings compared with the other computers?

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Re: OSTC2 Temperature reading
Posted by: robmason ()
Date: June 11, 2019 05:07PM

Hi Ralph - I've just downloaded a selection of my logs to SubSurface from my backup computer (Suunto EON) and compared to my downloaded OSTC2 logs. Max depth and min temp shown:

31/05/19
--------
Suunto = 28mins - 33.0m - 8.4 degrees
OSTC2 = 29mins - 32.7m - 10.4 degrees

29/05/19
--------
Suunto = 55mins - 21.4m - 8.8 degrees
OSTC2 = 55mins - 21.9m - 11.2 degrees

The diferences are fairly consistent. And honestly, the dives felt like 9 degrees! smiling smiley

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Re: OSTC2 Temperature reading
Posted by: heinrichsweikamp ()
Date: June 11, 2019 05:55PM

Hi,

The sensor in the OSTC2 is buried quite deeply into the housing and the plastic material is good insulator. It takes a while until the entire unit is on the same temperature then the water. The sensor itself is the same the other brands are using, you can expect +/- 1°C accuracy.

regards,
Matthias

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Re: OSTC2 Temperature reading
Posted by: Ralph ()
Date: June 11, 2019 06:23PM

To lift a secret, the primary job of the temperature sensor is not to measure the temperature of the water but the temperature of the depth sensor. That's because the raw reading of the depth sensor needs to be temperature-compensated. Actually, depth and temperature sensor are part of the same device, and each one is manufacturer calibrated and comes with a set of 6 individual compensation factors programmed into it. That's why i asked for the depths - if they match, the temperature sensor must match, too. The rest is as Matthias wrote, showing the water temp. is a by-product, and it's slower reaction is due the massive (=much material around the sensor) design of the OSTCs. Furthermore, the backlight and the CPU produce a little bit of heat that also keeps the inside of the OSTC a little bit above ambient temperature.

BR
Ralph



Edited 1 time(s). Last edit at 06/11/2019 06:25PM by Ralph.

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Re: OSTC2 Temperature reading
Posted by: VK ()
Date: June 12, 2019 08:28PM

I have experienced the same thing - OSTC giving about 2 degrees warmer readings than my back-up Suunto. Also, when ice diving in the winter I was pretty sure that the water temperature was quite close to 0 and the OSTC showed +2 degrees. The Suunto showed a temperature of ~0.

However, may I suggest a solution: why not have a temperature offset option in the menu so that the user could adjust the reading to reflect the true temperature?

An additional note: my old OSTC 2N agreed almost completely with the Suunto at all times, but the new OSTC 2 Tr show these +2 readings.



Edited 1 time(s). Last edit at 06/12/2019 08:29PM by VK.

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Re: OSTC2 Temperature reading
Posted by: Ralph ()
Date: June 12, 2019 09:06PM

Hi, there's already an adjustment option implemented, giving possibility to trim the temperature reading by -2.0 ... + 2.0 °C. It's just not made available through the "onboard" menu but requires to be set by external config software. In the same way, it's possible to slightly trim the pressure aka depth reading.

Ralph

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Re: OSTC2 Temperature reading
Posted by: VK ()
Date: June 12, 2019 09:38PM

Ralph, could you point me to the correct software? At least OSTConf doesn't seem to have that option. I'd like to adjust the reading because the difference annoys me greatly.

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Re: OSTC2 Temperature reading
Posted by: robmason ()
Date: June 14, 2019 10:29AM

Thanks All.

Ralph - I suspected the body of the computer would have something to do with this. It is a little disappointing that the temperature reading cannot be relied upon

I too will look into OSTConf, but could this possibly be calibrated at factory? I'm sure it would be possible to build up some reasonable figures for the average temperature deviation for the OSTC2 (and other models) across a predetermined time range.

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Re: OSTC2 Temperature reading
Posted by: robmason ()
Date: June 14, 2019 01:22PM

Thanks for the help.

I've just modified the "Temperature sensor offset: 049" setting to -2.0 using OSTConf v1.2.0.0

I'm going to keep a track of the deviation between my Suunto and OSTC2 to plot a more accurate average deviation. For now, -2 degrees is close enough.

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Re: OSTC2 Temperature reading
Posted by: Ralph ()
Date: June 14, 2019 05:06PM

Well, let's not over emphasis this. As well, it's not unreliable, but sure it has its accuracy limits - whereby HW never claimed any specific accuracy traits for the temperature measurement. As written before, the primary job of the temperature measurement is to provide for depth sensor compensation.

I don't want to disappoint you, but we won't put any additional sophisticated calculations in here. Too much would depend on the individual OSTC model: the Plus with it's aluminium body equalises a bit faster, the new Sport - also Aluminium - is air filled and as such slower (higher insulation), the old 2 and the cR have more mass and therefore are even more slower, then there are different PCB variants in all models with different sensor placement positions, and probably in the end there comes a diver wearing trilaminate dry suit with fully heated underwear...

If appropriate, we could widen the +/- offset range a bit.

Ralph

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Re: OSTC2 Temperature reading
Posted by: swissdiving ()
Date: June 14, 2019 05:51PM

Accuracy and precision from:
[labwrite.ncsu.edu]

Accuracy refers to the closeness of a measured value to a standard or known value. For example, if in lab you obtain a weight measurement of 3.2 kg for a given substance, but the actual or known weight is 10 kg, then your measurement is not accurate. In this case, your measurement is not close to the known value.

Precision refers to the closeness of two or more measurements to each other. Using the example above, if you weigh a given substance five times, and get 3.2 kg each time, then your measurement is very precise. Precision is independent of accuracy. You can be very precise but inaccurate, as described above. You can also be accurate but imprecise.

For example, if on average, your measurements for a given substance are close to the known value, but the measurements are far from each other, then you have accuracy without precision.

A good analogy for understanding accuracy and precision is to imagine a basketball player shooting baskets. If the player shoots with accuracy, his aim will always take the ball close to or into the basket. If the player shoots with precision, his aim will always take the ball to the same location which may or may not be close to the basket. A good player will be both accurate and precise by shooting the ball the same way each time and each time making it in the basket. 

Cheers,

Hansjoerg

--> 2N ¦ 2201 / 3892
--> OSTC4 ¦ 257 / 392 / 424

RTFM



Edited 6 time(s). Last edit at 06/14/2019 06:05PM by swissdiving.

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Re: OSTC2 Temperature reading
Posted by: robmason ()
Date: June 17, 2019 01:43AM

Thanks Ralph - I understand there will be limitations to what the sensor can achieve (especially how you describe its main purpose - that was insightful).

Personally, it's been helpful for me to understand, and I'm happy with the +/-2 offset. After diving this weekend, the -2 seems good enough for my purposes (measured against Suunto).

Cheers,

RobM

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Re: OSTC2 Temperature reading
Posted by: Ralph ()
Date: June 17, 2019 07:55AM

For all (nerds) who do like to know even more, the sensor used throughout all OSTC 2 family members is a MS5541C. Its datasheet can be found on www.te.com, it also gives information on how to calculate depth and temperature from the senor's raw values. Each sensor is individually calibrated by the manufacturer at 2 pressures and 2 temperatures, yielding 6 calibration factors, that are subsequently applied by the OSTC to the raw data on each sensor polling round.



Edited 1 time(s). Last edit at 06/17/2019 08:05AM by Ralph.

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Re: OSTC2 Temperature reading
Posted by: VK ()
Date: June 17, 2019 08:08PM

Hi folks, I use OSTConf v1.2.3 and it does not seem to have the option to adjust temperature offset. Is there any other osx/ios software that would have the option?

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