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Data recovery and disk repair questions and discussions related to old-fashioned SATA, SAS, SCSI, IDE, MFM hard drives - any type of storage device that has moving parts
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Helium level sensors - where/what are they?

September 1st, 2017, 17:52

I notice that HGST's helium drives have an additional SMART attribute (22) to monitor the Helium Level. Does anyone know how HGST does this?

Seagate's technology paper claims that HGST has no "digital environmental sensors" inside the HDA other than a thermistor (which is an analogue sensor). So how does Hitachi do it? Seagate, OTOH, has temperature, pressure and humidity sensors inside their HDAs.

I notice that the HDA connector of the WD80EFZX has 28 pins. That's quite a few more than the typical 20 or 22 pins. I wonder what the extra pins are for.

Internal HDA Environmental Sensors

Except for a low accuracy thermistor for temperature measurement, the HGST helium drive has no digital environmental sensors.

Seagate uses high-accuracy digital MEMS sensors for measuring temperature, pressure and relative humidity. These measurements are used to improve the reliability and read/write performance of the head-disk interface as well as to monitor the quality of the sealed-in helium environment.

"Decades of Proven Research Underpin Seagate’s Helium Drive":

WD80EFZX PCB-comp & HDA:


WD80EFZX Label:

Re: Helium level sensors - where/what are they?

September 1st, 2017, 18:19

I would bet on a pressure sensor. Either inside the drive ( why would seagate praise their competitors ? ) or some kind of membrane that permit the measuring of pressure from outside the drive ( then seagate would be right in saying there is not a sensor "inside" the drive, "forgetting" to say the sensor is on the outside.

Re: Helium level sensors - where/what are they?

September 1st, 2017, 19:18

Does the claim that there are "no digital environmental sensors" leave open the possibility that there is an analogue environmental sensor (aren't all such sensors analogue?)? If so, then AISI this contradicts the implication that the thermistor (for temperature measurement) is a digital component (it is analogue, but the signal conditioning electronics is both analogue and digital).

ISTM that Seagate is being deliberately vague. If Seagate knows how Hitachi does it, then why not say so? Seagate is very detailed and explicit when describing the mechanical aspects of HGST's technology but is strangely silent in regards to the He sensor.

Re: Helium level sensors - where/what are they?

September 2nd, 2017, 10:58

They describe the other´s, but are silent about theirs. Not exactly polite, isn´t it ?

Re: Helium level sensors - where/what are they?

September 2nd, 2017, 15:40

It appears that the temperature sensor (thermistor) may perform dual functions.

Method to detect helium leakage from a disk drive (US patent #434987):

A method to detect helium leakage from a disk drive enclosure is disclosed and claimed. A measurement electrical current is passed through a temperature sensor disposed within the disk drive enclosure. A reference electrical resistance corresponds to a reference temperature of the temperature sensor. A heating electrical current is passed through the temperature sensor. A heated electrical resistance of the temperature sensor, corresponding to a heated temperature of the temperature sensor that exceeds the reference temperature by at least 5° C., is determined. A value that corresponds to a quantity of helium within the disk drive enclosure is determined based on the reference electrical resistance and heated electrical resistance.

Basically the thermistor is biased with a current which is small enough to sense the ambient temperature but not large enough to result in significant ohmic self heating. Then the thermistor is biased by a much larger current which is intended to raise its own temperature via self heating. The resistance/temperature of the thermistor is then dependent on the amount of gas (helium) in the HDA. Presumably a higher than expected temperature (ie less convection cooling) means that some helium has leaked out. I would expect that the helium level would be calibrated against the thermistor resistance at manufacture.

Perhaps I'm too cynical, but I expect that Seagate would be using the same method if WD didn't own the patent.

Re: Helium level sensors - where/what are they?

September 4th, 2017, 18:12

Something like a Wheatstone bridge ?

I´m curious, wouldn´t using this kind of sensor in a place where the disk runs warmer than the usual ( like, you know, many places in the real world ) return wrong data ?

Re: Helium level sensors - where/what are they?

September 4th, 2017, 18:45

I don't think that a Wheatstone bridge is an appropriate description.

AIUI, the drive first measures the HDA temperature when the thermistor is at ambient temperature. Then it heats up the thermistor with a predetermined current and measures its temperature rise. The higher the increase in temperature, the lower the thermal conductivity of the gas. In other words, the drive is using the difference in temperature rather than the absolute temperature to gauge the current thermal conductivity of the gas. This figure is then compared against the thermal conductivity that was determined at the time of manufacture. If the value has fallen, then this means that some helium has leaked out.

The following table lists the "thermal conductivities of common gases at 0°C, 1atm":


Gas             Thermal conductivity (mW/m.k)

Hydrogen        168.2
Helium          142.2  *
Neon             46.5
Methane          30.2
Air              24.1  *
Nitrogen         24.0
Ethane           18.0
Ethylene         16.4
Argon            16.3
Carbon dioxide   14.5
Krypton           8.7
Xenon             5.2
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