HYDROS 21 no longer vented… any idea when this changed?

Are you using a sensor with bare wires at the end, or with the molded stereo plug?  Is it a red cable or a black cable?  We’ve bought hundreds of CTD-10 and now the Hydros 21 sensors (only the name changed when the company changed from Decagon to Meter, and there was no design change).  We bought a bunch earlier this year, and every one we’ve ever used has a small Gore-Tex sleeve about a foot from the end of the cable that provides the vent used for the pressure sensor atmospheric reference.  Even their new Generation 2 Hydros 21 sensor (due to be released soon) that we’ve been testing recently has the vent.  If your sensor doesn’t have a vent somewhere along the cable, then either it’s an error and it’s missing, or you’re using a bare-wire terminated cable and the end of the cable allows the air into the jacket to provide the reference (the newer cables have a dedicated air tube inside the cable whereas the old cables relied on the airspace between the wires).   Either way, I recommend contacting Meter Group tech support, they’ve always provided great customer support for us over the past 7 or 8 years that we’ve been using their sensors.  Did you try putting the sensor in a bucket of water at a constant depth and then logging depth over a few days to see if you see atmospheric pressure affecting the data?

Do note that the specifications says accuracy  0.5% of 10,000m or +/-50mm (edit  corrected to 50mm). This  implies that you want to keep it at a constant (20C?) while sitting in the bucket, and even then the apparent water level can wander up  +50mm  or down -50mm and still be within specification.    If the temperature changes then the accuracy specification doesn’t apply.

https://www.metergroup.com/environment/products/hydros-21-water-level-monitoring/

Water depth
Range: 0−10,000 mm
Resolution: 1 mm
Accuracy: ±0.5% of full scale at 20 °C

NOTE: Depth measurement accuracy assumes no abrupt temperature variations.

For environmentally significant flows (low water depth measurement, with high temperature variation) it has a lot of noise on the signal.

Great instrument for measuring borehole water pumping ground water that is usually a constant temperature and large variation in depth.

@fisherba – Thanks for sharing this! I’ll be very interested to learn what Meter tells you.

I see what you mean on their product page: “When used with the ZL6 data logger, you won’t require an additional sensor for measuring—and referencing—barometric pressure.”

However, both the sensor manual and the Integrator Guide in the Downloads section describe the vent for atmospheric correction, as expected — definitely an inconsistency.

FWIW, my most recent HYDROS 21 purchase was March 18th, and it’s got the vent. The part description on the Meter invoice says “40683-S CTD-10 (HYDROS 21) Sensor, Stereo end, 10 meters”. It’s got the new black cable, but I don’t know whether that indicates it’s a gen 2; the website only appears to mention gen 1. The only other change I can see visually is that the hook on the mesh Kellem cord grip could be more easily removed because it’s held on with a threaded link, rather than being clamped directly to the grip.

-Matt

Hi Shannon, do you know if they are using temperature compensation for the sensor on the Generation2. ? thanks

Sounds like a manufacturing error to me.  I’d suggest contacting Meter and tell them about it because I’ve never seen an unvented CTD from them and there’s no conceivable way it would work correctly without one.

@fisherba My March order has the stereo plug.