Voltage on Oven food probe?

Anova Precision® Oven
1

Update: Anova reached out to me and the oven is on its way back to their engineers.

Hi. I’m new here, but not to the APO - I’m on my 3rd one.

The first oven worked fine for about two weeks, then whatever one of the supports for the top element was supposed to be screwed into came loose inside the oven wall, and the top element sagged down to where it could have touched the food on the top rack.

The second oven seemed better-built, but a couple of weeks after receiving it, I was plugging in the food probe while touching one of the racks, and got a small shock. I subsequently measured the potential between the probe tip and oven wall at 32V; for the avoidance of doubt, I confirmed ground wasn’t floating by also measuring to the ground at my electrical panel, and also obtained the same measurement with a second food probe - the one from the first oven.

Anova replaced this oven too, though they messed up the shipping somehow and it took a few weeks to get here. I did ask whether there was some reason they would expect 30+ volts at the food probe, but they didn’t answer, just replaced the oven.

I just unboxed the 3rd oven, and just in the name of crossing i’s and dotting t’s, before running it at all I measured the voltage on the probe tip. It is 33V above ground / oven wall.

At this point I don’t know what to think, nor what to do. Neither I nor several coworkers fairly experienced in electronics design can see why the food probe tip should be tens of volts above ground, and it’s also concerning that it can supply enough current to give a perceptible, even slightly painful, shock. This is not going to kill a horse (or a housecat) but I can certainly see it leading someone to drop something very hot, and that’s not good. And to me, any stray voltage in a cooking appliance seems like a cause for concern.

If the food probe is not supposed to be energized, then I’ve just received two successive ovens directly from Anova that have the same safety defect. If the food probe is supposed to be energized, I’m just not sure what to think.

And there’s the matter of this coming on the heels of replacing the first oven because of a simpler, easier to understand defect that could still have started a fire.

I’ve reached out to Anova again, by phone and email, but I don’t really expect any response other than another offer to supply yet another replacement oven. What should I do?

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2

Follow-up: the literal, “tip-of-the-spear” tip of the probe is actually tied to “ring”, not “tip”, of the 1/8" phono plug and receptacle. The receptacle is metal, of course, so as it turns out it too is energized at 33 or so volts above the adjacent oven wall. If it weren’t for that rubber gasket, seems likely they’d short – especially when steam is used.

This can’t possibly be right, can it?

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3

The probe is a thermistor, the resistance of my probe is roughly 50 kOhm at room temperature and almost 6 kOhm at teawater temp i.e. roughly 90 degrees C. It is (probably) part of simple voltage divider circuit so there must be an applied voltage in order to measure the temperature. My probe is at 12 V right now. The fact that you got an electric shock is strange, I definitely don’t get on Is this happening every time you touch it?

4

I do understand how a thermistor works. I don’t however understand why there should be voltage between the exterior of the probe and ground (the racks, oven walls, etc. are at ground potential, as they should be).

If intentional, this seems like an exceptionally strange design choice. After all, the food being measured may be directly touching the oven wall or racks (particularly the racks) and any current leakage through that path will distort the measurement. Doesn’t make any sense to me.

The 12V you measured seems like a more likely design voltage for this circuit but I still can’t see what it would be doing on the probe tip (and, I assume, on the metal part of the probe receptacle that protrudes from the oven wall - I also see 32-34V there). But 30V+ seems quite high.

I work with a pretty good number of thermistors in industrial control applications, including “drop in” thermistors in metal housings (which we put in measurement “wells” in piping systems). If you cut one of these open, the thermistor itself is typically electrically isolated from, but thermally bridged to, the metal housing by a layer of thermally conductive epoxy. I’ve never found voltage on the exterior of one of these thermistor sensors.

Depending on the application these typically run at (or around) 12V or 24V. If the circuit was designed by oid-school analog control people then 15V or 16.5V or 25V. 32-34V just seems weird. It’s high enough to cause unanticipated current leakage though still low-voltage so not directly hazardous.

The 2nd replacement oven still measures 33V, as I noted, but no, it doesn’t give me a shock if I have dry hands. I was using steam in the 1st oven where I saw this problem, so resistances were probably lower and maybe that accounts for the perceptible zap I got.

I wonder if someone mis-designed the probe circuit in the oven so that the whole thing is unintentionally floating above ground, and some subsequent design change took that float from 12V, which I bet nobody would really notice, to 34V where it can give you a little zipzap like it gave me? Pure speculation, but I can see it happening.

5

I didn’t mean to question your knowledge, sorry if what I wrote made it look that way, my post was written before my caffeine kicked in. I do agree with you, normally the sensor part is isolated from the probe shield. I have no connection between the probe tip and any of the leads on the plug. I have 88 V, not 33 V, between ground and the ring like in your second picture. I also have around 14 MOhm between them. I believe it is time for Anova to step in and explain what is going on.

6

I haven’t dug out my voltmeter to ascertain my oven’s probe voltage but as Ragnhild has asked - Anova needs to respond

p

7

Should we expect the temperature sensor probe tip to be at potential?

8

Methinks they weren’t aware of this and are scrambling to find a response - medical/safety issues?

p

9

This is an isolated incident that we are in the process of investigating, and the customer is being taken care of via our support channels.

10

So the probe really should be at ground potential and I should contact support?

12

Yes, if you are experiencing this or any issue please contact support@anovaculinary.com. Appreciate it!