34411a: Permissible fan exhaust temperature - Page 1
Forum section: electronics / repair

author: hp3310a
author_rank: Regular Contributor
timestamp: July 11, 2025, 07:57:11 pm
content: I exchanged the fan of my new 34411a to a 30mm Sunon HA30101V3-1000U-A99 with a 100Ω series resistance. The fan runs at about 10.5V instead of 12V.I have the meter running on my desk for 3 hours doing DC voltage measuring. The temperature right at the fan output (I put the sensor thermocouple on the "hottest" of the 3 segments) is about 35°C (sometimes going up to 38°C for a couple of seconds) at non-constant ambient 24-25°C. Is that OK or too little cooling?

author: hp3310a
author_rank: Regular Contributor
timestamp: July 13, 2025, 04:40:35 pm
content: Since nobody answered I though, it might be a stupid question, so RTFMUnder General Specifications it says:Code:Operating Environment: Full accuracy for 0 °C to 55 °CFull accuracy to 80% R.H. at 40 °C Non–condensingSo if you operate at 40°C you will have an exhaust temperature >40°C, maybe 40.1°C maybe 50°C, certainly more than 38°C. Which means my fan must be sufficient.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 14, 2025, 12:08:22 am
content: Quote from: hp3310a on July 13, 2025, 04:40:35 pmNo. your whole deduction is nonsense. There is very little correlation between air temperature and the temperature of the hottest spot in your equipment. These days it's quite simple to measure the temperature of electronic parts with a IR camera. If you can't (or won't) afford one, then the amount of air that is moved though your equipment (before and after modification) is a decent indicator. Also, keep in mind that when electronics is "too hot" it usually does not break quickly, but it's lifespan and reliability is reduced. So you may see dead semiconductors and dried out capacitors in a few years time.

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 14, 2025, 12:19:53 am
content: So I had a look at what a 34411a is. Looks like this thing:I also find it quite inexcusable that a piece of equipment of that caliber has a small whining fan.But it does have a lot of empty space inside. You can put a much bigger (for example 80mm) fan inside and run it at lower rpm. You can make a guide from cardboard to force ventilation though the thing in a decent direction. With a bigger fan at lower RPM, you can get more airflow with less fan noise.But there is also a downside. Airflow in precision measurement equipment can (and does) make it less accurate due to thermocouple effects (each resistor basically has two thermocouples in "anti" series). When you have a 6.5 digit DMM such differences become quite significant.It would be a shame to wreck it by incompetence.

author: hp3310a
author_rank: Regular Contributor
timestamp: July 14, 2025, 04:35:59 pm
content: Quote from: Doctorandus_P on July 14, 2025, 12:19:53 amShoot first, ask questions later...Quote from: Doctorandus_P on July 14, 2025, 12:19:53 amIncompetence is a terrible thing... and apparently completely out of controlhttps://www.eevblog.com/forum/projects/agilent-34410a-vs-34411a-fan-noise/msg5884196/#msg5884196

author: floobydust
author_rank: Super Contributor
timestamp: July 14, 2025, 05:53:59 pm
content: Using a high speed/high airflow bladed fan and throttling it way down- runs it very noisy. Wails like aBansheeI think a lower speed/airflow fan throttled down can actually move more air. But you'd have to look at the vent area etc. to see why this performs poorly.I find it funny the voltage reference is directly inline with fan airflow on this series.We should heat the ref and cool it at the same time lol. Hint: do it like the HP3458, ref located in the far corner out of direct airflow.I imagine most of the heat is generated by the CPU, front panel processor, power transformer.There's no heatsinks on the big IC's, you'd have to find out where the temp sensor is. It might be built-in to an IC that makes heat.sorry, originally posted in wrong threadhttps://www.eevblog.com/forum/projects/agilent-34410a-vs-34411a-fan-noise/msg5978101/#msg5978101

author: hp3310a
author_rank: Regular Contributor
timestamp: July 14, 2025, 07:04:46 pm
content: Quote from: floobydust on July 14, 2025, 05:53:59 pmThe reference is protected from airflow by the shield, it isn't exposed to draft.

author: inse
author_rank: Super Contributor
timestamp: July 15, 2025, 04:27:26 am
content: Actually we don’t know how much engineering went into the airflow design.The rear panel seems so crammed that there was only room for a tiny whiny fan.A larger one could only be mounted outside with a kind of funnel or on the inside changing the airflow to sideways.But again, see initial statement.Nevertheless even the Master himself suggested to disconnect the fan, so _I_ would assume your series resistor solution should be fine…Think also that the meter was designed to be mounted into a rack, surrounded by other gear while it may be standing free on your bench.