HP3458A Temperature

[Barry Ward]

New post to avoid getting in trouble over on NU180" - A U180 drop-in replacement for the 3458A

A few questions:

Zondar said:
Where in the machine, or how, is the CAL175 temperature measured? I have a temperature probe on my previous board and find a substantial difference between that value and the temperature at the board (not unexpected). The temperature delta at the board is almost 2x the delta of CAL175 (which is distinctly higher in temperature, too). TCR's on the board would respond to the temperature at the board, obviously, rather than that of CAL175, so I wonder about pertinence.

What is the correct formula to compute temperature drift in ppm/C from CAL72? Is it just (cal72_1-cal72_2)/(temp1-temp2)*1,000,000? Adjust by dividing by cal72_1 as well? Is there another factor of 10 in there due to the 10V scale?

Also, what are people's experiences with the rising clock edge to date? I think most people are using the falling edge right now. Is it clear yet that we should stick with that?



I've wondered the same thing and also why have I not seen someone controlling the temperature instead of letting the tail wag the dog?

My former "parts unit" now workingish, that has an illegal Noctua NF-A6x25 FLX and runs 5 °c cooler than the hotter of the 2 other meters with Papst fans.
(Illegal=Noctua 12V fan running on the 15V unmodified 15V reg out)

Seems like a smart fella could generate a tempco table*, stick it in a micro, and modulate the fan speed (and maybe a gentile heater in the airflow).  Seems like Bob's your uncle.

EBM Papst 162L                  spec 12CFM @12V   26 dBA
Noctua NF-A6x25 FLX          spec 17.19 CFM @12V  (quieter too)  19.3 dBA


* (if you are fancy, use some coefficients, or other math magic, or God forbid: Analog.)

[aronake]

I tested one of these PWM temperature controllers:

https://item.taobao.com/item.htm?id=688785421807

and a 4 wire fan in one of my 3458a.

Some tweaking of the parameters and managed to get total TC of the 3458a reduced with 60% in range 20 to 27 degree C. So yes, it works, and makes the 3458a better from most measurement perspectives. It also reduces impact of filter getting dusty and reducing airflow.

[Kleinstein]

Regulating the fan is a 2 sided thing. One can keep the temperature at one spot in the meter more constant.
However the different parts of the meter are effected to a different degree by a change in the air flow.
This means that temperature gradients change with the airflow, while a constant air flow would keep the gradients about fixed and the whole meter changes with room temperature.
In an ideal world the meter is used in a room with relatively stable temperature, so that also the DUT sees a stable temperature.

[Barry Ward]

K,

It's about philosophy at this point, right?   I mean I think we should just try to make our meters as flat as possible at the intended use ambient.   

The DUTs have their own tempcos and that might be what we want to observe.
If either really need control, throw them in a chamber. (or 2)

We are at the edge of physics and it's super fun.   It's my pleasure to be among you guys here at the pantheon of precision.

B

[Kleinstein]

Changing the fan speed a little is likely not so bad, but much slower could cause some parts to overheat, eventhough the temperature sensor may still be OK. The hot parts are those where the fan is most important. The comprators and the fast OP-amp in the AC amplifier are known to run rather hot and are hard to replace.

[Dr. Frank]

New post to avoid getting in trouble over on NU180" - A U180 drop-in replacement for the 3458A

A few questions:

Zondar said:
Where in the machine, or how, is the CAL175 temperature measured? I have a temperature probe on my previous board and find a substantial difference between that value and the temperature at the board (not unexpected). The temperature delta at the board is almost 2x the delta of CAL175 (which is distinctly higher in temperature, too). TCR's on the board would respond to the temperature at the board, obviously, rather than that of CAL175, so I wonder about pertinence.

What is the correct formula to compute temperature drift in ppm/C from CAL72? Is it just (cal72_1-cal72_2)/(temp1-temp2)*1,000,000? Adjust by dividing by cal72_1 as well? Is there another factor of 10 in there due to the 10V scale?

Also, what are people's experiences with the rising clock edge to date? I think most people are using the falling edge right now. Is it clear yet that we should stick with that?



I've wondered the same thing and also why have I not seen someone controlling the temperature instead of letting the tail wag the dog?

My former "parts unit" now workingish, that has an illegal Noctua NF-A6x25 FLX and runs 5 °c cooler than the hotter of the 2 other meters with Papst fans.
(Illegal=Noctua 12V fan running on the 15V unmodified 15V reg out)

Seems like a smart fella could generate a tempco table*, stick it in a micro, and modulate the fan speed (and maybe a gentile heater in the airflow).  Seems like Bob's your uncle.

EBM Papst 162L                  spec 12CFM @12V   26 dBA
Noctua NF-A6x25 FLX          spec 17.19 CFM @12V  (quieter too)  19.3 dBA


* (if you are fancy, use some coefficients, or other math magic, or God forbid: Analog.)

TEMP? is measured via U21, on pdf page 50 in the CLIP, A1 board, top, left. That's an LM35DZ, please look for yourself, where it's localized on the PCB.
Don't fiddle around with the fan, as the 3458A is very delicate with varying temperature, if you want to go into sub ppm range, where it's intended for. Papst is fine.
You need to keep the room temperature absolutely constant, TEMP? will strictly follow with 12 ..20°C temperature rise, depending on how you set up your 3458A, i.e. on table top (best), or in a rack (worst), Best, because you have free air flow from the intake and out of the slots.
I run my lab mostly between 21 .. 23°C over the seasons, but constant to < 0.2°C over 24h, during a stability measurement. That feature of the basement room is much more important than any thermal experiments with the 3458A itself.
Keep the filter of the fan clean, and TEMP? as low as possible. Mine has always 33 ..35°C, which also gives longer life to these ELANTEC comparators.

Frank

PS: The first HP3458A I've used from end of 1989 onwards, had about a SER No 2823A03900, but unfortunately I did not write it down at that time.

[CalibrationGuy]

I agree with Kleinstein. It's all about the airflow and distribution. You need a minimum airflow to assure that you don't have hot/cold pockets. The solution is always to have a stable environment. Once you go sub ppm, even the air around the terminals matters.

TomG.

[Zondar]

Can someone help with this question, here or back in the original thread? Or else, if this isn't the proper way, then lay out the proper way to perform this sort of measurement and calculation? I have two boards with two different sets of resistors, and I'd like to see how important the TCR of the board's resistors are. But at the moment, I'm not getting sensible results.

What is the correct formula to compute temperature drift in ppm/C from CAL72? Is it just (cal72_1-cal72_2)/(temp1-temp2)*1,000,000? Adjust by dividing by cal72_1 as well? Is there another factor of 10 in there due to the 10V scale?

Also, any comments on which temperature reading is pertinent? Clearly the board's temperature is what dictates the board's TCR response, yet traditionally it's the CAL175 temp that people use. Note that I do have the machine with it's outer covers off, so it's not unexpected that the temperatures will vary.

[Kleinstein]

For the relative ppm one would need to divider by the CAL72 values of cause. So  (cal72_1-cal72_2)/CAL72_1/(temp1-temp2).
Ideally one would measure the temperature closer to the U180 board. However normally the main variations in the temperature are from warm-up and the room temperature. The room temperature would effect all temperatures about that same, just with a different offset depending on the position in the meter.
Not having the cover on would effect the relative temperatures - some get cooler and some may get warmer because of locally less air flow.

[Zondar]

OK, thanks, that's what I thought.

My results do not correspond to what others have reported, and the results from the two boards do not make sense relative to each other either. I'm puzzled.

[EE4all]

I wouldn't slow the intake fan speed, but I would consider trying to regulate the intake air temperature to a stable one.

I am currently contemplating using a chest style deep freezer that has lost its compressor as a lab chamber with a thermal control loop versus just discarding it. With some control, and added thermal mass inside, I could see keeping my standard and 3458a's much more stable. I would need to turn it on its side, so it would be like a well insulated cabinet.

[mendip_discovery]

I wouldn't slow the intake fan speed, but I would consider trying to regulate the intake air temperature to a stable one.

I am currently contemplating using a chest style deep freezer that has lost its compressor as a lab chamber with a thermal control loop versus just discarding it. With some control, and added thermal mass inside, I could see keeping my standard and 3458a's much more stable. I would need to turn it on its side, so it would be like a well insulated cabinet.

You could copy what TiN did,
https://xdevs.com/article/megatec/

[EE4all]

Wow, yes. Exactly! Of course he has done that already.   

With the old freezer, I can skip the enclosure making with insulating foam part.