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HP / Agilent 34401A hidden menu

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iMo:

--- Quote from: CBrandin on October 18, 2023, 08:21:22 pm ---iMo, Does your meter actually change anything when you switch to different THERMISTOR TYPE and TC/TYPE (assuming you have a thermocouple)? I think you can test it without needing different probes - just hook up what you have and see if readings change when you change TYPE selections. I tried that when I tested and nothing changed.

--- End quote ---

I will try later on and will report here..

A resistor 11.95k instead of the thermistor wired 2w:
Setting 6 "thermistor type":
2k2 ...  -9.21degC
5k ...     6.25degC
10k ...   20.65degC

PS: when you go to the thermistor type try to mess with all < > up down arrows to switch around..
I somehow doubt the 5k is the only selection, as I would go with 10k if they asked me to tell a single value..

iMo:
Are the calibration constants for the specific ranges somehow accessible?
My idea is to patch a single range (like +10V for example) with my new constant instead to go over the entire adjustment procedure..

alan.bain:
Well this depends on what you mean. The calib constants for the DC/AC/Ohms ranges are readily accessible (see attached file cal_values which shows all the "secured" cal constants in my 34401 with some notes on what they are - base address in RAM for each constant is in the second column, but the bytes need reversing e.g. 5415 is address 1554, addresses are obviously specific to the 7-5-2 firmware).  But basically as expected for most ranges an offset and multiplier.  The AC notes are cryptic and really require some additional explanation.

But the temperature conversion is pretty unadjustable at least in rev 7-5-2. It performs a raw measurement using the meter as usual and then calls the table intepolation on the final result of the measurement.

You can see the raw measurement with DIAG:POKE 29,0,1 (don't expect much enligtenment, this is probably the same as selecting OHM or VOLT depending on sensor type and making a reading). 

For interpolation data for a 5K sensor (Ytbl-D758.txt, really a CSV, but I cannot upload csv). This is as dumped from firmware image, but as I recall first colum is resitance in ohms * 10 and second column temp deg C * 1000.  This data is hardcoded into the firmware as an interpolation table (notes suggest at 2D6F2 and 2D758 for x and y and interpolator is at 0x28A6F).

CBrandin:
iMo, thanks for the response. This confirms my suspicion that firmware after 7-5-2  more thoroughly supported temperature functions.

I'm positive about the 5k thermistor issue (I tested thoroughly). It seems like an odd choice that they would have picked 5k instead of 10k, unless you consider the fact that HP was selling the E2308A thermistor at the time - which is a 5k thermistor.

In my testing I tried all the thermistor settings and they all resulted in the same (correct) results with a 5k thermistor and failed with 10k thermistors (I tried a few 10k thermistors). It appears that alan.bain discovered the same quirk.

I only have K thermocouples so I couldn't test other types, but I did discover that results for all the TYPE options resulted in the same (correct) readings with a K thermocouple.

It looks like they coded the menu options before they coded the firmware to support them all.

CBrandin:
I looked up thermistor data for 2.2k, 5k, and 10k type B thermistors (which is what I think HP supported). They all have the same beta value. HP got a little lazy with the 7-5-2 firmware as all they would have had to do is scale the ohm readings for the 5k thermistor temperature table to support 2.2k and 10k thermistors.

I know this is some heavy math :), but here goes:

Table lookup OHMS = measured OHMS * 5 / thermistor TYPE KOHMS - a whopping one line of code!

in fact, source code wouldn't even have to add one line of code.  Instead of table_lookup=measured_ohms, they could have changed it to table_lookup=measured_ohms*5/thermistor_type_kohms.

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