Mettler TM15 RDT PT100 mecanical  temperature display 1970 test teardown - Page 1
Forum section: electronics / testgear

author: oz2cpu
author_rank: Frequent Contributor
timestamp: November 02, 2023, 11:20:49 am
content: this one i just have to share :-)my latestMettler TM15 RDT PT100 temperature display 1970 test teardownThis is one of them WOW how cool type of display readout tech :-)mecanical counter, gears, stepper motor, feedback potentiometers,bridge, PT100 sensor, AC drive and compare with ref potentiometer,to figure out if motor needs to go forward or reverse, amazingly nice construction, and it really works.video linkhttps://youtu.be/DJZji0WJ-Ho

author: MK14
author_rank: Super Contributor
timestamp: November 02, 2023, 11:36:42 am
content: Nice video, and very interesting piece of equipment.I'm wondering, what the display shows if the temperature goes negative?Also, as it seems to show ' [ o ] C', can it be switched to a Fahrenheit scale 'F'?Edit: Putting some table salt, onto the ice (ideally with little or no, extra water, as it dilutes the salt action), should create temperatures below zero Centigrade.

author: TomKatt
author_rank: Frequent Contributor
timestamp: November 02, 2023, 12:19:07 pm
content: Great video! Subscribed!They had several numeric display technologies in 1970 (planar, nixie, numitron etc)... I wonder why they went with a mechanical display ?

author: MK14
author_rank: Super Contributor
timestamp: November 02, 2023, 12:30:39 pm
content: Quote from: TomKatt on November 02, 2023, 12:19:07 pmI think it is because its circuit action, is basically analogue (comparison, between the measured temperature, and the potentiometer, which moves along with the mechanical counters, if I have following things correctly), so it doesn't need the digital circuitry for the other display types, which would have been very expensive, at that time (or even non-existent, depending on when this design was first used).I.e. If this technology was old enough (when it first came out), it could pre-date, integrated circuit counters and stuff.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: November 02, 2023, 12:53:49 pm
content: >I wonder why they went with a mechanical display ?I think it is due to the compare potentiometer is insanely accurate and linear,converting potentiometer position to a type of readout that is non mechanicwould add all sorts of not so accurate detailsI also like the fact that this unit display the last known value, when power is missing

author: TimFox
author_rank: Super Contributor
timestamp: November 02, 2023, 01:52:23 pm
content: The technique is similar to analog chart recorders, where the pen position is encoded by a potentiometer, whose voltage output is compared to the analog input.I remember old 40-turn "servo" helical potentiometers that had linearity on the order of 0.01%.

author: MK14
author_rank: Super Contributor
timestamp: November 02, 2023, 02:04:21 pm
content: Quote from: TimFox on November 02, 2023, 01:52:23 pmWow!I'm surprised the potentiometers, don't keep wearing out, in such applications.I suppose such instruments were used a lot, before digital logic became common place.

author: oz2cpu
author_rank: Frequent Contributor
timestamp: November 02, 2023, 05:04:43 pm
content: exactly, those potentiometers is mostlikely what define the cost of this unit !BOURNS the largest blue model

author: MK14
author_rank: Super Contributor
timestamp: November 02, 2023, 05:39:56 pm
content: The following free PDF handbook, seems to nicely fill in the details. For anyone who wants to know more about precision (and other) potentiometers, like I did.https://www.bourns.com/docs/technical-documents/technical-library/corporate/OnlinePotentiometerHandbook.pdfThey seem to have helped fill in the gap, for creating powerful (e.g. control and measurement) functionality, until cheaper electronics (especially integrated circuits), could fill in the void, in later years.Edit: Also for trimmers:https://www.bourns.com/docs/technical-documents/technical-library/trimmers/technical-articles/trmrpmr.pdf

author: Vgkid
author_rank: Super Contributor
timestamp: November 02, 2023, 09:05:33 pm
content: Thanks for the teardown, I have never seen a thermometer with a mechanical digital displsy.