What RTD probe for antique Winsco temperature meter? - Page 1
Forum section: electronics / testgear

author: zal42
author_rank: Contributor
timestamp: March 28, 2016, 02:23:09 am
content: I picked up a Winsco Model 4101 SRM Temperature Indicator from an electronics recycler for almost nothing. This is from the early 1950s and is a thing of beauty. Take a peek:http://www.plccenter.com/en-US/Buy/WINSCO/41010600FIt's in great condition, and after replacing a knob and replacing the (mercury!) battery with something that is legal (I went to the trouble of building a LiPo power source that closely replicates the discharge curve of the original battery. It only seemed respectful), it appears to be in perfect working order. However, it's also pretty clear that when it says "RTD" on the probe jacks, it does not mean a PT100 platinum RTD.Does anyone have any idea what type of sensor this thing might take?

author: SeanB
author_rank: Super Contributor
timestamp: March 28, 2016, 09:00:52 am
content: Why would it not be a 2 wire pt100 or pt1000 probe. Only probes that will safely do 600F. Try using a 100R resistor across the terminals and see if you get a reading.

author: zal42
author_rank: Contributor
timestamp: March 28, 2016, 12:39:23 pm
content: Quote from: SeanB on March 28, 2016, 09:00:52 amI did, and I do! But the reading is seriously off. Originally, I assumed the meter was broken, so I cracked it open for a repair. Fortunately, the circuitry is VERY simple -- it's just an unbalanced Wheatstone bridge, and contains no active parts. Just the battery, meter, and resistors. The resistors were fun to figure out -- I didn't think they were resistors at first because they looked weird and were covered with unusual markings. After a bit of searching, I finally found datasheets on them. They're special-made US military 1/2 watt, 1% resistors. Their special quality is that they drift less relative to temperature than others around in 1952.So I tested every component individually, tested the characteristics of the circuit as a whole, and everything appears to be correct. So I'm confident that the meter is working as intended.The problem is one of scaling -- the circuit is expecting a resistive device with VERY different characteristics than a PT100. In the first place, it wants a device that acts more like a thermocouple than a PT100, in that it needs the resistive change in the probe to be a lot larger than a PT100 gives. Also, the resistance that it considers 0 degrees is a lot different.A dead short across the terminals makes the meter read full scale minimum, and full scale maximum happens somewhere in the neighborhood of 3k.One other thing of interest, although I don' t know how important this is, is that the printed scale on the meter is almost, but not quite, linear. I don' t know if you can tell from the photo I linked to -- I didn't even notice it for a few days, but the scale compresses slightly as it approaches maximum value.

author: SeanB
author_rank: Super Contributor
timestamp: March 28, 2016, 03:48:32 pm
content: PTC glass bead thermistor then, and probably not a common one at all, around 300R at 25C. Likely it can be reconnected to use a 2 wire Pt unit, and still be reasonably accurate.https://www.thermistor.com/sites/default/files/QTG12.pdfQuick search gives this.

author: zal42
author_rank: Contributor
timestamp: March 28, 2016, 04:42:36 pm
content: Quote from: SeanB on March 28, 2016, 03:48:32 pmYou sir, are both a scholar and a gentleman. Thank you!I actually thought about modifying it to take a pt unit -- it does seem relatively easy to do -- but I decided that I'd prefer to keep it unaltered to the greatest degree I can. It really is a well-preserved antique, and as such it's essentially a curiosity item.This thing is older than me, by fewer years than I care to contemplate. A part of me figures that if it remains in good working order, then I can too.