Path: news.eternal-september.org!eternal-september.org!.POSTED!not-for-mail
From: Bill Sloman <bill.sloman@ieee.org>
Newsgroups: sci.electronics.design
Subject: Re: NTC thermistor failures
Date: Thu, 10 Jul 2025 17:36:03 +1000
Organization: A noiseless patient Spider
Lines: 37
Message-ID: <104nqhc$ofs3$1@dont-email.me>
References: <104nm40$nfql$1@dont-email.me>
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8; format=flowed
Content-Transfer-Encoding: 8bit
Injection-Date: Thu, 10 Jul 2025 09:36:13 +0200 (CEST)
Injection-Info: dont-email.me; posting-host="d985da792e68be1e0889dd9999d93b19";
	logging-data="802691"; mail-complaints-to="abuse@eternal-september.org";	posting-account="U2FsdGVkX1/5+73mhJCrXJ71sN0JtpvNJLjt19z44TI="
User-Agent: Mozilla Thunderbird
Cancel-Lock: sha1:HNc3iO3IvPcmrOj4cZ5N9rvDplk=
X-Antivirus-Status: Clean
Content-Language: en-US
In-Reply-To: <104nm40$nfql$1@dont-email.me>
X-Antivirus: Norton (VPS 250710-0, 10/7/2025), Outbound message

On 10/07/2025 4:20 pm, Don Y wrote:
> I'm looking to purchase some thermistor-based probes to
> monitor the performance of the HVAC system (return and
> supply duct temperatures along with liquid and suction
> lines.  Maybe the temperature of the compressor, etc.)
> 
> As part of my research, I look at the problems people
> (consumers) report with the units used in their systems.
> 
> I would have thought NONE of these would fail:  they are
> out of the way so not likely to be physically disturbed;
> often encased in a protective sheath (e.g., probes);
> shouldn't be operating beyond their design constraints
> (unless poorly designed); etc.
> 
> Even the pigtails from such devices should be reasonably
> safe from disturbance!
> 
> Yet, I see folks replacing them and proud that they
> did so without calling in the HVAC contractor!
> 
> Are there other "wear" factors that come into play?
> What sort of service life (in a conservative design)
> should I expect?

About the only failure mode that I'm aware of is drift - thermistors are 
sintered metal oxides, so there are gaps in the material and stuff could 
diffuse in.

The good ones are enclosed in glass beads - Yellow Springs were the 
first to introduce them - and those drift a lot less than their 
predecessors. I had my nose rubbed in this back when I was working at 
Kent Instruments in England in the early 1970's, and I've used them from 
time to time since then.

-- 
Bill Sloman, Sydney