thermistor - ratings and information?

BobbyG

I need to get a failing sensor, because the manufacturer is saying the existing control isn't available and I need to "upgrade/revise" the entire control.  All I have is a bad sensor (the little 3/8" diameter X 1" sensor with two leads - similar to a tekmar 071) but I am having trouble finding any information on how they are listed.



I have the ohms resistance charts on tekmar sensors, and various boiler manufacturers sensors that they use, but the one I need to replace puts out 3,480 ohms @ 74.5 deg. F (the reason I know is the unit has both a supply and return sensor and only the supply is bad).



question #1: how are sensors rated?  (I think I remember where it is the resistance at a certain temp. - but I can't find anything in writing to validate my sometimes questionable memory)



question # 2:  can someone give me a direction on brands or how I can find a replacement sensor?



Thanks in advance for any help.

Tim McElwain

Thermisters

are basically a thermostatic resistor. They will change resistance based on temperature. What is the make and model number of your equipment? Do you have the Installation and Operations Manual (usually somewhere near the equipment, unless someone took it)? The I and O manual will usually have a chart giving resistance versus temperature.

BobbyG

almost there!

http://www.lochinvar.com/_linefiles/CF-CH%28P%29-I&S-02.pdf



page 26 and 29 list a 4.8 ohm/F

what does ohm/F mean? Does it mean 4.8 ohm @ o deg. F?



If someone is familiar with the T775 control, did they ever make a change or will these sensors work with any age T775 control?



http://customer.honeywell.com/techlit/pdf/62-0000s/62-0265.pdf



These look like they are rated (1K at 0 deg. C) so I'm not sure what the 4.8 ohm/F means on my unit literature.



Tim McElwain - thanks for your help!

Tim McElwain

The answer to your question

as I read it is the 4.8 ohms is the resistance per degree F. To the best of my knowledge the T775 works with the sensors listed on the spec sheet. Note on page 2 of that sheet it lists in Table 1 other compatible temperature sensors.



Try contacting Lochinvar for a direct answer to your question.

BobbyG

thanks Tim!

the first contact told me they didn't have any information on the sensors.  The recommendation was to upgrade/revise the control.  I will try to contact some others at Lochinvar.



Thanks for your help - I really appreciate it!

Bob

BobbyG

I'm an idiot! lol

I didn't look at the parts breakdown closely enough.



The temperature sensor is listed, but not on the diagram......



http://www.lochinvar.com/_linefiles/CF-CH401-751%20Parts%20List.pdf



I'm sorry to take up your time with my making it harder than it should have been.

steamer7676

thermister rating

bobby t775 is a very common honey well control. there are several different types of thermistor sensors 1k,3k,10k type2 and 3 and many  others. a Honeywell control dealer should be able to help you. i know Lochinvar as do many manufactuers like to use proprietary parts . if you have a Honeywell bas dealer in your area  they might be able to help you find the correct sensor . i don't think Honeywell has made the old tan t 775s for a while now but you should be able to find a sensor for it .