Everything I always wanted to know about CHT gauges, and less |
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Everything I always wanted to know about CHT gauges, and less |
76-914 |
Feb 14 2012, 10:59 AM
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#1
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Repeat Offender & Resident Subaru Antagonist Group: Members Posts: 13,502 Joined: 23-January 09 From: Temecula, CA Member No.: 9,964 Region Association: Southern California |
In the past 9 month's I have bought, installed, removed and/or re-installed 2 VDO CHT gauges and 3 sending units. This is a brief account of what I've learned.
Last spring I bought a "used" VDO CHT gauge (from a (IMG:style_emoticons/default/poke.gif) unnamed member (IMG:style_emoticons/default/biggrin.gif) )at the GnR flesh roast. At $20 this seemed like a good deal. I was unable to locate a VDO sender so I ordered another brand from Dakota. Once installed I discovered that it did not work. (IMG:style_emoticons/default/sad.gif) I had opened the ring terminal to 14mm and believed that my doing so might have ruined the sending unit. So I ordered another another from Dakota but this time with the proper end (14mm). I avoided this one previously because of the 14' lead. It is made long for the bus application. But before I could install this one I got side tracked by some erroneous advice or article (I forget) which convinced me that I needed a matched set. I contacted the VDO supplier and found that they were still available. I had been told they were no longer available originally. Once my VDO arrived it was installed. Much to my chagrin the gauge still did not register anything. I realized that my "used" gauge was trash at this point and ordered new gauge. Once installed and coupled to the VDO sender the new gauge showed signs of life. (IMG:style_emoticons/default/cheer.gif) Or so I thought! I thought that I would take a quick run up the road to Ron's (aka Slits) and enjoy my "now" working CHT gauge. It was a coolish California morning but it appeared the car was running a tad bit warm. After talking with Ron, I believed that I needed to recheck the timing, fuel pressure, valve gap and cooling flaps. The return trip was tagging 400F if I ran over 50 mph. OK, what did I expect? I built this engine w/o any prior experience in type. I contacted Geoff (aka Bleyseng) whom had rebuilt the MPS regarding the mixture. After a few pm's I decided to look at the CHT a little closer. Geoff had asked if I had ever calibrated the gauge. Well, no. Who in the right mind would mfg a CHT gauge that was not accurate? The answer; everyone. More especially VDO! I put a pot of water on the stove and got it to the boiling point. I checked the temp of the water with a mechanical candy thermometer against 2 digital ones on different multimeters. Almost 25F difference between the lot. (IMG:style_emoticons/default/bs.gif) I finally realized I should determine what the boiling point was at my altitude ([212F]-[elev x .0018] which was 209F. Once that was determined I began to take readings from the gauge with all the different configurations that I could assemble from my nest of parts. e.g. VDO sender only, Dakota sender only, VDO w/ factory length leads then with factory leads cut to length (which they say not to do (IMG:style_emoticons/default/bs.gif) ) then the same set of tests w/ Dakota sender and different lengths of Dakota's factory leads. What I found were temps that ranged from 250F with the VDO gauge, sender and factory leads, to 210F with the VDO gauge, Dakota sender and Dakota leads. I had a set of Dakota leads which were cut to length that gave the same reading; 210F. To summarize, I found that; It doesn't matter if you enlarge the ring terminal. It is there to transfer heat to the thermocouple. The thermocouple consists of a cold junction and a hot junction. The ring terminal is the hot junction and the other end is the cold junction. The two wires are of dissimilar metals and creates a difference in voltage that is measured between the hot & cold junctions with a determined set point. Herein lies a problem. The thermocouple and gauge are calibrated using 72F as a set point. Is there a place in the Hell hole that stays at 72F. As such the gauge will proportionately register higher than actual temps when ambient temps are below 72F, and lower than actual temps when ambient temps exceed 72F. Simple math but something you should be aware of. Cutting the leads between the thermocouple and gauge has no effect on the readings, as far as I could determine w/ 50 degree increments. Each thermocouple has it's own particular set of values, though Dakotas only varied 2 degrees between themselves. VDO's sender is a hunk of junk. The best news??? My CHT's are normal. Whew! |
ConeDodger |
Feb 14 2012, 11:05 AM
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#2
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Apex killer! Group: Members Posts: 23,588 Joined: 31-December 04 From: Tahoe Area Member No.: 3,380 Region Association: Northern California |
Lot's of good information here but the one lesson that stands out to me is if you bought a compensated aircraft quality gauge and sender in the first place you might have spent less time and money getting the information you seek...
Happens all the time with the car hobby. (IMG:style_emoticons/default/dry.gif) |
tradisrad |
Feb 14 2012, 11:19 AM
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#3
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Senior Member Group: Members Posts: 985 Joined: 11-September 06 From: San Mateo, CA Member No.: 6,815 Region Association: Northern California |
Ratwell has a good write up on his site.
I also question the reliability of my VDO CHT, but unless it really spikes I am not going to worry to much about its accuracy. I have seen it hit the 400+ mark. I have looked at the Dakota senders and I probably just need to drop the $25 on one. |
SUNAB914 |
Feb 14 2012, 01:14 PM
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#4
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Senior Member Group: Members Posts: 698 Joined: 29-December 08 From: Fredericksburg VA Member No.: 9,880 Region Association: South East States |
Sorry for stupid question but what is "Dakota" or leads? is this someone that makes them? Good info though.
Thanks |
tradisrad |
Feb 14 2012, 01:18 PM
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#5
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Senior Member Group: Members Posts: 985 Joined: 11-September 06 From: San Mateo, CA Member No.: 6,815 Region Association: Northern California |
Digital Dakota is a company that makes gauges. Leads is in refrence to the length of the thermocouple "cable".
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MBowman325 |
Feb 14 2012, 01:20 PM
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#6
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Member Group: Members Posts: 465 Joined: 14-July 05 From: Oklahoma City, OK Member No.: 4,413 Region Association: Southwest Region |
I would bet he's talking about Dakota Digital (makers of things guage-like and related)
Leads are a different way of refering to "wires". (I have heard that a lead is technically different from a "wire", but don't recall details at this point) |
rwilner |
Feb 14 2012, 03:13 PM
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#7
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No Ghosts in the Machine Group: Members Posts: 953 Joined: 30-March 10 From: Boston, MA Member No.: 11,530 Region Association: North East States |
Here's the CHT gauge I have installed in my car. It is temp compensated and they sell the proper 14mm spark plug ring terminal also.
I believe it's pretty popular here on the world site and relatively short money. CHT Gauge |
aharder |
Feb 14 2012, 03:14 PM
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#8
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Advanced Member Group: Members Posts: 4,391 Joined: 6-September 11 From: Dallas Texas Member No.: 13,524 Region Association: Southwest Region |
Well I just bought new VDO CHT, OP, OT and Volt Gauges so I'm glad this got posted before I got it all back together (IMG:style_emoticons/default/biggrin.gif) Looks like I'll be buying the 18' Dakota thermo coupler and the 323 055 oil temp sending unit (IMG:style_emoticons/default/mad.gif)
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76-914 |
Feb 14 2012, 03:53 PM
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#9
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Repeat Offender & Resident Subaru Antagonist Group: Members Posts: 13,502 Joined: 23-January 09 From: Temecula, CA Member No.: 9,964 Region Association: Southern California |
Sorry I rushed this but had to get out early today. I should have been more clear on this, clearly! Leads are the wires that run between the thermocouple and gage. I had an A/C quality CHT in m y plane. I doubt that it was much more accurate than any other J or K thermocouple system other than the head was tapped for the sender. If you could find a CHT based on thermistor technology, I would say you had an accurate temp measuring device. We have used these in commercial boilers and water heaters using a Delta T config with excellent results. Thermistors are a bit slow on temp drop but this is not a problem when dealing with water heaters. I don't think it's as important to order a thermocouple and leads from Dakota as it is to calibrate your CHT before install. If it '"x" degrees off just note it. However, with 50 degree increments, it was difficult for my "old" eyes or my failing thought process to be accurate. (IMG:style_emoticons/default/biggrin.gif) AHARDER: I've got an extra set of Dakota leads and thermocouple. If you need one pm me. I've also got an extra set of VDO if you don't mind shitting yourself every time you look at the gauge.
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