Alternative Cyl Head Temp Method

philinjax · 01-08-2002, 12:12 PM

Joe Recard and jmcleod's posts to the thread "914 Thermostat.. does yours have one" both mentioned CHT sensors: A little story.

One day while fooling around on my bench with a stock center console oil temp gauge, I hooked it up to a CHT sensor and applied 12 Volts. The gauge read zero. I applied heat to the sensor with my hot air gun and the guage needle started to go up. When I held the heat on (it is a 1500 watt gun) for awhile, the gauge needle went up to close to full scale. So what does this imply? I think that if you have carbs, you could hook up the stock CHT sensor to an oil temp gauge. You wouldn't know what the temp reading is but, you would have a very inexpensive, relative reading, CHT gauge. Of course, we don't know what the heck the center console oil temp gauge reads either.
Phil

pbanders · pbanders's Garage

The CHT is a negative-temperature-coefficient resistor (NTR). The resistance when at room temp is about 2.5 K ohms, and when hot (>250 deg F) it's 100 ohms or less. Most other temp sensor elements used on cars are NTR's or some type of thermocouple (less common). I don't know exactly how the temp gauge works (it probably measure current), but it probably assumes the sensor is a NTR and that's why a CHT works with it.

Brad Anders

philinjax · 01-08-2002, 02:19 PM

Brad,
Yes, you are correct, both the oil temp sensor and the CHT are thermistors, although with different resistive ranges. The stock oil temp gauge in the optional center console is simply a miliamp meter with a fixed range.
I'm refurbishing my wife's teener at the time but, when I get back to my project car, I may use two stock temp gauges, one wired for oil and one for CHT. I will calibrate the gauges so that they accually read temps.
Phil

jabez · 01-08-2002, 02:32 PM

Since both you guys sound like you know what you are talking about, I'll ask an off the wall question. I got the combination gage that was standard on the early 911 from a teener in a junk yard a while back for $30. I liked the idea of being able to actually see oil pressure and oil temperature. I tried hooking up the oil pressure gauge to the standard VDO pressure switch/sender and it read about 1/2 actual value. Could I install a resistor in line to compensate for this, or do I have to buy the 911 sender? If I do have to buy the sender, do any of you 911 experts know if they have the combination switch/sender type? This combination gage has a spot for the oil pressure light as well as oil pressure gage.

GS

pbanders · pbanders's Garage

I may sound like I know what I'm talking about, but when it comes to gauges, I don't know squat. Dave Darling seems to know all the ins and outs of the gauge equation.

Dave at Pelican Parts · Dave at Pelican Parts's Garage

Not all, but some...

There are two ranges of aftermarket VDO senders for the oil pressure. Sounds like you have the 150 PSI sender, and you need the 80 PSI one.

A friend once told me, "All analog meters are actually ammeters." He seems to be right so far. They (almost?) all work by running a current through an electromagnet and using that to pull the (spring-loaded) needle. More current, more magnetic force to overcome the spring force.

The context of the remark was that a voltmeter is really just an ammeter, but it applies to most analog meters I've found...

--DD

jmcleod · 01-09-2002, 09:17 AM

so the temperature transducer is probably either a thermoresistor (temperature affects resistance) or a thermocouple (temperature produces a voltage) that work with Ohm's Law (V = I * R) to affect a current that is passed thru a coil of wire wrapped around a permanent magnet to deflect a needle attached to the coil to deflect in front of a calibrated scale. so if the meter is calibrated, "any" sensor could be used with the meter to indicate oil or head temperature.

philinjax · 01-09-2002, 10:40 AM

The sensor must be matched to the current range of the meter.
In some cases, a resistor of appropriate value can be added in line that will bring a sensor range within the display range of the meter but, it will always distort the temp. vs resistance curve beyond design spec.
Of course, thermistors can't be interchanged with a thermocouples in a given circuit.
Phil