Over the last two weeks we've shown you how to install thermocouple intake air and exhaust gas temp probes. But how do you see how hot or cold they are? To display the temps, you need to use a device that will take the input of a K-type thermocouple. And it isn't just a simple meter - K-type thermocouple displays need to have internal cold junction temperature compensation and fancy amplifier circuits if they are to be accurate.
Hand-Helds
The easiest way of seeing the output of the probes is to use a multimeter. Many high-level multimeters come with direct thermocouple inputs - you plug the thermocouple into the special socket on the meter and set it to 'temperature'. The LCD display then shows you what temp the thermocouple is experiencing. If you search hard, it's even possible to find cheaper multimeters with this feature - eg Cat no Q1051 at Altronics (www.altronics.com.au) costs just A$30 (A$20 on special as I write!) and will cope with inputs of up to 1370 degrees C. Mounting one of these on the dash of a competition vehicle gives a ludicrously cheap and accurate thermocouple read-out - just the thing for measuring EGT on a budget race car!
Even if your multimeter doesn't have a direct temp read-out, you can plug a thermocouple adaptor into it. The direct voltage output of a thermocouple is very small (just millivolts) and the thermocouple adaptor amplifies this voltage and applies the required compensation. The output of the adaptor is usually 1 millivolt per degree C, which means that - for example - when the thermocouple is experiencing a temp of 51 degrees C, the output of the adaptor is 51mV. Plug the adaptor into the meter, set the multimeter to read in millivolts, and the display will say 51. If the multimeter has an auto-ranging facility, it might read out 0.051 (as here), or 0.510 or 51.00 - but it's still pretty easy to interpret. Thermocouple adaptors cost about A$75 from electronics stores, or you can go here http://www.extech.com and pick one up for US$29.
Alternatively, you can buy a handheld meter designed specifically to show the output of thermocouples. These meters are often used in motorsport where they're matched to a tyre temp probe, but they're also available (more cheaply!) from electronics stores. Single input thermocouple meters cost from about A$100, while a dual input design like the one shown here costs A$170 from Dick Smith Electronics (http://www.dse.com.au). An advantage of a dual input meter like this one is that it has the ability to subtract one temperature from the other (eg to find the temp drop across an intercooler), can store maximum readings, and freeze the display. As with most thermocouple meters and adaptors, the display can be in either Fahrenheit or Celsius units.
Dashboard Displays
But what if you want to build in a dashboard instrument, rather than have a handheld meter floating around in the cabin or strapped to the dash? There are a few approaches that can be taken. Many industrial process control, refrigeration and air conditioning systems use a digital display matched to a K-type thermocouple. Unfortunately, most of these meters are mains-powered and so are not amenable to being mounted in a car - not without a long extension cord, anyway! However, there are some around that are small, neat, effective - and work on 12 volts.
The best that I have been able to find is the Carel IR32 series of controllers. The Carel IR32V2L000 uses a 13mm high green 3-digit LED display, can measure temps up to 999°C, and has an in-built over-temp alarm. It can even trigger an on-board relay when a certain temp is reached, allowing the switching on and off of accessories like an intercooler water spray or similar. The controller works on 12-24 volts and uses the input of a single K-type thermocouple (so you'll need two controllers if you want to display both intake air and EGT). I used a Carel controller in a turbo car for 12 months and can vouch that it's a superb instrument.
A drawback of the Carel controller is that, like all LED displays, it cannot be read in directsunlight. As a result, it needs to be recessed in its dashboard mounting - a bit like a LED digital clock. The Carel IR32V2L000 controller has a retail price of around A$260 - go here (http://www.carel.com/) for your nearest distributor.
If you're happy to fit a 5 volt regulator (simple to do, and covered next week) you can use the Extech Instruments Big Digit Temperature Panel Meters .
Cheap and good looking, these thermocouple displays are available in red or green LED displays and cost just US$79 each. They use a one-eighth DIN cut out (45 x 96mm) and are only 35mm deep. This is an extraordinarily low cost for a standalone K-Type thermocouple meter, but the display reads up to only 800 degrees C - so if you're intending on running very high EGTs, this may not be high enough. As with the Carel controller, you will need two displays to measure two temperatures.
And if you're even more electronically inclined, you may have realised that the output of the multimeter adaptor (described above) can be fed straight into a LED or LCD digital panel meter. These panel meters are available very cheaply from electronics stores (from about A$30) and, with the input from the thermocouple adaptor, can be configured to read out temp in the same way as the multimeter. However, there's an even better (ie cheaper!) way of doing this which we'll cover next week.
And what about the guinea pig Audi? To make things difficult, I wanted the new temperature display to match the colour and type of digital displays already present in the car. The Audi - like many cars of the last five years - uses what are called negative LCD displays. Rather than have black digits on a silver background (as conventional LCD displays do), the car uses light coloured liquid crystal digits on a dark background. These displays are of course backlit (so that they're visible at night) and have much the same appearance in all lighting conditions - direct sunlight, at dusk, and when fully backlit. These types of display sometimes use negative red, negative green or negative amber - the Audi displays are negative amber. So what did I want? - a negative backlit amber LCD display!
But there aren't any K-Type thermocouple meters with negative backlit amber LCD displays available anywhere in the world, so I had to make my own. But with me being a bit of a duffer when it comes to electronics, it had to be pretty simple to produce. Next week we show you how to do it, with the resulting temp display available in pretty well any colour and style you like, and also having the ability to take multiple thermocouple inputs.
Next week: making your own thermocouple display.
TempScreen: Part 1 - Installing the Intake Air Temp Probe
TempScreen: Part 2 - Installing the Exhaust Gas Temp Probe
TempScreen: Part 4 - Building a Custom Temperature Display
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