When I first joined the lab, I watched Dr Know set up a PC to
accept measurement data. After opening up the case, he first had
to decide which device he could do without in order to free up
a PCI slot, then he had to untangle some wires and wiggle a data
acquisition card firmly into place before closing it up again.
After that came the fiddling around to allocate an IRQ that didn't
conflict with something else, before he was able to connect up
the sensor and calibrate the device. It seemed a lot of fuss just
to link up a little thermocouple and get some temperature measurements
into the PC.
USB has changed all that. If the data acquisition you have to
do is relatively straightforward and uncomplicated, there are a
number of small, inexpensive, auto-calibrating devices that simply
plug in to one of the USB ports dotted around your PC. Some of
these are designed specifically to connect thermocouples and other
temperature sensors to PCs via USB, and here I shall look briefly
at products from three manufacturers: National Instruments (www.ni.com),
Measurement Computing and Pico
Technology (www.picotech.com).
Thermocouples are far the most frequently used temperature sensors;
they're inexpensive, rugged and operate over a broad temperature
range. Pico's TC-08, NI's USB-9211 and Measurement Computing's
new PMD-TC are all designed for thermocouple measurements. They're
all plug-and-play devices and draw their power needs directly from
the PC via the USB port, so set-up couldn't be simpler.
The TC-08 is a basic but very capable unit with 20-bit resolution,
and a UK list price of under £250. The PMD-TC offers greater
accuracy and 24-bit resolution; its UK list price hadn't been finalised
at the time we went to press, but UK distributors Adept Scientific
expect it to be around £200. Unlike the other two, the NI
unit doesn't have on-board cold junction compensation, and has
four thermocouple input channels while the TC-08 and PMD-TC each
has eight; it is also more expensive, at £275.
If accuracy is your primary goal, it's not just the choice of
interface that matters; the accuracy of the sensor is also significant.
The electrical resistance of metals such as platinum is a constant
and easily measurable function of temperature, and resistance temperature
detectors (RTDs) rather than thermocouples will usually be your
choice of sensor when accuracy is your prime requirement. Or, if
sensitivity is your main concern, a thermistor may be a better
bet - the higher resistance of its coated semiconductor can deliver
sensitivities of ~200 O/∞C, though it's only really an option
if you're measuring lower (sub-300∞C) temperatures.
There's a new generation of temperature measurement modules that
allow you to connect these different types of sensor (thermocouples,
RTDs, thermistors) to different channels. That means you can mix
and match whatever sensors you need, without requiring additional
signal conditioning. The Pico PT-104 offers four simultaneous channels;
the just-released PMD-TEMP from Measurement Computing caps that
with eight temperature input channels - they're configured in pairs,
so you have to use the same type of sensor in a pair, but this
is unlikely to be a problem for users. Accuracy specifications
are impressive.
The PMD-TEMP is available in the UK at around £300; the
Pico unit is more expensive, around the £400 mark. Both come
with decent-quality data logging software and drivers for standard
application packages such as LabVIEW and programming languages.
The PMD-TEMP also includes the SoftWIRE graphical programming interface
for Visual Studio .NET (www.softwire.com) - why this clever, elegant
and extremely easy-to-use package isn't better known among design
engineers beats me, especially as you can now get it for free from
www.adeptscience.co.uk.
Oh yes, a final piece of advice: if your PC hasn't already got
USB2 ports, buy yourself a USB2 adapter card. You can pick them
up for as little as £10 (try www.amazon.co.uk) and the speed
gains are well worth it.
Prices quoted in this article were taken from the respective suppliers'
Web sites and are given exclusive of VAT.
Next month we'll depart from the Excel theme, and take a look
at the recently launched Data Analysis Extension Pack for Mathcad.