Yikes!cYou are entering the most competitive field in industry. There are literally thousands of temp controllers.
The commercial refrigeration field almost never uses three/four wire devices. They really go for two wire.
osmosis' -50 to +150degC is pretty much it.
Your Ni has a higher temp coeff which helps.
The the most coveted of industrial temp controllers was the GoldLine made by a company that also made swimming pool controls and actually grew from that field outward. Unfortunately that company stumbled over digital pool chlorination and doing it by just adding salt to their gizmo which cracks it into chlorine. They can then creep the Cl into the pool 24/7 instead of the normal, big blasts of it, causing wild chemistry swings. In their infinite wisdom they dropped their comprehensive temp control line completely, and nearly instantly.
Replacing it as the simple, most inexpensive biggest bang for the buck digital controller for temp, in the industry is the 'Ranco'.
It's nicely done. (I just hacked 4 of them for a job) You should pick one up and dissect it. It uses the industry favorite - the thermistor.
Thermistors limit the maximum temp and are the main reason designers head to RTDs.
With your RTD you can easily do 1/4F but no one will care and you immediately add display costs doing it, as you then need a decimal and another place.
Most people in industry/commercial realize that any space bigger than a domestic refrigerator will not be one uniform temperature. Corners may be warmer, there may be some stratification, warmer areas, colder where the evaporator outlet is, etc. So sub degree control is pretty pointless. It's also is much harder on all the equipment.
Hence the 1F or 1C most commonly seen.
Whatever you do, you'll probably want a factory calibration, as the Ni1000 are out of the box about +/-2 degrees. You will probably want to allow calibration by the installer too. The typical situation is the unit is installed, run to setpoint, then a hand held thermometer is put in the most rational location. The thermostat display is then adjusted match the thermometer. This takes into account everything previously mentioned about cooled space inhomogeneity.
If you want a more accurate assessment you'd need to put your proposed controller into a more specific field for us. Like wine cooling, or blast freezing, etc.
Keith Cress
kcress -
