24-channel RTD board
APOLLO is going to go nuts about temperature measurement. For example, each time-to-digital converter (TDC) will have five RTD sensors placed within it (one at air intake, three near the RC portion of channels 15, 8, 2, and one at exhaust). Right now APOLLO only intends to use one TDC, but this may change in the future.
The RTDs are 1000.0 ohms at zero °C, and change by 3.85 ohms per °C. We need signal conditioning electronics to send exactly one milli-amp through each RTD and scale the resulting voltage to some sensible analog value like 0.0 V at 0.0 °C, and 0.1 V/°C slope. The result can be fed into our A/D PCI board (64-channel single-ended or 32-channel differential).
The conditioning should be built compactly in a box with two potentiometers (gain, offset) for each channel. Two 25-pin D-connectors can connect to two different sets of 12 RTDs (e.g., 12 in laser environment, 12 in cabinet/IL-enclosure). The output can be either a single 25-pin D connector (for single-ground sigle-ended) or two 25-pin connectors for differential measurement. Since the box will presumably be located near the computer, it may not be unreasonable for them to share grounds and run differentially.
The RTDs may be far away, and while we will probably use shielded cabling, we should put appropriate filtering onto the inputs and outputs so that the measurements are very slow to change (few-second time constants).
A list of potential locations for the 24 RTDs follows. The first 12 are in or around the laser.
As you can see, it's not hard to fill out a list of 24. Maybe it should be 32! What would we do if we wanted another TDC? I can think of a few other places in the laser environment (motor, APD) where we might like to know the temperature.