Hi Mike,
I've been using what might be regarded as expensive (£85) pressure transmiters from RS Components, part no 455 4624. These are for the range 0 to 10 Bar (approx 0-150psi) and give a 4 to 20mA output, which can be put through a suitably scaled ammeter and/or into the analogue input of a PIC.
The advantage of using a transmitter is that the data, once "inside" the PIC, it can be used for control and also indication, either on a LCD and/or, as on "Vital Byte", a servo driven indicator that can easily be seen from the shore.
Frankly, I only tried using the pressure for control during the early experiments and now only use it for start-up and monitoring from the shore, in which case a 2 inch diameter (tyre pump gauge?), traditional pressure gauge would suffice. The signal is also used to shut the gas valve if the pressure exceeds 60psi, but this has rarely been used in practice.
I see the Maplin’s digital gauge (A12JR) is +/-1psi accurate, which is more than adequate for our application for start-up and monitoring purposes, though it would be a bit small to see from the shore.
Whichever pressure device you use, you should ensure that a "swan-neck" (u-tube) is placed between the steam manifold and the gauge, so that a slug of cold water condenses and prevents the steam from over-heating the gauge/transmitter.
I also use a pressure transmitter to monitor the fuel gas cylinder pressure. This signal again is fed into the PIC where it is used as a form of fuel-gauge (a switch is used to change-over from the servo indicating steam pressure to that of indicating gas pressure); this is very useful for extended runs (an hour) and you want to keep an eye on the available fuel supply. The gas pressure is also used to trim the steam temperature control loop so that, when a fresh cylinder is installed and the gas pressure is high, the gas valve doesn’t open too vigorously. This is a bit of icing on the cake regarding temperature control and I think you could initially get away without using it. A conventional 2 inch gauge (0 to 60 psi) indicating gas pressure would be very useful to have (no “swan-neck” required).
A couple of old foot pumps would be a good source of useful size gauges. Polly Model Engineering do a range of gauges and also Technobots on
http://www.technobotsonline.com/pneumatics/pressure-gauges.html. do a very reasonably priced range of gauges.
Since the “flash boiler” is not legally considered a pressure vessel, it doesn’t require a pressure gauge nor a safety valve, since the amount of “stored energy” is so low.
Ian.