For the water side, I want to monitor water temperature and pressure in and out of the heat pump. Pressure gauges are available with suitably small range so as to get some accuracy. But when I search for temperature gauges, either by themselves or in a tridicator (T&P together), the temperature range is usually a couple hundred degrees, intended for water boilers. A GSHP water loop runs with a smaller change in temperature, perhaps only 8-10 degrees F. Does anyone know of a good gauge for the purpose, which won't break the bank? Ideally, I'm really only interested in pressure and temperature differences. Is there such a thing as combined dT and dP, a "quadricator" (I'm not holding breath on that one!)? Flow I already have covered, with an inline B-W rotameter.

Inexpensive digital thermometers can offer a high degree of accuracy for a modest price in spite of the range.
Using seperate instruments for measurements always offers an op for misread as they may not agree.
j

I'm pleased with $10 Topfin aquarium thermometers available at PetSmart. I strip back the heat shrink on their probes and use HVAC foil tape to attach them to lines of interest. Add a rag or bit of pipe insulation to minimize ambient effects and you'll get within a degree or two.

Posted By DickRussell on 08 May 2011 12:13 PM
For the water side, I want to monitor water temperature and pressure in and out of the heat pump. Pressure gauges are available with suitably small range so as to get some accuracy. But when I search for temperature gauges, either by themselves or in a tridicator (T&P together), the temperature range is usually a couple hundred degrees, intended for water boilers. A GSHP water loop runs with a smaller change in temperature, perhaps only 8-10 degrees F. Does anyone know of a good gauge for the purpose, which won't break the bank? Ideally, I'm really only interested in pressure and temperature differences. Is there such a thing as combined dT and dP, a "quadricator" (I'm not holding breath on that one!)? Flow I already have covered, with an inline B-W rotameter.

What brand/model pressure gauge will you be using? I'm interested in the same thing, but hung up on the pressure gauge issue.  Prefer to have a digital output for monitoring purposes.

Scewter, I haven't decided on T or P yet. For pressure, I'm really interested in just pressure drop, as a crude indicator of exchanger fouling over time. Admittedly, there can be significant increase in water side fouling factor long before fouling results in significant increase in pressure drop. I already have the gauge at the pressure tank as a rough measure of supply pressure. I'm thinking that I'll pipe up  small line with valve from each of the in and out piping, tee those together downstream of the valves, and then add a single gauge. I could get pressure of both sides with a single gauge that way.

For temperature, I may go with screw-in type or tape-on/insulate (eg. TopFin). I haven't decided yet. I don't really need recording on that, yet anyway. The idea of WEL is appealing, however, and I would like to accumulate KWH to the heat pump.

Maybe this:

http://parts.digikey.com/1/parts/950838-sensor-diff-press-29psi-max-mpx2202dp.html

If you're interested to the point where you want to record measurements for historical reference, have some charts to see trends, and have a picture of real time operation, all available via a web screen, then probably your lowest cost alternative with the least fuss is using a WEL monitoring system.  Still, while this is relatively inexpensive compared to alternatives, it's still got some expense to it that doesn't make it worth while if your interest is just casual and occasional curiosity.

The WEL unit itself costs $406.

Temperature sensors cost $15.

Two temp sensors plus the WEL is all you need to monitor EWT, LET, and DeltaT.

Pressure sensors are expensive, on the order of $200+.  The WEL readily accommodates either 0-10 VDC or 4-20 ma versions.  Two of them, to provide DeltaP, would be needed.

Installation of the sensors to HDPE pipe is also expensive if not done at time of installation.  (BTW, if you're going to go to the expense of retrofitting in pressure sensors, I'd go to further expense and put in a flow sensor while you're at it.)

Now you've got everything needed to get continuous and accurate BTU and BTU/h data.

Permanent or long term insertion of sensors (temperature or pressure) into PT ports is not recommended - the seals in the ports are not designed to have something constantly inserted, and thus will probably start leaking.

Unless there is unusual reason for the need for continuous pressure monitoring, I don't think the cost of instrumentation pressure sensors makes sense for a residential situation.  Instead, get yourself an accurate pressure gage, and manually measure the pressures as desired via the TP ports.  I use a 4" Ashcroft 'Duralife Plus' gage, at approximately $80.

Hope this helps.

Best regards,

Bill