An automatic water temperature control valve (TCV) is a highly desirable darkroom accessory. Unfortunately, good ones are expensive. With the decline in traditional darkroom activity, TCVs and photographic water control panels now show up as surplus or auction items quite frequently. One of the best TCVs is the Lawler 9700 series, also sold by the firms listed in the title. These valves are capable of superb temperature regulation, but are also prone to a variety of problems. Long ago I attended RIT and every darkroom was equipped with one. They were unpredictable, impossible to adjust accurately, and I assumed this was simply normal operation. Only when I acquired my own did I learn exactly how they worked, and how to transform them from a constant annoyance, to a valuable tool that improves process consistency. This document applies to the model 9700, but the general concepts should useful for different variations of the valve.

The first order of business is to go to the Lawler web site, www.lawlervalve.com, and download the Installation & Maintenance Manual for your particular TCV. Lawler's distributors will sell you any parts you may need, but try and recondition as much as possible. The basic o-ring kit is inexpensive and that's one item you should always keep on hand.

The Lawler manual covers basic testing and operation very well, and you should perform their checks exactly as outlined. There are, however, some subtleties not covered, and that is the reason for this document. I will assume you've read the manual, and will not go over the basic operation or construction of the valve here. I'll also assume some familiarity with tools, that you have the right ones for the job, and you understand that the mark of a good craftsperson is that they leave no marks.

It isn't necessary to do a permanent installation, in fact the need for cleaning and maintenance makes behind-the-panel mounting undesirable. The valve should be equipped with inlet strainers and check valves to prevent back flow and mixing in the water system. The best ones are supplied by Lawler and, hopefully, are already installed on your TCV. A standard male garden hose coupling can be attached to each check valve inlet. Mount the valve to the wall, using a board with a support block at the bottom and a capture block at the top. The capture block is nothing more than a piece with a hole to accept the vertical outlet pipe, split, so the valve can be easily removed. Use standard flexible clothes washer hoses to connect from the fittings to your hot and cold water supplies. If the installation is near the clothes washer, use inexpensive "Y" connectors to attach at the same point. Using this method, I was able to have my valve mounted and operating in less than an hour, with no significant plumbing work.

The TCV outlet pipe should have a thermometer fitting, and possibly a valve downstream to control flow. The newer 9700s have a flow control valve on the cold water inlet check valve. I don't know which is the better arrangement from a performance standpoint.

A common process thermometer found on these TCVs is a 3 ¾" diameter, 40-140° F dial type, with a large bulbous stem that seals in the vertical outlet pipe with an o-ring. These thermometers are extremely fast acting, and can be adjusted by unscrewing the window, holding the axle with a screwdriver, and carefully rotating the friction fit pointer. They are not the worlds most accurate thermometer, suffering from some long term inconsistency, but they are ideally suited for the application due to their quick response. Note that these thermometers may act erratically when subjected to large temperature changes, but this is normal operation.

A useful addition, if you use an outlet valve, is a pressure gage. Mount this somewhere on the outlet pipe, ahead of the outlet valve. My average water pressure is a low 30 psi, so I use a 0-60 psi gage. Use whatever your maximum water pressure is, plus 100%. You need the extra range to handle water hammer and pressure surges.

This is a deceptively simple part of the TCV, contained under the large hex cap at the lower front of the valve. If your TCV fails the tests in the Lawler manual the pressure shuttle is certainly sticking. Even if your TCV passes those tests, the shuttle may still have too much friction to perform well. Symptoms include sudden jumps or vibrations on your pressure gage not caused by the water supply, or oscillations in the control temperature. These oscillations are usually too fast to register on the thermometer but easily felt by hand.

The pressure shuttle must be absolutely free to move in its bore. It must be free from water deposits, and the o-ring must be exactly the right size. It is best to work on the pressure shuttle with the temperature control knob removed. Use a penny to unscrew the central screw (avoids marks), then pull off the knob and brass nut. With the water lines shut off, remove the hex cap covering the pressure shuttle. Catch the water with a small bucket if necessary. The end of the pressure shuttle is threaded ¼-20. Use any ¼-20 bolt as a handle to pull the pressure shuttle out of the valve. After much neglect, it may be difficult to remove. Pull harder. The pressure shuttle operates in a plastic liner. This liner seals to the valve body with two o-rings and may be nearly impossible to remove, or it may simply come free with the pressure shuttle. If it doesn't come out, we'll decide whether to remove it shortly.

Warning: All the valve parts tend to have sharp edges that will cut the o-rings. Use extreme care and a bit of o-ring lubricant to prevent damage to o-rings. Do not force anything, as all parts should slide together easily. If something appears to be hung up, stop and figure out why. Inspect all used o-rings carefully for cuts and dents.

Remove the o-ring from the pressure shuttle and clean off any water deposits. The shuttle is plated, so don't file or stone it unless absolutely necessary. The plating may be worn off in some areas, but that shouldn't affect operation. Without the o-ring, slide the shuttle in and out of its bore. It should have no resistance what-so-ever. None. If the shuttle binds, and it appears in good condition, you should remove the plastic liner. This is risky business, as a new one will have to be purchased if you damage it. The liner has 12 internal water ports, and these can be used to grab it and pull it out. One method, admittedly crude, is to use two medium nails. Grab the pointed end of the nails with Vise-Grip™ pliers, so the heads of the nails are a couple inches away. Insert the heads into the bore, then expand them sideways into the first set of ports. Press something tapered between the nails so they cannot close up again. Lock the pliers tightly. You must keep the tapered item in place so the nails cannot close up. Now, attempt to pull the liner out of the valve body. Do not use excessive force that will damage the port slots if another liner is unavailable.

Inspect the liner and o-rings. A common problem is o-rings that were cut and jammed during a previous installation. This makes the liner egg shaped and causes it to bind the pressure shuttle. It should spring back round if correctly installed with new o-rings. The liner should install easily- twist and wiggle it until it slides in. Use lubricant. Do not force it, as the o-rings will be cut and you'll end up right back where you started. Note that the liner is oriented with the two o-rings toward the front of the TCV. Test the fit of the pressure shuttle again. Proceed only when the pressure shuttle can move freely.

Using lubricant, install an o-ring on the pressure shuttle. Insert it back in the liner and check the fit. You should feel that the o-ring has entered the bore, but it should barely compress. There should be almost no friction when the shuttle is moved. If you detect more than the slightest amount of "stick-slip", the valve will not work well. Clean the o-ring groove, find a slightly smaller o-ring, or otherwise determine the cause of the problem. When all is free, reinstall the hex cap. It seals with an o-ring and does not have to be excessively tightened. You may want to test the valve at this point, since many problems seem to originate with the pressure shuttle.

The mixing valve piston must be in proper adjustment, clean, and free to move. The rubber "hot seat" gasket should be in good condition, and the thermostatic element must be working properly. The critical parts are stainless steel and are very long lasting.

The piston tends to have sharp edges which can cause it to hang or stick slightly as it moves in the bore. Using a very fine file at 45° , or a piece of fine silicon carbide paper, smooth the edges of the piston until they are no longer knife-like. You don't want to round the edges, just prevent them from digging into the bore.

The piston assembly consists of the piston itself, a threaded rod, spring, the hot seat gasket holder and gasket, a special nut, and a tapered spring that attaches to the nut. The assembly will have been assembled using a thread locking compound, so first time disassembly will require a bit of effort and subsequent cleaning. Remove the nut, then the hot seat gasket parts. It isn't necessary to remove the rod unless you decide to reverse the spring as described later. You may want to use a mild thread locking compound (local auto parts store) after determining final settings, but I haven't found it necessary for intermittent home processing use. In a commercial application it should definitely be used.

The setting of the hot seat gasket height is extremely important. As the gasket is moved away from the piston, the overall flow rate of the valve will be reduced because both the hot and cold supplies will be more restricted for any given setting. At the same time, the gain of the control system will go up. In other words, a very small motion of the thermostatic element will have a large effect on the balance of hot and cold water. Most amateur darkroom operations require low flow rates, so the hot seat gasket can be fairly extended. Press the piston all the way in and examine the cold water ports at the top of the bore. My valve worked best with very little of the ports visible, maybe .015" or so. You should be able to find a setting where the output temperature remains steady in spite of input fluctuations, and reacts predictably to changes of the control knob position. If you can't get the flow you need, move the gasket closer to the piston. If regulation is poor, move it further away. Listen to the water flow and watch the pressure gage to be sure the TCV isn't oscillating or unstable. The TCV should return very close to the last temperature setting each time you use it and small adjustments of the control knob should yield small predictable changes in temperature. There should be a bit of overshoot, then reasonably fast settling to the final temperature.

Clean off all debris before reassembling. Do not over tighten the four slotted screws holding the body together and use a cross tightening pattern so the cover is brought down evenly.

No guarantees on the following. The information was obtained from measurements on the two older Kodak PX-9700 series valves I own, but little seems to have changed over the years. Note that these valves are offered in different flow ranges, so port sizes and pipe sizes may vary. Mine are 10 gpm with ½" fittings. Modern 9700s come in several flow rates with different internal part sets.

The pressure shuttle o-ring appears to be a Parker 2-013 size, the liner appears to use two size 2-018. In a pinch, these are often available at local hardware stores.

The temperature control knob o-ring is a square cross section and should be secured from Lawler.

Parker O-ring Lube works well for assembly purposes. Though I worry about a blob of grease leaving the TCV and contaminating my film or prints, it hasn't been a problem. Use sparingly!

The hot seat gasket should be 0.563" in diameter and 0.066 thick. The experimentally inclined may find improved regulation with a replacement made from 1/16" hard Teflon, or similar material.

The copper rod that actuates the piston should press evenly on the end of the spring. If the end of the spring isn't well formed, forces will tend to cock the piston and increase friction. You can unscrew the center rod to remove the spring and examine each end for the best surface. Install the best surface towards the copper rod.

C. Hoffman, 5/6/03